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Fire Control, Ranging, Dispersion


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13 hours ago, bojan said:

Problem with RMG is that it is useful only at the distances of APDS/APFSDS battlesight range. Then you start wondering "why bother with it at all", like Israelis decided those were not worth the effort

I dusted Rob Griffin’s book of Chieftain a couple weeks ago; IIRC the RMG was initially good out to 1500m and later improved ammo raised it to 2000m. Firing APDS at 1400m/s you could probably engage anything stationary out to 1500m without any help. Of course, it would be handy for HESH but with M392 APDS and M456 HEAT (1100m/s from the L7/M68) I can understand they Israelis not being too impressed by the RMG

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21 minutes ago, Gorka L. Martinez-Mezo said:

Well, when I was a young lad playing Microprose’s Gunship in the mid 1980s the T-72B was still known as T-74..... and I had no idea T-64 even existed. So it’s pretty evident basic intelligence was lacking.....

 

on the issue of “invulnerable” Soviet armor, probably a solid hit by L52/M728 or L15 APDS surely would cause some internal damage. Still, the Soviets could kill NATO tanks at any practical range with a single hit from either APFSDS or HEAT.... scary

Yeah, that makes me chuckle to think back on it as well.  😄 In fairness they did put T64 in M1 Tank Platoon. What wonderful sims those both were.

12 minutes ago, Gorka L. Martinez-Mezo said:

I dusted Rob Griffin’s book of Chieftain a couple weeks ago; IIRC the RMG was initially good out to 1500m and later improved ammo raised it to 2000m. Firing APDS at 1400m/s you could probably engage anything stationary out to 1500m without any help. Of course, it would be handy for HESH but with M392 APDS and M456 HEAT (1100m/s from the L7/M68) I can understand they Israelis not being too impressed by the RMG

I cant find Bobs book now, and ive no reason to doubt him because he knew his onions. OTOH on Page 66 of George Forty's Chieftain book (still worth picking up even though it came out in 1979) the RMG tracer burnout was at 1800 metres (which seems to agree with the Chieftain RMG manual if my memory serves) with a 'recently introduced'  new tracer round that gave ranges out to 2500. Though I suspect witnessing strikes out at that range made your eyes go out on stalks.

I think where RMG worked best was in quick reaction shoots. That was the kind of engagements CAT tended to simulate, and RMG seemed to work quite well on those occasions.

 

 

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3 hours ago, Stuart Galbraith said:

People assume from the results CAT it was inaccurate. It wasnt. It was slow, which is why, due to the mediocre stab, they prefered to fire from static fire positions. Which was British Army doctrine anyway. Yes, the report probably was overegging what an IFCS or CCS could do,but reaching out to 3000 metres at a target on the move should have been perfectly doable from a static fire position. The Digital Version was certainly reaching out that far in 1991.

It was not inaccurate, but T-72M is not inaccurate either in theory. Problem with both is "return reticle on target manually after ballistic solution was achieved", is not that it only slows down engagements, but also made firing at moving targets more problematic. Yes, it could be done, but doing it with any sort of regularity @ 3500m is... optimistic to say at least. Compared to fully automated FCSs of the every new tank at that moment it was, well, not stone age, but early iron age at best. :) Hell, even Soviets saw the light, T-64B/B1 had fully automated FCS in 1976, and T-80B also had it. They just could never produce enough of those to afford it for "2nd line" T-72s, so those had same crap as C1. Well, worse one, w/o lead counter, which sucked even more for engaging moving targets.

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Well it saves APDS rounds for one thing.

Yes in theory (if first shot hit is not achieved by battlesight engagement), but it also extends time of engagement, increasing chance that tank will be hit. It was also not w/o other problems (it is not affected the same way as real projectile by the crosswind, temperature, air pressure etc). I come from a position that practically everyone tried it and yet... gave up on it. Except Soviets ironically who installed DShK/KPVT in the T-10 series for exactly the same reason, but those fired much slower AP initially so it was considered a good addition for a while... They (liking simple solutions, and RMG is one of the simplest) considered it for T-64 but found no point in it with APFSDS. So they went with optical RF, same way Americans and Germans went.

