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.

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".

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.

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

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").

Thank you for the "complete" drawing. I had asked the Swiss for GP11 bullet diameter, ogive radius and boattail angle. They kindly sent me a sanitized drawing with all other dimensions and all tolerances removed. So the only dimensions left were R 70, diameter 7.84, overall length  35.2 and 19 degrees boattail angle (9.5 degrees when expressed as the usual half angle). 

Thank you very much, very interesting info.

Possibly relevant:

Whilst the French had not seen any technical information on the .280" round they were of the opinion that the stopping power of the ADE ammunition was likely to be inadequate. They therefore believed that NATO ought to adopt the .30" calibre and made it clear that France would support the US in questions relating to small arms.90

90 Report on the Small Arms Conference held in the USA on 2nd and 3rd August 1951, CAB 21/3465,
NA.

 - Ford (2008) p.191

Of course, it is the opinion of entirely different people from those who chose 7.5mm in the first place given the time lapse. You could also interpret "stopping power" in terms of energy rather than caliber (though harder to square with "had not seen any technical information").

To switch the topic to the familiar .280 British saga, rereading Ford was interesting. While he misses the trajectory and special purpose bullet concerns that most other versions emphasise (and seems to be inaccurate in his claims about retained energy), he gives a lot of references.

One interesting titbit is that the goal of a select fire rifle (thus presumbably the Ideal Caliber Panel and the .270) was itself a compromise to appease British marksmanship enthusiasts. The operational researchers just wanted to give everyone Stens, which were considered as good as a Bren up to 300 yards (pp.117-120)! 

Also Churchill couldn't resist chiming in:

When it was pointed out that the British infantryman had been expected to use .303" ammunition for 50 years and that it was time for it to be replaced, 'Mr Churchill replied, with a smile, that we had used the long bow for very much longer than 50 years'. (p.194)

'When I was at Omdurman I rode with a sabre in one hand and a revolver in the other' to which
Slim retorted, 'Not much standardisation there Prime Minister'. (p.197)

And half a century before the (British) NRA opposed the SMLE because it wasn't the ideal match rifle (p.99). 

The ".30" refered in the text is the .30-06, and France was OK to use this round for rifles and MGs, should the US decided to push this round into NATO standardisation process.

The ".30" like the T-65 was less well received, and France transition from 7.5x54 mm to 7.62x51 mm was a very slow process.