SEMIAUTOS - SHOULDERS GETTING BUMPED UPON CHAMBERING

[Edsel]

https://www.300blktalk.com/forum/viewtopic.php?t=102073

Haven’t really seen much discussion on this topic elsewhere.

In a 5.56x45mm AR, letting the bolt fly loose onto a round can set the shoulders back by as much as 0.004” to 0.005”, depending on the make of brass (Federal GMM and Bulk .223 Remington in those 300 round boxes - they’re soft!).

In effect, if I had insufficiently headspaced brass at, say +0.004”, and release the bolt onto a chamber with headspace measured at +0.001” - the brass (empty OR loaded) will, more often than not, still chamber.

Note: Relative measurements were obtained using Forster Case Gauges and an L. E. Wilson Case Gauge Depth Micrometer. For example, Case Gauge 1.4636 allows the bolt to close with light finger pressure sans ejector. This Case Gauge measures at exactly 0.000” using a Headspace Gauge and the Micrometer pictured above. All sized brass is made to approximate the same headspace of 0.000” +/- 0.002”.

Since I know the exact headspace of my chamber - and that the act of chabering a round also bumps the shoulders fairly effectively - why not FLS and headspace brass to match the chamber’s headspace as close as possible, instead of following the usual recommendation of -0.003” to -0.005” less than that of a fired case (and we know how insufficient those values can be with overgassed rifles)?

At the moment, my brass matches the headspace of my chamber by +/- 0.002”, and I haven’t had any reliability or accuracy issues over the last 420 rounds (this AR is a range toy). I’ll only have to worry about “squishspacing” the oversized 0.002”, not the undersized half.

Any negatives to this approach?

 

[straightshooter1]

https://www.300blktalk.com/forum/viewtopic.php?t=102073

Haven’t really seen much discussion on this topic elsewhere.

In a 5.56x45mm AR, letting the bolt fly loose onto a round can set the shoulders back by as much as 0.004” to 0.005”, depending on the make of brass (Federal GMM and Bulk .223 Remington in those 300 round boxes - they’re soft!).

In effect, if I had insufficiently headspaced brass at, say +0.004”, and release the bolt onto a chamber with headspace measured at +0.001” - the brass (empty OR loaded) will, more often than not, still chamber.

View attachment 8643895 View attachment 8643896

Note: Relative measurements were obtained using Forster Case Gauges and an L. E. Wilson Case Gauge Depth Micrometer. For example, Case Gauge 1.4636 allows the bolt to close with light finger pressure sans ejector. This Case Gauge measures at exactly 0.000” using a Headspace Gauge and the Micrometer pictured above. All sized brass is made to approximate the same headspace of 0.000” +/- 0.002”.

Since I know the exact headspace of my chamber - and that the act of chabering a round also bups the shoulders fairly effectively - why not FLS and headspace brass to match the chamber’s headspace as close as possible, instead of following the usual recommendation of -0.003” to -0.005” less than that of a fired case (and we know how insufficient those values can be with overgassed rifles)?

At the moment, my brass matches the headspace of my chamber by +/- 0.002”, and I haven’t had any reliability or accuracy issues over the last 420 rounds (this AR is a range toy).

Any negatives to this approach?

Variability!

With the bolt slamming shut and moving the shoulders back .004 to .005, is that going to happen for every cartridge? I think not. When sizing cases, it's hard enough to get shoulder bumps consistent in a .001 range, even when annealing is being done. Matching headspace by +/- .002" means you've got a .004" leeway for variance, which sounds close enough to me. Trying to go to .000" on case head space just doesn't sound like a wise approach for a semi auto.

 

[45-90]

A small part of the measurement error is the variations in the case head....which is not completely flat on any case fired or new ...with any brand.
So thats one area you can not control, and you keep measuring your headspace and it fluctuates a bit, and even seems to change when you index the same case off the high spot and it drops a half thousandth or so...

Even If Annealing Is Done! It does nothing to make the case heads flat.

So bump the auto loader shoulder back, for easy chambering about. 001" to .002" minus and if chambering changes this slightly no big deal it's already chambered ready to shoot, and still capable of 1/2" 5 shot groups...quite often.
The bullets also move forward on chambering, .0005" to .007" depending on bullet weight, neck tension, and crimp in AR rifles.