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I can only go by what ive read, the gun was accurate enough that with a good gunnery it wasnt really necessary. OTOH, the Israelis wanted RMG fitted anyway. Go figure.

There you have it. And it is not gun "accuracy" it is a hit probability at range with fast rounds.

Israelis might have wanted it, but they also promptly removed them when harsh reality hit.

 

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We had just joined the EEC and started throwing our weight about. So no, not really so strange. :D

Or they have shared it, but it was ignored since it was contrary to a dominant thinking at the moment and would raise a lot of unpleasant questions. Corporate culture (and militaries and DoDs are most blatant examples of it that can be found on the God's green earth) is like that.

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4 minutes ago, Stuart Galbraith said:

their usage of RMG ... I think it was probably the cheapest, easiest solution to the problem before Laser rangefinders. Its more solid than optical rangefinders,

I'm a bit skeptical, TBH. I think the decision to field RMGs was made by underinformed officers. This is based on a number of papers I read (I no longer remember where, exactly, but it was a scanned typewriter page by some ballistics boffin giving his technical assessment of the military requirement to develop a ranging round that would behave "ballistically identical" to the main gun round. This was apparently done after the decision to use a ranging MG was already made, and "all that's left to do" was to develop a cal .50 round to do the magic.

You could sense his incredulity in the non-technical passages where he explained in simple words that the requirement wasn't difficult, but impossible to reach. I have personally experienced that superior officers were even unfazed by mathematical proof of the impossibility of a given task, congratulating us on the "great work", followed by the order to keep working on it.

RMGs were cheaper than stereoscopic rangefinders. But in the relevant range bracket they were definitely not better, or faster to work with, particularly not in moving engagements. They were cheap, and that's all there is to say about it.

Incompetent officers weren't a rarity. The inventor of the SABCA fire control system which I interviewed some 15 years ago in Australia told me that when they presented the computerized fire control system for the Leopard with its clearly superior performance, the General in charge went ballistic and told everybody that he didn't want no f'in' computers on his tanks, and that they were a useless gadget that would make crews incompetent in gunnery (and would possibly turn them gay or something).

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24 minutes ago, Ssnake said:

....Incompetent officers weren't a rarity. The inventor of the SABCA fire control system which I interviewed some 15 years ago in Australia told me that when they presented the computerized fire control system for the Leopard with its clearly superior performance, the General in charge went ballistic and told everybody that he didn't want no f'in' computers on his tanks, and that they were a useless gadget that would make crews incompetent in gunnery (and would possibly turn them gay or something).

Military likes the stuff that works and are skeptical toward new stuff. They actively push down those trying to champion innovation. It is a worst corporate culture imaginable for introducing new things. Well, unless there is a serious war, where everyone is "Give us something better now, or even better yesterday". Just like any big companies, and turned up the 11. :)

Edited by bojan
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Just now, bojan said:

Military likes the stuff that works and are skeptical toward new stuff. They actually push down those trying to champion innovation. It is a worst corporate culture imaginable for introducing new inventions. Well, unless there is a serious war, where everyone is "Give us something better now, or even better yesterday". Just like any big companies, and turned up the 11. :)

or the US, where they went from "we want stuff that works" to "we want stuff that may not work but is so bleeeeding edge the rest of the World will be amazed"

 

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That is because they found that lobbying brings them nice paycheck after they retire. It is a corruption at it's ugliest form.

