Shoulder or case mouth?

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Demi-human

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For those with a preference, which is prefered?

In a choice between two AR cartridges of similar magnitude, one with a shouldered case, the other straight walled. What benefits or deficits does your choice provide you?
 
So... AR... shouldered vs. straightwall... aren't we doing 458soc vs. 450bm in another thread already?

In a semiauto rifle, I want a shoulder, assuming sufficient shoulder is offered to ensure headspace.
 
Yes, I was reading that thread when this question came to my mind. I tried to state it more ambiguously than a choice between those two cartridges, to make it more about functionality or preference. However, I thought it may have something to do with the AR platform as well. I did not want to derail the other thread, so here we are.

I am more wondering why one would feel one way or the other. There is not a wrong answer, I was just wondering why?
I don't want to seem dense, but I am still mostly new at this. Is one more reliable than the other? More consistent head spacing? Easier loading one through a magazine than the other? Would your choice be different if the rifle was different?

If it just looks cooler to have a shoulder, and that is what one prefers, that's fine too.
 
Shoulder seating provides some small degree of forgiveness to case headspace being a bare scoch too long.
Mouth seating is a hard stop.

I don't like hard stops in gas guns. :uhoh:
 
"Why?"

For me, the comparison of headspace designs have to have context. Different horses for different courses, and exceptions to every rule, and all that... Rim-spacing cartridges don't make much sense in a bolt gun, shouldered rounds have challenges in a revolver, and mouth spacers have their challenges in big bore, semiauto rifles. Any of it can be made to work, but some take more work than others.

For a semiauto with a relatively big recoiling cartridge, I favor a shoulder when one can be had - contact (x), consistency, and forgiveness, and the opportunity to decouple my crimp from my headspacing. Case mouths move a lot more than shoulders throughout their life, and take a lot more touches than shoulders. Shoulders blow forward and are sized back, that's it. Case mouths stretch out, shrink back, are sized in, compress forward, are trimmed back, cut square, deburred and chamfered to a bevel, squeezed into a crimp... If I want to be sure my BCG and spring don't shove my bullet deeper into my case; in a shouldered round, I don't have to play games, just crimp and be merry.

For the context of the two, there's not so much difference. The 450 Bush isn't a straightwall, it's tapered, but a guy has to size the base/body appropriately to match their chamber, else the brunt of the feeding is caught by the case mouth. The shoulder on the 458soc isn't large, and it equally has to be positioned correctly, albeit with a bit more forgiveness. There's no balancing act with shouldered rounds, however, so there's no real consideration for headspacing vs. neck tension to fight bullet set back or crimp jump, and no messing with collet crimp dies to give sufficient tension AND mouth lip.

Mouth-spacing rounds run fine when loaded properly, but I prefer to have the shoulder.
 
I don't like hard stops in gas guns.

It shouldn't be a hard stop.

The 450B is a tapered round (as are the other pistol rounds for the AR which headspace on the case mouth). It's only a "hard stop" if the reloader oversizes the round, leaving the base undersized so the case wall doesn't make contact before the case mouth "hard stops."
 
It shouldn't be a hard stop.

The 450B is a tapered round (as are the other pistol rounds for the AR which headspace on the case mouth). It's only a "hard stop" if the reloader oversizes the round, leaving the base undersized so the case wall doesn't make contact before the case mouth "hard stops."

The round sort of has to hard stop no matter what in an AR. The bolt has to slam the case against whatever it is head spacing on (rim, shoulder, case mouth, belt, etc) hard enough that the extractor can snap over the case rim and still have enough momentum and/or spring force to lock the bolt into battery.
 
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The round sort of has to hard stop no matter what in an AR.

When a guy talks about "hard stop" in terms of headspacing, below is an analogy to compare what I described above:

It's only a "hard stop" if the reloader oversizes the round, leaving the base undersized so the case wall doesn't make contact before the case mouth "hard stops."

You're running full speed, then jump and slide. Picture sliding into home plate vs. sliding across the ice of a hockey rink into the wall. One means dirty pants, the other means a broken ankle.

The tapered case and a bottleneck shoulder is a much "softer" stop, applying friction over a much greater area, than the "hard stop" of a case mouth slamming into the rim of the chamber throat. The "wedging" and dragging deceleration of the case into the chamber take in a lot of force.
 