So its a good thing in a fighting rifle that the design will force some slightly long headspace cartridge into battery ready to fire...killing the enemy...no "click", when your life depends on "bang".
And if it bumps the shoulder back .004" max in the experiment it still fires and would be similar to new commercial brass cases.
I use LC military brass in the AR 15, but lots of times on blaster ammo any 223 case, range pick up.
And run it as fast as 1080 rounds per minute, in a "gaming" gun 5.56 Nato chamber, Colt barrel 20" cut to 16.5" Hbar then fluted, increased gas port dia, with rifle gas, ultra light bolt carrier, so no hammer follow through, super fast cycling, low recoil.
It works!...with reloads, loaded on a Dillon progressive, shoulders set back a bit, not much else matters, for this endeavor.
It gets super hot, piles up brass, like on a battlefield...no maintenance during shooting, no misfires or jams.
There is something to like about the 5.56, slow down it's still fairly accurate.

 

[Edsel]

A small part of the measurement error is the variations in the case head....which is not completely flat on any case fired or new ...with any brand.
So thats one area you can not control, and you keep measuring your headspace and it fluctuates a bit, and even seems to change when you index the same case off the high spot and it drops a half thousandth or so...

Even If Annealing Is Done! It does nothing to make the case heads flat.

So bump the auto loader shoulder back, for easy chambering about. 001" to .002" minus and if chambering changes this slightly no big deal it's already chambered ready to shoot, and still capable of 1/2" 5 shot groups...quite often.
The bullets also move forward on chambering, .0005" to .007" depending on bullet weight, neck tension, and crimp in AR rifles.

So its a good thing in a fighting rifle that the design will force some slightly long headspace cartridge into battery ready to fire...killing the enemy...no "click", when your life depends on "bang".
And if it bumps the shoulder back .004" max in the experiment it still fires and would be similar to new commercial brass cases.
I use LC military brass in the AR 15, but lots of times on blaster ammo any 223 case, range pick up.
And run it as fast as 1080 rounds per minute, in a "gaming" gun 5.56 Nato chamber, Colt barrel 20" cut to 16.5" Hbar then fluted, increased gas port dia, with rifle gas, ultra light bolt carrier, so no hammer follow through, super fast cycling, low recoil.
It works!...with reloads, loaded on a Dillon progressive, shoulders set back a bit, not much else matters, for this endeavor.
It gets super hot, piles up brass, like on a battlefield...no maintenance during shooting, no misfires or jams.
There is something to like about the 5.56, slow down it's still fairly accurate.

How does this figure when the high points of the case heads get swallowed up by the sizing process anyway?

Let’s say it’s twice what you stated, a full 0.001”.

You don’t correct it, but is it such big deal in an AR?

I guess I should rephrase my question as follows:

If my lazy Saturday afternoon rifle swallows up all of my insufficiently headspaced range pickup handloads and still lands the shots where I want them, should I bother FLSizing the brass to -0.003” to -0.005” shorter than the chamber’s headspace?

 

[45-90]

How does this figure when the high points of the case heads get swallowed up by the sizing process anyway?

Let’s say it’s twice what you stated, a full 0.001”.

You don’t correct it, but is it such big deal in an AR?

I guess I should rephrase my question as follows:

If my Saturday afternoon rifle swallows up all of my insufficiently headspaced range pickup handloads and still lands the shots where I want them, should I bother pushing the brass to -0.003” to -0.005” shorter than the chamber’s headspace?

It's another variable as to why the measurement floats slightly on your headspace comparator when measuring with the caliper, on the high spot, off the high spot, the same cartridge or several cartridges, then push and twist the caliper to get the same reading... plus the much larger added shoulder spring back.
Which can be helped with annealing.

This is about accurate measuring and why it's not exact...for informational purposes for those trying to be exact.

Has nothing to do with functionality, as I use my AR 5.56 as stated. I just bump em back a thousandth or two, load the plinkers on a Dillon progressive. And they all go bang...range pick up who cares if ya lose them brass.
Also load some for accuracy but they get the same brass treatment.
Keep on doing what your doing and don't worry about it.

