7,5 mm MAS Autofrettage


#1

A couple day’s ago I found this French round.
It has no extractor groove and a lead bullet.

Perhaps somebody could tell me what I have.

Thanks
Dutch


#2

I think I’ve seen similar rounds in other calibers, something called an “auto-frettage” cartridge. They have something to do with proofing barrels.


#3

There’s a brief mention of Autofrettage rounds in Chris Punnett’s “An Introduction To Collecting .30-06” on this forum’s homepage. Have a look under Proof cartridges.


#4

There is a photo of the 30-06 version of this type of rimless proofing rd in the Intro to 30-06 link, second from the left:


#5

I have one of the 284 Winchester Proofs without a rim (most collectors have one) but never could understand why they are made that way. Maybe someone can explain? The case goes through the header and headstamp die so why is the rim cut off??

Ray


#6

Hello to everybody!

Yes, it IS an Autofrettage round.
This kind of cartridge, rather uncommon, was used to test the unfinished barrels in the factories, and supposed not to be found outside…

You will note that the actual calibre of the lead cylindric “bullet”, in this case, is not 7.5 mm but 7.29 mm, the shooting test being made before rifling.

Such ammunition was, as a matter of fact, used to better the steel mechanical properties of the barrels. I will try to explain the story, but, of course, our friends having some backgrounds in the theoretical mechanics will praise it better…

The Autofrettage rounds were based on a well-known mechanical principle used in heavy mechanics, to increase, thanks to an artificial manipulation, the metal ability to support higher strains than when it leaved the rolling mill.

To obtain this result, the metallic sample is stretched to a value slightly superior to its elasticity limit, then left to go back to its precedent condition.

The resulting permanent deformation will modify the ratio lengthening/traction in a linear way, allowing to work in this lengthened straight part of the curve, with the result of higher usable strains.

Today, the system is no more in use, as the barrels are made through cold hammering, which increases the metal resistance through shingling.

It must be noted that autofrettage was not only used in France, as several specimens are also known in the USA (Cal.30" Govt).

Anyway, an interesting specimen.

Cheers!

Philippe


#7

Excuse me, but I forgot to precise that all the Autofrettage rounds observed in 7,5 mm Mle 1929C (or 7,5 mm MAS) were made by Cartoucheie de Valence, and so-headstamped (VE).

Ph.


#8

Frettage is still done on artillery barrels, but with oil pressure working the metal.
Soren


#9

Here’s one in 7.62x54 (no “R” I guess) Was formed from a .303 round. I’ve never understood what it was. Thanks for the info!