A case with shoulder will naturally have more powder capacity for that same caliber as a straight-walled (or gentle taper) will being the diameter of the body needs to be larger to have a place to put the shoulder. Properly done I don't think it matters that much if you headspace on the mouth or shoulder...either will work. But with equal length cartridges of equal caliber...the shouldered case 'should' have a ballistic advantage due to the larger powder charge.
 
When a guy talks about "hard stop" in terms of headspacing, below is an analogy to compare what I described above:



You're running full speed, then jump and slide. Picture sliding into home plate vs. sliding across the ice of a hockey rink into the wall. One means dirty pants, the other means a broken ankle.

The tapered case and a bottleneck shoulder is a much "softer" stop, applying friction over a much greater area, than the "hard stop" of a case mouth slamming into the rim of the chamber throat. The "wedging" and dragging deceleration of the case into the chamber take in a lot of force.

There is less than 1.0 degree of included angle to the taper of a 450 Bushmaster case. I have a hard time seeing that slowing anything down before the case mouth hits the end of the chamber especially if either or both (cartridge or chamber) are near max tolerance off there nominal dimensions (ie big chamber, small cartridge).
 
The math isn't so hard, see for yourself...

I though I just did? :) The angle of the wall of a 450 Bushmaster is 89.55 from vertical .9 degree included angle. Per SAAMI specs a max diameter cartridge is still .001 inch smaller in diameter at the mouth than a minimum spec chamber. No one makes max dimension cartridge and very few make minimum dimension chambers. At the opposite end of the specs a minimum case mouth would be as much as .011 inch smaller in diameter than a max diameter chamber. My 450 BM reloads pass the "thunk" test in both my case gauge and my rifles chamber. It's going to hard stop on that case mouth.
 
No one makes max dimension cartridge

Every cartridge I reload for my chambers are matched to the chamber. Many of the chambers and my dies are cut with precision matched reamers, for that very purpose.

Friction is what friction is - your BCG has clearance dimensions in your upper, but it still exhibits drag, and they're not even tapered... the math to which I referred isn't dimensional, it's friction...
 
Every cartridge I reload for my chambers are matched to the chamber. Many of the chambers and my dies are cut with precision matched reamers, for that very purpose.

Friction is what friction is - your BCG has clearance dimensions in your upper, but it still exhibits drag, and they're not even tapered... the math to which I referred isn't dimensional, it's friction...

But there is something pushing the BGC against the upper to create the friction, the hammer, the rounds in the magazine, the cam pin trying to move in its path, etc. Once the round is out of the magazine and into the chamber there is nothing there to really pushing the round into the wall of the chamber to generate any appreciable friction. No normal force, no frictional force. There is no doubt some dynamic/inertial forces that are generating case/chamber friction but compared to the other frictional forces in the system the case sliding in the chamber is pretty negligible. The round stripping out of the magazine plays the largest role in slowing the BGC down.

You could test this. With no magazine in the gun Let the bolt shut on an a round fully seated in the chamber compare that to the time it take to close with a round in the chamber just far enough to let is keep itself in the chamber and more or less aligned.
 
Thank you kindly for the replies gentlemen. I think I have an understanding of it now.

Thank you especially, @Varminterror, for the clear and concise synopsis of the issue. This was precisely what I was looking for.

I have no opinion yet. They are all just cases to me. I am sure more experience will change that.
 
I have no opinion yet. They are all just cases to me. I am sure more experience will change that.

Very apt. They ARE all just cases, and I'll really be surprised if your opinion changes - I don't look at this one as "one side of the fence or the other," but rather a few tools in a toolbox which can all be used for the job. Between these two, I favor the 458 not for the shoulder, but for the selection of .458" rifle bullets over the .452" bullets (Note in the other thread, @CraigC made the same comment, but favoring the .452" 450B for the cheaper pistol bullets for practice/plinking - that one is a fenceline). The shoulder is a small bonus. I have, enjoy, shoot, and hunt with cartridges which headspace on the rim, shoulder, belt, and case mouth, even firearms which don't have cases at all... All of it works.
 
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