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Why did the French choose 7.5mm?   Ammunition <20mm

Started 24-Aug by hobbes154; 1315 views.
hobbes154

From: hobbes154

24-Aug

(I was going to put this in the For Your Amusement discussion but it was getting a bit long.)

Does anyone have any good English or machine-translatable info on why the French chose 7.5mm caliber? I find this interesting because it's sort of a preview of 7.62x51 being chosen over .280, and it's clearly not driven by inertia since it involved a change from 8mm.

My starting point is Emeric's paper which details a lot of French experiments with high velocity 6-7mm rounds before WW1 (trying for 800m point blank range against a 160cm target!), and the adoption of 7mm Meunier that was derailed by the war.

However it just states "Development of a new cartridge in 7.5 mm started just after WWI (called “X1”), using the body diameter of the Swiss GP11 round and the case length of the 7.9 mm German round" without any further explanation of why that caliber was chosen. 

So, why did they pick 7.5mm? It wasn't an existing French caliber, and the GP11 isn't any better than various French bullets for ballistic coefficient or form factor. While "the 7.5 mm Mle 1929C load delivered a flatter trajectory up to 800 m than the previous 8 mm Mle 1886D, with less recoil and a case geometry more compatible with automatic loading, so the main requirements for a new small-arms ammo were fulfilled" I would have thought those requirements could be attainable with a smaller caliber with even less recoil (and weight). Or alternatively, they could have stuck with 8mm and just moved to a rimless case without needing to change their barrel and bullet manufacturing straight away, phasing in lead bullets and light/heavy loadings as desired.  

The biggest problem with the 6-7mm experiments was powder quality, however these issues continued with the early 7.5mm rounds, even with access to German powders (and it didn't stop them nearly adopting 7mm Meunier).

Was it the long-range "artillery" capability represented by the heavy Mle 1933D, which I assume could not be matched in a smaller caliber (see e.g. the Swedish trials which found 7.9mm superior to 6.5mm at ranges over 2400m). However if this was so important, why was it only introduced in 1933? And my understanding is it could/should not be fired out of standard 7.5mm weapons anyway, so it might as well have been a separate caliber as with the Swedish or Italian 8mm.

Nathaniel F used to emphasise AP, incendiary and tracer favouring .30 over smaller calibers. But again, this page says these loadings for the final 1929 version of the cartridge were only adopted in 1935. (Admittedly the early 1924 version - 7.5x57 not x54 - had them from the start. I don't know if they tried to make anything other than ball for the 7mm Meunier.)

I guess another factor is they weren't worried about full auto from rifles at that time.

So there's a lot of possible reasons but it would be interesting to know which was actually true! 

 

EmericD

From: EmericD

24-Aug

Well, I don't have a definitive answer to this question, and could only list some "facts" to paint a comprehensive picture!

  • Around 1900, the French army was convinced that the best technical solution was a high velocity 6 mm cartridge using a >100 gr bullet with sectional density above 0.24 lbs/in², unfortunately, French powders were too fast for this application,
  • Tests were resumed with 6.5 mm bores (and bullet in the range of 0.23 lbs/in²), with large capacity cartridges (6.5x60 & 6.5x61 mm), unfortunately, French powders were still too fast for this application,
  • Finally, the 7x59 mm cartridge was designed to be able to launch a steel bullet (~0.21 lbs/in²) at ~1000 m/s, the powders were nearly OK for this task, but the rifles were not,
  • The French army was aware of the German 7.9 mm S patrone, and it's superior ballistics and performances between 0 and 800 m,
  • When WWI started, the 7x59 mm was "downloaded" (and shortened) to make the "stopgap" 7 mm Meunier,
  • During WWI several contracts were made with the US to produce 8 mm Lebel rifle ammo, not a single lot of those ammo were rated "good for war" and all were used for training,

The experience of WWI was clear, the French army needed a new, modern cartridge, that could be easily produced by our allies if needed, so we had to revert to conventional FMJ bullet, hence the transition from the Mle1898D bullet to the FMJ Mle1932N bullet and modification of rifles and MGs.

All developments prior to WWI were devoted to find the "ideal" rifle round, but WWI experience showed that the squad main asset was the "automatic rifle", so the emphasize was to be put on the development of such a gun to replace the "Chauchat". A semi-auto rifle (or even a bolt-action) could be designed later to fire the same round than the LMG.

The Swiss ballistic school was highly regarded in France and we maintained close contact before WWI and after, so the starting point of the new cartridge after WWI was a "magnumized" version of the 7.5 mm GP11 (remember that the French army wanted a high velocity round before WWI without success, and we needed something with a higher MV than the 7.9 mm S Patrone at least for national pride). The 7.5 mm caliber used a 7.8 mm bullet, which was close enough to the US .30" so in case of a new war, the US could produce the new 7.5 mm cartridge without much problems. Remember that the .30-06" M1 cartridge started it's development when a 7.5 mm GP11 bullet was loaded into a .30-06 case...

