7.62x54r rim design

[bob4wd]

Does anybody know why the back side of the rim is beveled on this cartridge? I suppose it's to help it feed onto the bolt face, but I don't really know if that's the case. I can't off hand think of any other major cartridge that's made this way?

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[browningautorifle]

Perhaps for the extractor to cam over?

[jmoore]

Here's the French 8mm rifle rounds. The one with the groove is for spritzer type bullets and the Lebel's tubular magazine. Fairly beveled design as well. But very different loading methods! Tube mag or en-bloc clips (Lebel, Berthier), versus charger loaded magazine with cartridge separating device for the top round in the Russian rifle. But all have a ring about the bolt face.

[earlwb]

It is quite likely that the Russians used the 8mm Lebel cartridge (8x54R) as a reference for their 7.62x54R round. Thus they lightly beveled the head of the cartridge in the same manner. The question is probably more correct if one asks why the French beveled the head of the 8mm Lebel cartridge. Of course that leads into the fact that the 8mm Lebel was a necked down cartridge based on the 11mm Gras cartridge. So the beveled head rim comes from it ultimately.

Anyway, the Gras and Lebel cartridges were designed for use in a tubular magazine and the tapered rim likely had to do with more reliable feeding in this case. In the French rifles the tubular magazine is a bit tricky in how it operates and feeds fresh rounds to shoot into the chamber.

[RCS]

The bevel head rim design was adopted in 1930 by the SovietsAttachment 76282Attachment 76283Attachment 76284

photos show some early Russian 7,62x54R mm cartridges

[earlwb]

I think that browningautorifle has a point. The extractor would more easily slip over the beveled rim on the cartridge when single hand feeding a round into the rifle. So that may really be the reason.

[bob4wd]

I think that browningautorifle has a point. The extractor would more easily slip over the beveled rim on the cartridge when single hand feeding a round into the rifle. So that may really be the reason.

Yeah, this reason does seem to make the most sense. It does beg the question,though, if it's such a good idea (and I think it is), why didn't everybody do it? One would think that a gun designed for the rigors of combat would use every trick available that might make it more reliable.

[Parashooter]

The Mosin is a push-feed design. Fed either from magazine or directly into chamber, the extractor has to snap over the rim as the bolt is closing, so the domed or chamfered rim is helpful. In controlled-feed designs (like Enfields and most Mausers) the rim slips under the extractor claw while rising from the magazine and the bevel isn't critical. Some controlled-feed designs (such as U.S. M-1903 and Lee-Enfields) have strongly beveled extractor claws designed to ride over single-loaded cartridges with minimally-chamfered rims.

(The U.S. Krag is a push-feed and most .30/40 rims have a strong chamfer. Norwegian Krags are controlled-feed and 6.5x55 cartridges have minimal chamfer, but the extractor is designed to work with direct-chambered rounds as well.)

[Patrick Chadwick]

I can't off hand think of any other major cartridge that's made this way?

There was a large family of European BPCR cartridges that originated with the epochal Mauser "A" base design from ca. 1871. They all had bevelled rims to help the extractor slip over the rim. In the 1st generation cartridge rifles, the engagement relied on the springiness of the extractor itself, so "Snap" is an unlucky word, as that is exactly what some old extractors have done, and replacements are "hens' teeth" parts to find. A "family tree" would look something like:

M1871---M71/84---11 mm Kropatschek and derivatives
..........---Gras 1874 ---Lebel 1886---Mosin-Nagant 1891

There were many civil versions: 9,5x47R etc. Most were bolt-action rifles with a hook-like extractor that had to pass over the rim to engage. Block-action rifles (Sharps, Rolling Blocks, Martini...) had extractors that sat in front of the rim, and thus did not have to slip over it.

[matthanne1]

You also have the considerations of MGs that used that cartridge like the DP-28, a beveled cartridge probaby was the best fit that worked for all.