doublegunshop.com - home Forums General Discussion Classic & Custom Single Barreled Sporting Rifles @ doublegunshop.com (Somewhat OT) Thrust mechanics rimmed vs. rimless

A question I've had for some time. I've noticed that break-open rifled arms traditionally favored rimmed ctgs. And many contemporary ones still seem to.

The traditional explanation (after rimless rounds became widely available, that is) was that rimless extractors were complex and unreliable. Clearly we have got past that long ago (except in stopping rifles where perfect is the only standard).

Another is that equivalent rimless rounds exert more thrust against the breech face than their rimmed counterparts, say a 7x57 vs. a 7x57R. So the RELATIVELY weak break open actions are stressed and worn less by rimmed cartridges, all other things being equal. Apparently this is because the larger surface (head + rim area) of a rimmed cartridge distributes the thrust force more widely over the breech face than a rimless ctg. which has only the area of the head.

Is there anything to this latter argument? (I understand it takes a degree in mechanical engineering to give a REAL explanation, but is it true as I've stated it?). Even true-ish?

Yours in ignorance, not bliss,

Mike Armstrong

I think that is a good question, I have always been uncertain too. The basic deal is evident: back thrust is case head area x pressure, which is expressed as pounds per square inch! So a Hornet generates about as much back thrust as a 32-40 schuetzen load, both of which are nearing the Low Wall limit. My problem is yours, I don't know whether to use the whole case head area (bigger for rimmed) or just the area of the base of the powder column. I have read it both ways. If it is just the powder column, you would think there would be no real back thrust difference between the case types. I do know that the rimmed versions (7x57R vs. 7x57) are normally loaded down a bit, possibly because of the rifles they are used in. Maybe a real engineer can explain it to us.

Thanks, 1878. I know there are multiple issues here. Have also noticed that the rimmed versions seem to be loaded lighter, but wonder if that is just a cautious approach by the loading companies who suspect that there may be some very old (and some very cheap) break open actions out there.

Hard for me to understand why the rimmed case wouldn't transfer some of the thrust laterally thru the rim to the receiver face, since there must be SOME motion by the case. (In the "perfect" case and action there wouldn't be, but in the real world both do move at least fractionally. Or why would we resize?).

What got me thinking about these issues is a new Savage/Valmet combination gun in 12x.308, and reading discussions about appropriate .308 handloads for it....and shooting a friend's Merkel 16x16x7x65R, basically a rimmed .280.

Perhaps it is as much tradition and marketing (knowing their customers) as engineering.

I think you use the entire case area, rim & all, for the back-thrust calculations.

I certainly don't know, but I thought for example case stretching due to head space problems was an example of the case spreading the pressure to the chamber wall and not just back thrust.

I always thought long heavy tapers and rimmed cases were designed to overcome the lack of camming forces that a bolt action could apply to a cartridge or fired case.

This is a very important question, especially if you're converting a firearm to a different caliber than it was designed for.

I always ran under the assumption that the rearward force was figured by the square inches of area times the pressure of the inside diameter at the base. Because that's hard to do I always just measure the base. I have never use the rim in any calculations.

I'm not sure if I'm right or not and just don't have time to look it up in my books but if anyone is going to convert a firearm I would certainly get some good information before I did it.

What Michael said.

A rimmed case will always exert the same TOTAL thrust as an otherwise-identical rimless one, but the rimmed case's slightly larger head area will always result in slightly less thrust per square inch where it touches against the breech face. In a bolt rifle or falling-block SS this thrust difference is immaterial since the breeching is tighter and there is usually some camming action upon closing. However with a break-open rifle there's little camming action upon closing and so the dimensions must allow for a slightly-dirty action closing upon a slightly-oversize cartridge.

Eventually this slightly looser & more-flexible design will result in a battering of the action internals as compared to the bolt or falling block rifles. Might take a thousand rounds, might take a hundred thousand, probably won't ever become noticable at all in most instances. But the fact remains that the rimless case exacerbates this battering tendency by concentrating its thrust upon a smaller surface area than the rimmed one.

There's also the fact that a rimmed case will always be stronger than an otherwise-identical rimless one, simply because of the added hoop strength provided by the rim as opposed to the extractor groove.
Regards, Joe

Thanks, guys. Joe, that is a very clear explanation of what I always (dimly) believed. Nice to hear it from somebody with more experience than I'll ever have.

I realize this only becomes a REAL issue when you are contemplating a cartridge conversion, as Michael says. But the theoretical explanation has always bugged me a little.

(My new .308 to the contrary, I'm a confirmed user of rimmed cartridges. I think this started because as a kid of six I could easily understand exactly how a rimmed extractor worked, but not always a rimless one....old prejudices die hard.).