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44 minutes ago, Stuart Galbraith said:Yes, you are right I fully admit. I kick myself not asking Bob Griffin more about their usage of RMG whilst I could. I think it was probably the cheapest, easiest solution to the problem before Laser rangefinders. Its more solid than optical rangefinders, and less requiring of maintainance and training. OTOH, its telling the MOD removed them as soon as they got a good TLS, despite troopers wanting to keep them to use as a secondary Coax. Personally Ive never been keen on optical rangefinders when it was seemingly proven in pakistan how much longer it took to get a solution than it did the 3 round technique in what I think were 20 pdr centurions.

IIRC the Indian Centurions had no RMG, but they used the three shot ranging technique used by the British at the time with their 20pdr guns. The RMG worked similarly but saved main gun ammo. Will try to add the RMG graticle from Bob’s book on Chieftain

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5 hours ago, Ssnake said:

With .25mil shot dispersion, the 1SD impact area is a .5mil diameter circle. That fits almost exactly your target silhouette. It's .02mil too little vertically but has .1mil margin left and right, so the hit likelihood would still be at least 68%, probably over (if barely) 70%. I deliberately lowballed the estimation in my first post.

68.3% fall into the 1 sigma bracket

95.5% fall into 2 sigma

99.7% within 3 sigma.Standard_deviation_diagram.svg

Well perhaps you're right, but I still doubt that the graph wasn't calculated from pure mechanical dispersion alone. If you take a look at the probability of hit calculations from the Trilateral Gun trials, 120mm HEAT should have a Ph of just 0.1 at 3 km, even lower than depicted in the graph. 

unknown.png

And in this context, they were specificall dealing with dispersion alone, not other factors.

unknown.png

(images courtesy of wiedzmin)

 

So again, maybe you're right, but this doesn't totally add up.

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38 minutes ago, Ssnake said:

Incompetent officers weren't a rarity. The inventor of the SABCA fire control system which I interviewed some 15 years ago in Australia told me that when they presented the computerized fire control system for the Leopard with its clearly superior performance, the General in charge went ballistic and told everybody that he didn't want no f'in' computers on his tanks, and that they were a useless gadget that would make crews incompetent in gunnery (and would possibly turn them gay or something).

I’m not surprised; when Spain was developing the Lince project (a baby Leopard 2) top brass asked for a simplified FCS as the EMES 15 was “too complicated” for the average soldier..... at a time when botj Spain and Germany were conscripted armies. I guess German 18 years olds we more intelligent than Spanish generals.

Curiously enough, the Mk7 FCS being introduced into M48s at the time used the IFCS style moving reticle with seems to be more difficult and less user friendly to use vice EMES 15

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22 minutes ago, Interlinked said:

Well perhaps you're right, but I still doubt that the graph wasn't calculated from pure mechanical dispersion alone. If you take a look at the probability of hit calculations from the Trilateral Gun trials, 120mm HEAT should have a Ph of just 0.1 at 3 km, even lower than depicted in the graph. 

I'm now more convinced than ever.

Such a low low hit probability figure can only be explained with their asumption that the range to the target is NOT known with precision. They seem to use a range estimation bracket and then look at the trajectory of the round to see what range error is still tolerable.

A very flat trajectory like APFSDS with high muzzle velocity and minimal drag allows for very generous error margins, at least on fully exposed targets (with the resulting high hit probabilities), but the steeper the trajectory becomes the greater the need for accurate range measurement. Conversely, if you fix the range error, you make the hit probability your variable.

 

This goes to show that you can't be careful enough looking at papers and diagrams and that you have to check the underlying methodology, otherwise you're going to compare apples not even with oranges, but internal combustion engines.

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1 hour ago, Ssnake said:

RMGs were cheaper than stereoscopic rangefinders. But in the relevant range bracket they were definitely not better, or faster to work with, particularly not in moving engagements. They were cheap, and that's all there is to say about it.

The main advantage of RMGs (and arguably their only) was the much easier integration into the tank. Due to the lenght of optical rangefinders being in excess of 2 meters, they could not be placed into the frontal section of the turret without sacrificing a good ballistic shape (well sloped turret front). This is why the Chieftain has no optical rangefinder and why the rangefinder in the M60A1 and similar American tanks is operated by the tank commander, rather than the gunner.