 

[Old Man with Gun]

I'm not sure what your goal is. Is it a theoretical perfection or are you unhappy with the groups you are getting? Have you tried FLR and noticed any change in group size?

Many FLR for the AR and see no benefit in bumping shoulders, except -big maybe- brass life.

It may not matter what approach you use, results will always be the same.

 

[918v]

Reliability. If you set the shoulder lengths on your rounds intending the gun to bump them to the desired amount you’ll run into failure of the bolt to rotate into battery sometimes. Other times the bolt will close but hard and you’ll eventually break one of the lugs. Then you may have partially rotated bolt where the firing pin drags and doesn’t set off the primer reliably. Not worth the bs.

 

[Doom]

Unlike a bolt action rifle the Autoloading process imparts a great deal of kinetic energy into the cartridge itself and as the cartridge enters the chamber the only thing that stops it is the shoulder itself. The energy is dissipated by deforming/reforming the shoulder slightly on each chambering. The bolt face itself may or may not come into contact with the case head but the ejector spring will be compressed to imparted another force that will be absorb by the case as the bolt/ejector override the cartridge rim.

The phenomenon that's observed here is simple physics. It is using velocity/kinetic energy to resize the base in the camber as opposed to force in a resizing die.

It's part of the price that is paid for having a 600 round per minute firing rate. You can play around with the datum dimension all you want but in the end do you really want to deal with stuck case for some extremely minor improvement in accuracy, that probably is lost in myriad of other factors affecting precision?

 

[45-90]

I'm not sure what your goal is. Is it a theoretical perfection or are you unhappy with the groups you are getting? Have you tried FLR and noticed any change in group size?

Many FLR for the AR and see no benefit in bumping shoulders, except -big maybe- brass life.

It may not matter what approach you use, results will always be the same.

Its about perfection that's basically unattainable, with intertwined variations. Which some strive for, and try to give as measurements to a half hearted expertise scientific degree, where it doesn't matter, and all the variables are not accounted for...as I have investigated much of this, and even passed other information they have not considered, but would have a fine or slight variation on their findings, or measurements that do not matter...especially in general use.

I get good groups with ARs, where good groups are needed...I do not tolerate inaccurate shooting rifles, where accuracy is a main goal.

 

[Edsel]

Variability!

Reliability.

Simple enough, thanks

The phenomenon that's observed here is simple physics. It is using velocity/kinetic energy to resize the base in the camber as opposed to force in a resizing die.

We also hear "thermodynamics, metallurgy, engineering, and aerospace grade" left and right

 

[Edsel]

I'm not sure what your goal is. Is it a theoretical perfection or are you unhappy with the groups you are getting? Have you tried FLR and noticed any change in group size?

Cases are FLSized, as mentioned in the first post.

I guess it’s like, why squeeze the brass down so much, when the rifle does it for you only as much as it needs (or seems to need) anyway.

 

[Doom]

I guess it’s like, why squeeze the brass down so much, when the rifle does it for you only as much as it needs (or seems to need) anyway.

The first requirement to have a reliable firearm is that the ammunition chamber correctly. If is doesn't you are screwed. If I took your example of setting the datum what could happen if I rode the charging handle a little bit when loading the first round from an uncharged bolt? What could happen if all off the tolerances aligned and the setback wasn't enough? What happens if the chamber gets a little dirty?

You can set the datum any anyway you see fit. It may work or it may not. But I don't think that you are going to find much consensus with your proposal.

 

[Edsel]

The first requirement to have a reliable firearm is that the ammunition chamber correctly. If is doesn't you are screwed.

It’s a lazy Saturday afternoon range toy, as mentioned in the earlier post…

Overgassed, overthis, overthat, loose tolerances and whatnot - that’s for service firearms.

You can set the datum any anyway you see fit. It may work or it may not. But I don't think that you are going to find much consensus with your proposal.

Figure I’ll do just that.

Still need to look into whether a tight - fitting cartridge would accelerate bolt lug wear, as 918v suggests.

I was under the impression that a loose cartridge imparting a brisk impulse on firing would do this, not the other way round.

 

[918v]

Look up neck sizing for the AR-15 and you’ll get a lot of different opinions and entertainment.