The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: Semi-automatics

Thread Tools Search this Thread
Old July 5, 2023, 12:21 PM   #1
Senior Member
Join Date: October 9, 2009
Location: North Alabama
Posts: 8,110
headspace in straightwall AR ctgs

The nearby thread re: the .400 Legend, and its relatives the .450 and the .350 all have (or appear to) straight case walls. We see this in pistol cartridges, which headspace on the case mouth. This got me thinking....(always dangerous, sometimes expensive).

.....I thought relying on the case mouth for headspace in a rifle cartridge was considered a bad idea/design, though I cannot say why, seems like I read it somewhere?
bamaranger is offline  
Old July 5, 2023, 12:44 PM   #2
Senior Member
Join Date: July 1, 2001
Posts: 6,147
Well, it is not exactly headspacing on the mouth the way you might think. The extractor is holding the rim back. The ejector is pushing forward. It is rotating to hold the case mouth or bullet against the chamber. This all straightens out upon firing. The case mouth headspace is only used to resist the primer strike which it should be able to do as well as every other straight wall rimless. I make it sound all crooked, but case to chamber fit is quite tight.

Something like a 458 Lott is slightly better with its belt.

To me, the real negative is you cannot crimp it much or the mouth won’t catch the chamber. This can be a big issue with bid bullets and high recoil. Not so much with 350 Legend.

Last edited by Nathan; July 5, 2023 at 12:50 PM.
Nathan is offline  
Old July 5, 2023, 01:12 PM   #3
Senior Member
Join Date: March 2, 2014
Posts: 11,409
Not so much with 350 Legend.
Yes it can--both myself and meheavy experienced case mouth "camlock" on bullets when the mouth protruded past the neck setting in the chamber.
"Everyone speaks gun."--Robert O'Neill
I am NOT an expert--I do not have any formal experience or certification in firearms use or testing; use any information I post at your own risk!
stagpanther is offline  
Old July 5, 2023, 02:49 PM   #4
44 AMP
Join Date: March 11, 2006
Location: Upper US
Posts: 28,105
.....I thought relying on the case mouth for headspace in a rifle cartridge was considered a bad idea/design, though I cannot say why, seems like I read it somewhere?
I'm sure you did read that somewhere. It's been in print for generations, and not entirely without valid reason.

Here are some points to consider, (and are general, not AR-15 specific)

Rifle rounds are generally longer and often use heavier bullets in the same calibers as pistol rounds. Gun designs differ, and many things common to older designs don't apply quite the same way to newer ones.

IF the gun design feeds the case rim up under the extractor that's one thing. If it doesn't, and relies on the pushing the case into the chamber and then the extractor snapping over the rim because the case is stopped by its headspace point, that is a different matter.

Long and heavy (compared to pistol rounds) rifle rounds get a greater "running start" going into the chamber, and they have greater inertia to keep going until stopped. IF what stops them is the case mouth headspace ledge in the chamber, consider that rifle rounds hit that ledge "harder" than pistol rounds and sometimes its hard enough for the straight rifle case to push past the "stop point", which is never a good thing.

With gun designs that are variations of controlled round feeding, this is rarely an issue, and indeed, depending on details of individual guns and ammo clearances, cases may never actually touch the chamber headspace ledge, and effectively headspace off the extractor.

Specific to the AR design, its not a controlled round feed system. Rounds are pushed ahead of the bolt face until they get stopped in the chamber, and then the bolt finishes it final movement ensuring extractor engagement.

System works just fine with bottle necked cases, SHOULD work ok with straight wall rifle rounds too, but sometimes doesn't work as well as it should.

In the old days, designers didn't have a century + of experience about what works best, and what doesn't to look back on when designing new rounds or gun systems.

For example, every pistol round Browning designed before the 9mm Luger was made used a "semi rim" for headspacing. Browning wasn't convinced headspacing on the case mouth was reliable enough. The 9mm Luger proved it was, and the rounds Browning designed after that were true rimless cases.

The first true magnum rifle rounds (H&H) while bottlenecked didn't have very much in the way of large shoulders and so H&H added the case belt, for reliability, and that was in bolt action rifles, not semi autos.

The .400 Whelen (essentially a straight .30-0t6 case) got a notorious reputation for not headspacing reliably on the tiny, narrow bit of case mouth it had.

Additionally, large bore straight rifle cases with their heavy slugs requre a degree of crimp to function well in repeaters. Even taper crimping squeeses the case mouth against the bullet some, and this also reduces the "shelf area" of the case mouth slightly.

Its a known issue, and can be overcome or its likelihood reduced, when enough thought is put into the design, but its not simple, ABSENT something other than the case mouth to headspace on.
All else being equal (and it almost never is) bigger bullets tend to work better.
44 AMP is offline  
Old July 7, 2023, 02:18 AM   #5
Senior Member
Join Date: September 28, 2013
Posts: 3,535
I tried to load m1 carbine round with spritzer bullets. I could do it for single feed. In order to fit that round in magazine, I had to seat the bullet low into the brass. The round couldn't feed. There was void between the brass mouth and the bullet ogive. The "naked" brass mouth nagged on everything it saw. I could make it feed by trimming the brass shorter. But then the round couldn't headspace. Headspacing in extractor? I don't play that game, so end of the rope.

Got into similar situation when I was monkeying with 22TCM9R. Couldn't find the 40gr bullet armsco specifically makes for this invention of theirs. The HP bullet for .218 Bee was a little too long. I trimmed the brass neck shorter and seated the bullet lower. It worked. I made the round headspace on shoulder.

Straight wall brass link brass length to headspace, and hence removes one degree of freedom. Not ideal. I like bottle necked brass, even for pistol rounds; .30 Mauser, .357 sig, 22TCM etc. They look better.


Sent from my SM-N960U using Tapatalk
tangolima is offline  

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 12:47 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2023, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2021 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent:
Page generated in 0.06426 seconds with 8 queries