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49 minutes ago, Gorka L. Martinez-Mezo said:

IIRC the Indian Centurions had no RMG, but they used the three shot ranging technique used by the British at the time with their 20pdr guns. The RMG worked similarly but saved main gun ammo. Will try to add the RMG graticle from Bob’s book on Chieftain

Yes, as said they used the 3 round technique. The only 20 Pdr Centurions that hard an RMG would have been ones that got converted back to 20pdr from 105mm (you only unscrew the barrel and change the racks) to use old ammunition for training, and the Aussie ones, presumably they  wanted a .50 for a coax in Vietnam. So yeah, the Indians were still using the old 3 dot technique.

1 hour ago, Ssnake said:

I'm a bit skeptical, TBH. I think the decision to field RMGs was made by underinformed officers. This is based on a number of papers I read (I no longer remember where, exactly, but it was a scanned typewriter page by some ballistics boffin giving his technical assessment of the military requirement to develop a ranging round that would behave "ballistically identical" to the main gun round. This was apparently done after the decision to use a ranging MG was already made, and "all that's left to do" was to develop a cal .50 round to do the magic.

You could sense his incredulity in the non-technical passages where he explained in simple words that the requirement wasn't difficult, but impossible to reach. I have personally experienced that superior officers were even unfazed by mathematical proof of the impossibility of a given task, congratulating us on the "great work", followed by the order to keep working on it.

RMGs were cheaper than stereoscopic rangefinders. But in the relevant range bracket they were definitely not better, or faster to work with, particularly not in moving engagements. They were cheap, and that's all there is to say about it.

Incompetent officers weren't a rarity. The inventor of the SABCA fire control system which I interviewed some 15 years ago in Australia told me that when they presented the computerized fire control system for the Leopard with its clearly superior performance, the General in charge went ballistic and told everybody that he didn't want no f'in' computers on his tanks, and that they were a useless gadget that would make crews incompetent in gunnery (and would possibly turn them gay or something).

The thing is, and this is the point that seems to get missed, the British Army had been using the RMG for some years before it was introduced on Chieftain. It had already been introduced on Centurion for some years before  (id have to exactly check, but im guessing between the late 50's the early 60's). With the newly introduced MOD being so penny pinching (hence the reason why the miserable L60 was never replaced) I find it hard to believe they would ever have introduced the RMG into Chieftain, if they were not satisfied it was at least part of a solution to a problem. For example, there was an IR detector stalk for Chieftain that was seldom employed, and never seems to have even been carried in later tanks. There was the meterological stalk for IFCS that was ONLY carried by command tanks, because the MOD was so miserably tight to buy them for every machine. And then there is the II swapsight for the gunner that seems to have been bought in very small numbers and never actually issued to chieftain crews in the mid 1980's. This was an organisation that thrived on skimping on costs. To me it doesnt fit that they would carry on with RMG unless it was filling a particular need. And indeed, the speed with which they removed it when they had a TLS points to this.

As for faster, I can only point to what Ive read about the experience at CAT, where Chieftain crews say they were quicker on the draw than other nations tanks. Now perhaps there was some obscure CAT rule where they were gaming the system (it may be they counted .50 rounds on target as an engagement for example), but as they won one year and ran up the next year, then it was clearly doing something right. And this would have been going up against tanks like M48 and leopard 1 that used an optical rangefinder.

For longer range engagements, yes, I could imagine an optical rangefinder has many advantages. The disadvantages I would imagine are space it takes up in the turret, cost, training, and potential vulnerability to shellfire. By comparison, hatever the flaws in RMG, if you could unjam it and you had ammunition, it would work. Its pretty much soldier proof.