The 7.5x58 mm was designed from the start using 3 different bullets, a light one (following the lines of the German "S" bullet), the GP11 for control, and a heavier bullet for long-range fire (in parallel to the work done on the new 8 mm Lebel heavy FMJ bullet that will become the Mle1932N).

The light bullet (9 g @900 m/s) became the Mle1924 and was used in the FM Mle1924. Unfortunately, the powder had it's own problems and after a few month in a dry & hot climate, the pressure and velocity went off-scale (MV above 950 m/s were recorded after the powder lost 1% of humidity).

As a first movement, the powder load was reduced to avoid over-pressures after "hot & dry" storage, reducing the loading density of a powder that was already difficult to ignite.

In 1929, the case was shortened to regain loading density and the 7.5 mm Mle 1929C was born.

JPeelen

From: JPeelen

24-Aug

A fact that Emeric points out in his message is one of the most overlooked in modern small arms history: the Swiss and French 7.5 mm bullets have practically the same diameter as cal. .30 bullets (7.8 mm). 

hobbes154

From: hobbes154

24-Aug

EmericD said...

Well, I don't have a definitive answer to this question

OK, I'll give up then! So to your knowledge there was no formal study of 7.5mm against different calibers the way the Swedes did for 6.5 vs 7.9mm or the British later did in the Ideal Caliber Panel? That's quite surprising given all the pre-WW1 experiments you detail.

EmericD said...

Finally, the 7x59 mm cartridge was designed to be able to launch a steel bullet (~0.21 lbs/in²) at ~1000 m/s, the powders were nearly OK for this task, but the rifles were not

So they viewed 7mm Meunier as a technical failure (even the downloaded "stopgap" 7x57 version), not just interrupted by the war? That would explain a lot.

EmericD said...

All developments prior to WWI were devoted to find the "ideal" rifle round, but WWI experience showed that the squad main asset was the "automatic rifle"

Did this have any documented impact on cartridge choice? E.g. more emphasis on AP/I/T rounds? Or more worry about overheating (thermal load) or barrel wear which might also favour a bigger bore?

EmericD said...

close enough to the US .30" so in case of a new war, the US could produce the new 7.5 mm cartridge without much problems

If you wanted commonality wouldn't it make sense to go the same bore as well - I thought barrel making (boring) was a bigger problem than bullets (e.g. 7.92x33 and 7.62x39 were made in the full power rifle bore diameter despite using very different bullets)? Or was the WWI experience that only ammo was in shortage? I suppose introducing a new weapon in wartime is a different problem from keeping existing weapon types supplied.

EmericD said...

and the 7.5 mm Mle 1929C was born.

And you could argue it's the best all round rifle caliber cartridge of the time (aside maybe from the powder/primer chemistry) - lighter & less recoil than 7.92 Mauser or .30-06 while still having competitive trajectory in the light loading and range in the heavy loading, rimless unlike .303 and 7.62 Russian, more versatile for MGs than the various 6.5s. But it's somewhat anticlimactic if the process was just "well our 6-7mm didn't work very well, and here's this nice Swiss bullet (but inferior BC to Balle D!) in a size that's convenient for the Americans to make, but we'll keep the Swiss and not American bore because ... we like the metric system?"

hobbes154

From: hobbes154

24-Aug

Do the bullets/rifling have to be constructed differently to deal with the tighter squeeze? But this suggests otherwise https://en.wikipedia.org/wiki/7.5%C3%9755mm_Swiss#Reloading.

That's a remarkable amount of tolerance in what we usually think of as a very precise system! Though I note the GP11 diameter is technically .306 not .308 ;)

However I now see the 7.5 French bullet diameter is .309 or 7.84mm! So if I understand correctly, the rifling is 4.3% smaller than the bullet where with .308 in a .30 bore it is only 2.6%.

nincomp

From: nincomp

24-Aug

The SAAMI specs of the 30-06 allow use of bullets from 0.309" and 0.306" diameter, so they were both technically in spec.

EmericD

From: EmericD

25-Aug

hobbes154 said:

So to your knowledge there was no formal study of 7.5mm against different calibers the way the Swedes did for 6.5 vs 7.9mm or the British later did in the Ideal Caliber Panel? That's quite surprising given all the pre-WW1 experiments you detail.

I don't think that they had the time to do that, as the work on the 7.5x58 mm started just after WWI and by 1922 all the details were already seated.

