7.62x54r rim design

[ireload2]

I think the large diameter rim is a problem for large capacity magazines and causes them to be banana shaped. This arc at the back of the magazing causes difficulty with large rims feeding around the curve. The curve makes the rim virtually thicker. Putting a large chamfer on the edge of the rim helps the base of the case feed around the curve easier. I doubt that it has as much to do as much with a bolt rifle compared to how it feeds in automatic weapons with either belt feed or long curved magazines. Think about French ammo going around that silly Chauchat magazine.


The 6.5X53R Dutch Mannlicher case drawing has a large radius across the base that accomplishes the same thing. Yet most people make these cases from non-chamfered .303 brass that have no chamfer and wonder why they do not feed very well in the Mannlicher clips.

[Patrick Chadwick]

The examples I cited on my previous post were all bolt-action rifles introduced long before large magazines (or even magazines at all !) became standard for military rifles and long before the introduction of automatic weapons. Rimmed cartridges headspace on the rim. Practical tolerances of 19th C. cartridge manufacture and chamber dimensioning required a fairly substantial rim compared with modern so-called rimless cases - which do have a rim, but it is recessed so that it does not normally protrude outside the base (yeah, yeah, I know about the Arisaka). Since these modern cartridges do not headspace on the rim, the (recessed) rim only has to be large enough for the extractor to engage reliably.

"I doubt that it has as much to do as much with a bolt rifle compared to how it feeds in automatic weapons with either belt feed or long curved magazines."

Sorry, I cannot agree. The 30-round .30M1 carbine magazine also has the banana-look. And the strongly curved Bren magazine takes rimmed .303 rounds that do not have a chamfer on the rim. So while a chamfer may help in such cases, it is apparently not vital.

[ireload2]

Back when many of these rounds were designed there were a lot divergent ideas about what was vital and no so vital. The French machined solid bullets, the Brits used propellant in strings that burned out early rifling forms, the Germans could not figure out the depth of rifling or the bullet diameter, the US used 3 different rounds and eventually switched to a pointed bullet. The Brits eventually used the 8X57 rounds in the BESA machine guns used in tanks while they stole the Czech ZB vz 26 design for the Bren gun. The Japanese had 3 different 7.7 rounds and one was nothing more than 7.7X57R (.303 British).

There is also institutional constipation. Once you have a design for a small arms and ammo and a significant inventory of both built many countries cannot afford to make a change.
This inability to change may be loss of face by an influential person or persons or organization, political fall out, budget constraints or national inertial and a poor economy.

In regard to the rim design some countries were not very forward thinking. The 7.5 X55 Swiss, 7.92x57 German and 6.5 Carcano rimless rounds are approximately the same age as all of the other rimmed dogs designed, The basic case design of the Swiss round is still excellent today.

The .30 Carbine round has a small diameter case head and therefore a small chord across the head making it a poor example just as the 5.56 is a poor example compared to a magazine full of Lebel or X54R rounds. The Bren gun was a stolen design adapted to an older round that could not be changed, I am sure due to national inventories of existing ammo and weapons. I am also sure the Brits would have been happier to not have been using both cartridge and rifle designs that originated at the end of the black powder era.

The examples I cited on my previous post were all bolt-action rifles introduced long before large magazines (or even magazines at all !) became standard for military rifles and long before the introduction of automatic weapons. Rimmed cartridges headspace on the rim. Practical tolerances of 19th C. cartridge manufacture and chamber dimensioning required a fairly substantial rim compared with modern so-called rimless cases - which do have a rim, but it is recessed so that it does not normally protrude outside the base (yeah, yeah, I know about the Arisaka). Since these modern cartridges do not headspace on the rim, the (recessed) rim only has to be large enough for the extractor to engage reliably.

"I doubt that it has as much to do as much with a bolt rifle compared to how it feeds in automatic weapons with either belt feed or long curved magazines."

Sorry, I cannot agree. The 30-round .30M1 carbine magazine also has the banana-look. And the strongly curved Bren magazine takes rimmed .303 rounds that do not have a chamfer on the rim. So while a chamfer may help in such cases, it is apparently not vital.