 

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1 hour ago, bojan said:

They used them for extended range engagements, beyond capabilities of 7.62mm coax, taking advantage of FCS, for a targets that did not warrant the use of the main gun round. There was no reason to use it as RMG since by that time they were actually issuing hand-held LRFs to a TCs of the tanks that did not have LRF integrated into FC computer.

No, they werent using them as RMG, the story I heard is they were using them to kill snipers in tower blocks. It was a lot more friendly than lobbing a HEAT round into them I guess.

I would love to see the report of what the Israelis thought of the RMG on Chieftain Mk4. Ill have to have a word with Marsh Gelbart and see if he saw anything on it.

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1 hour ago, Ssnake said:

I'm now more convinced than ever.

Such a low low hit probability figure can only be explained with their asumption that the range to the target is NOT known with precision. They seem to use a range estimation bracket and then look at the trajectory of the round to see what range error is still tolerable.

A very flat trajectory like APFSDS with high muzzle velocity and minimal drag allows for very generous error margins, at least on fully exposed targets (with the resulting high hit probabilities), but the steeper the trajectory becomes the greater the need for accurate range measurement. Conversely, if you fix the range error, you make the hit probability your variable.

 

This goes to show that you can't be careful enough looking at papers and diagrams and that you have to check the underlying methodology, otherwise you're going to compare apples not even with oranges, but internal combustion engines.

But the massive reduction in hit probability for HEAT is explained by the deficiencies of the projectile design, which causes dispersion to increase drastically at long range.

Take a look at how the increase in dispersion from 2 km to 3.5 km relates to the hit probability graph?

Edited by Interlinked
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I'm sorry, but that's not supported by the facts. The dispersion of the 120mm DM12A1 HEAT-MP-T is substantially lower than that of APFSDS rounds of its age, in the region of .15 ... .18mil - and that's when fired from a comparatively short smoothbore gun. If at all, the hit likelihood should go substantially UP if nothing but the round-to-round dispersion were considered as the limiting factor for hit probabilities.

The main difference is that the HEAT round is 33% slower and comes with more drag, therefore the lower hit likelihood once that you introduce error in range estimation/measurement and crosswind influence.

The last round fired on our master gunner's course was a HEAT round vs a hard target at 3,800m range with, admittedly, crosswind dial coaching from range control, fired by the tank commander. It hit dead center.

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Soviet tank HEAT also had lesser dispersion than APFSDS. HEAT runs into problems once it becomes transonic, but that is at a range that is non-issue for tank gunnery.

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14 hours ago, Ssnake said:

I'm sorry, but that's not supported by the facts. The dispersion of the 120mm DM12A1 HEAT-MP-T is substantially lower than that of APFSDS rounds of its age, in the region of .15 ... .18mil - and that's when fired from a comparatively short smoothbore gun. If at all, the hit likelihood should go substantially UP if nothing but the round-to-round dispersion were considered as the limiting factor for hit probabilities.

The main difference is that the HEAT round is 33% slower and comes with more drag, therefore the lower hit likelihood once that you introduce error in range estimation/measurement and crosswind influence.

The last round fired on our master gunner's course was a HEAT round vs a hard target at 3,800m range with, admittedly, crosswind dial coaching from range control, fired by the tank commander. It hit dead center.

unknown.png

.15 ... .18mil is only true at shorter ranges, perhaps up to 2 km only.

Quote

Soviet tank HEAT also had lesser dispersion than APFSDS. HEAT runs into problems once it becomes transonic, but that is at a range that is non-issue for tank gunnery.

Yes, that's true for Soviet HEAT with the large pop-out fins. 105mm and 120mm HEAT with the long tail boom and fixed fins start to destabilize at around ~660 m/s.