I've got a report from february 1922 already detailing the ballistic results achieved with the X1 bullet (11.8 g), the GP11 bullet and the X1c (short) bullet, and this report refers to previous results achieved with the GP11 bullet.

By the way, the twist of the French 7.5 mm is one turn in 270 mm, which is also the twist used by of the Swiss 7.5 mm cartridge, and absolutely not required to fire the X1c (later Mle1924C) bullet.

My understanding is that after WWI the priority was put to make an automatic rifle firing a modern cartridge, and to make it fast.

The previous work to replace the 8 mm Lebel started in 1893 with the 6 mm APX, and by 1914 (21 years later!) not a single weapon or working cartridge were available.

hobbes154 said:

So they viewed 7mm Meunier as a technical failure (even the downloaded "stopgap" 7x57 version), not just interrupted by the war? That would explain a lot.

I don't think that the 7 mm Meunier was considered a technical failure, as you can find studies on a new 7x59 mm using the slender R43 FMJ bullet dated from 1923-1924. The case body diameter of this 7 mm was the same as the 7.5x58 mm, and the R.43 bullet seems to be a scale-down version of the Mle1898D, in a FMJ form, sharing some attributes of the later Mle1932N & Mle1933D bullets (estimated weight around 150 gr based on the bulelt drawing, SD>0.26 lbs/in²).

By the way, if you scale-down the 7.5 mm X1 bullet weight from 7.8 mm down to 7.3 mm (7 mm bore), you end with a 149 gr bullet which is very close from the estimated 150 gr of the R.43 bullet.

Maybe the 7.5x58 mm was found less challenging than the 7x59 mm?

Given the bad experience with the 7.5x58 mm (revealed too late after development), maybe the 7x59 mm flaws did showed during testing?

hobbes154 said:

Did this have any documented impact on cartridge choice? E.g. more emphasis on AP/I/T rounds? Or more worry about overheating (thermal load) or barrel wear which might also favour a bigger bore?

No, I have nothing that could explain the choice of 7.5 mm vs. 7 mm, but I don't think AP/I/T rounds were a factor because there was a parallel program to developp a 9 mm heavy MG for dismounted infantry use, to deal with planes / light vehicles threat.

hobbes154 said:

Or was the WWI experience that only ammo was in shortage? I suppose introducing a new weapon in wartime is a different problem from keeping existing weapon types supplied.

By the way, the French army introduced several "stopgap" infantry weapons during WWI, like the Mle1915 Chauchat automatic rifle and the Mle1917 RSC semi-auto rifle.

I think that "outsourcing" ammo to avoid shortage was really the issue during this first high-intensity conflict.

hobbes154 said:

But it's somewhat anticlimactic if the process was just "well our 6-7mm didn't work very well, and here's this nice Swiss bullet (but inferior BC to Balle D!) in a size that's convenient for the Americans to make, but we'll keep the Swiss and not American bore because ... we like the metric system?"

France used 0.15 mm grooves (not 0.1 like other countries), so the bore diameter was 0.3 mm less than the bullet diameter, so the logical choice was 7.5 / 7.8 mm which gives the flexibility to use US .308" bullets.

By the way, late production of 7.5 mm Mle1929C cartridges were even loaded with the 7.62 mm NATO Mle1961 bullet instead of the Mle1924C bullet, and 7.62 mm NATO barrel were chambered from blank 7.5 mm barrels.

As a sidenote, the land-to-land diameter of the "7.62 mm NATO" UK L1A1 Self Loading Rifle is reported to be 7.54 mm, with a 7.80 mm groove-to-groove diameter, so technically it's a "7.5 mm bore" (same thing for the HK417, which is technically using a 7.5 mm bore to fire the 7.62 mm NATO cartridge).

So, it's not "we like the metric system" but more closely "we want to use 7.80 to 7.84 mm bullets, with 0.15 mm deep grooves, so we need a 7.50-7.54 mm bore".

In reply toRe: msg 7
EmericD

From: EmericD

25-Aug

EmericD said:

No, I have nothing that could explain the choice of 7.5 mm vs. 7 mm, but I don't think AP/I/T rounds were a factor because there was a parallel program to developp a 9 mm heavy MG for dismounted infantry use, to deal with planes / light vehicles threat.

Additional data: AP/T rounds were studied before 1928, and the reduced capability of the AP round was a question raised when the case capacity was reduced from the Mle1924 case to the Mle1929.

JPeelen

From: JPeelen

25-Aug

On the the Swiss military drawing I have, the GP11 bullet nominal diameter is given as 7.84 mm, which is .3087".

EmericD

From: EmericD

25-Aug

And the nominal diameter of the Mle1924 C bullet found on the drawings is 7.82 mm (0.3079"), with a minimum allowed of 7.80 mm (0.3071") and a maximum of 7.85 mm (0.3091").

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