Rechamber 9.3X72 to 9.3X74

[okie98]

Has anyone done this? What are the pros and cons? Thanks for your input.

[germanguns1234]

There aren't any pros, just cons. Don't do it. The gun won't take the pressure.

[Axel E]

Here is what I wrote about this subject on another forum:
Rechambering an old drilling from 9.3x72R to 9.3x74R is about as bright an idea as rechambering an equally old Smith & Wesson .38 Special Military & Police to take the .357 Magnum. The CIP max pressure for the x72 is 2000 bar, while the x74 allows for 3400 bar.
The problem here is not chamber wall thickness. Up to the 1920s the Suhl gunmakers used hi-temp brass brazing to assemble the chamber area of the barrels. This was ok for bp pressure cartridges, but sometimes the steel in the critical chamber area was "burned" and brittle. This was the reason for the design of such drilling cartridges like the 8x57R 360, 8x58R S&S, 8x72R Brenneke. For the innocent customer the "8mm" designations sounded the same as the then modern high power 8x57I or IR cartridges, but those "drilling" numbers were loaded to much lower pressures and ballistics. I have seen the sorry remains of an old drilling that was post-1990 rebored, rerifled and rechambered in Suhl from 6,5x58R S&S to 8x57IRS. The wall thicknesses appeared to be ok to all persons involved, the master gunsmith, the master barrelmaker and the proofhouse officials. At the proofhouse, on firing the prescribed 8x57IRS proofload, the chamber area blew apart. The breaks looked like coarse textured broken cast iron. It's the same problem like that with the low-numbered Springfields of the same time span: Most of them take lots of use and abuse, but some are brittle and may shatter if overstressed even slightly.

[Axel E]

Though both cartridges are nominally "9.3 mm", there is a difference in bore dimensions too:
As the 9.3x72R is a black powder number, merely loaded as a "Nitro for black" load, the barrel dimensions were designed with deep grooves to shoot lead bullets or very soft copper jackets. So the modern CIP minimum barrel dimensions are:
Bore/land diameter .344", groove .364", rifling twist one turn in 16.5".
Many old, pre-1940, barrels are even undersized by these standards.
On the other hand, the 9.3x74R was a smokeless cartridge from the start, using heavier, steel jacketed bullets. The minimum dimensions from the start are: bore/land .354", groove .365", twist 14".
Shooting 9.3x74R factory level , hard jacketed bullets through a 9.3x72R dimensioned barrel is likely to raise pressure to a dangerous, maybe destructive level.

[mike ford]

okie98,
Even if you could find a modern enough gun to stand the pressure and had it reproofed for 9.3x74R, the rifling twist would likely be too long to stabilise the longer bullets in the 9.3x74R. The bullet weights/form in the 74R are 258gr spitzer to 293 gr spitzer versus 193 gr flat point in the 72R( per Axel, twist 1 in 14 vs 1 in 16.5)
Mike