Edited by Interlinked
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Let's not get sidetracked here. The argument was that the hit probability figures must have been based on nothing but ammunition dispersion, and I've demonstrated that

a. this can't be the case for APFSDS as the given likelihoods are not even remotely close to target dimensions and ammunition dispersion, and

b. it can't be even less so for HEAT since

b1. it's dispersion is lower than APFSDS (we can debate if it is indeed as low as .18mil) so hit likelihood figures based on dispersion alone would rather have to go up

b2. even if we discount b1, the dramatic drop in hit likelihood at longer ranges can only be explained by looking at the depths for which the rounds trajectories intersect with the elevation of target silhouettes (the steeper the trajectory, the smaller the tolerable range error that still gives you a hit)

 

These were the only points I argued. Feel free to start a new debate where we look at individual round/gun combinations and how dispersion figures change over in-flight distances. But that would warrant an entirely separate thread about the intricacies of exterior ballistics in general.

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48 minutes ago, Ssnake said:

Let's not get sidetracked here. The argument was that the hit probability figures must have been based on nothing but ammunition dispersion, and I've demonstrated that

a. this can't be the case for APFSDS as the given likelihoods are not even remotely close to target dimensions and ammunition dispersion, and

b. it can't be even less so for HEAT since

b1. it's dispersion is lower than APFSDS (we can debate if it is indeed as low as .18mil) so hit likelihood figures based on dispersion alone would rather have to go up

b2. even if we discount b1, the dramatic drop in hit likelihood at longer ranges can only be explained by looking at the depths for which the rounds trajectories intersect with the elevation of target silhouettes (the steeper the trajectory, the smaller the tolerable range error that still gives you a hit)

 

These were the only points I argued. Feel free to start a new debate where we look at individual round/gun combinations and how dispersion figures change over in-flight distances. But that would warrant an entirely separate thread about the intricacies of exterior ballistics in general.

Well, no, not at all. But I'm trying to point out that dispersion alone explains the form of the graph. If we include ranging errors, the hit probability would be drastically worse than the graph shows. You can see it from the hit probability of an M60A1 with both APDS and HEAT. Note how the hit probability with HEAT falls to 0.1 by 2 km, and essentially falls to 0 long before reaching 3 km. 

ihWvsef.jpg

Why compare 105mm to 120mm? Because the dispersion of 105mm HEAT at 2 km is 0.18 mils while 120mm HEAT is 0.23 mils. You would expect the hit probability to be a bit higher, at least up to 2 km, but it isn't. So you see just how massive of a difference that ranging errors make. 

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2 hours ago, Gorka L. Martinez-Mezo said:

This graphic shows first hit using the rangefinder and FCS? 53% probability at 1500m with APDS may be worse than T-62 firing APFSDS without any RF equipment 

The TRADOC bulletin never specifically stated that the hit probabilities were calculated with the coincidence rangefinder taken into account, but it would be very strange if they weren't. They wanted to show the M60A1's long range advantage over the T-62, after all.

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58 minutes ago, Interlinked said:

The TRADOC bulletin never specifically stated that the hit probabilities were calculated with the coincidence rangefinder taken into account, but it would be very strange if they weren't. They wanted to show the M60A1's long range advantage over the T-62, after all.

Not very impressive results given the expense of creating and building the M60 FCS plus training the crews, maintenance..... In this case, I would have to side with Stuart: RMG would be better here! : )

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The clincher for me is, an optical rangefinder needs a specialised engineer or someone qualified to repair it. An RMG breaks, its a doddle to bolt in another and get an armourer to fix it.

Yes, I would agree totally that an optical rangefinder gives theoretically better results at longer range. I have to ask how often that really outweighed all the disadvantages, in turret space, maintainance issues, and training.

Did I read somewhere that a percentage of recruits could not use optical rangefinders, due to some occular reasons?

Edited by Stuart Galbraith
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1 hour ago, Stuart Galbraith said:Did I read somewhere that a percentage of recruits could not use optical rangefinders, due to some occular reasons?

IIRC, some 20% of people cannot use a stereo RF, initially favored by the US in the M47/48. Coincidence RF are on paper less precise but easier to use.

in any case, keeping the RFs and the assorted FC gears and cams in good shape took a lot of maintenance work and care

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