Measuring Gas Gun Shoulder Bump?

[RegionRat]

The tape trick works well enough to tell you what you want to know or expect from brass expansion.
If you don’t have the Go Gage, you can run the same method on the brass case with the most cycles or longest shoulder datum.
The difference between using an actual Go gage versus a sample of your brass, is the feel (stiffness) difference.
A sample of brass can be softer and less stiff than a solid column of steel, so you will want more finesse with respect to forcing the bolt closed.
If you have the inclination, you can also temporarily remove the ejector so that force isn’t part of the study.

 

[searcher]

The wide range in brass length as measured from the shoulder as you did - is normal for factory ammo. Factory ammo must be made to fit all chambers - so you are going to have very "generous" clearances in the chamber. These loose chamber conditions hamper brass from fully blowing the shoulder forward, especially when the base of the cartridge is not solidly supported - so you get the various measurements for many reasons. Because the brass is not all the same thickness, doesn't have the same value of annealing, not all supported the same at ignition, exact powder charge, etc., etc. Brass will spring back a tad when fired - but not that much. So, it is pretty safe to assume - looking at your brass - that the 1.5555" measurement "could" be your blown out brass measurement because it is far from the shorter measurements and there are many that are the same. I would look very carefully at the sole longer piece at 1.5560" because is only one piece and could measure that way because of a ding when ejecting or whatever. I'd load up your next batch to be 1.553". That is pretty tight for an A/R, as you will usually get .001' or so spring back after it is loaded for a while - leaving you .001" clearance +/-. After shooting these pieces, measure your fired brass. If they stay at the 1.555" - it is pretty solid that is the length you want to use to reduce from when bumping in the future. If the brass grows LONGER - you need to record the longer measurement as your new baseline. Regarding safety if you don't bump enough - as long as you bump ANY amount shorter than the brass was - you should not worry. Think M-16 and the side bump (forward assist) for slamming gritty dirty ammo and a chamber full of carbon to close on a round that doesn't want to go. Believe me - you are far from that. In reality - if you were to not bump enough and your shoulder was .002" LONGER than your chamber, you would not realize it when your bolt slams it forward when chambering a round. It will absolutely push that soft brass forward .002" without slowing down. It does put a bit more wear and tear on your bolt cam and locking lugs and that isn't how you want to run your final ammo - but you could. So those who say "the sky is falling" when you don't have .004" clearance, that might be sage advice - but the design of the gun is quite forgiving. So I'd not worry about "blowing yourself up" over a few 000.

 

[jelenko]

you would not realize it when your bolt slams it forward when chambering a round.

To this point, I've had brass that is .005 over the length of the chamber chamber just fine in an AR. Once I discovered they were longer than the chamber, I went back and sized them so the max case headspace was just at the chamber length.
I do clean the chamber after every range session - which can be up to ~ 130 rounds. Over the 50,000 or so rounds, haven't had a failure to feed.

 

[243winxb]

M16, Ar15 must have bolt locked to fire. If action opens to soon, head to datum measurement of fired brass, will NOT be correct to the chamber.

Take factory ammo fired brass. Set a .010" gap between shell holder & bottom of FL die. See if the sized brass will allow the lugs to lock. Bolt should not lock.

DO NOT let the bolt slam on brass placed in the chamber. This will size the brass, pushing the shoulder back.
Move sizing die down .002" at a time, till bolt locks in place.

 

[243winxb]

I guess my most important question is if there is not enough bump can it cause OOB kaboom in an AR? Or will it just not fire?

Not fire & round may be jammed in action.

Excessive crimp may bulge a shoulder, this causes jams/action locking problems.

Taper crimp die, if available.

 

[243winxb]

I've had brass that is .005 over the length of the chamber chamber just fine in an AR.

I

DO NOT let the bolt slam on brass placed in the chamber. This will size the brass, pushing the shoulder back.

.005" is a bit much. Have seen almost .002" set back, when M16 bolt is allowed to slam into brass sitting in the chamber.

The firing pin strike from a Savage Axis 223 bolt action can set the case shoulder back .006" Found this out when a primer failed. The 2nd pin strike, sets the shoulder back even more.

 

[clowdis]

You have to full length size .223 in order for it to cycle the bolt on an AR consistently. Is there something special about a .204 that you can size cases .002 under and still have reliable function?

 

[gdbleb]

DO NOT let the bolt slam on brass placed in the chamber. This will size the brass, pushing the shoulder back.

Not sure how you can avoid this in highpower slow fire. I noticed the extra setback during chambering when I wasn't setting back the shoulder .004 or more for the AR. Spoke to Frank at Compass Lake and he said it was normal. Just did .004 - .005 in the press and chambering stopped the extra setback.
Curious about the variance of the once fired brass in the OP. That's quite a range. I only see a .001 variance from a CLE in .223.

 

[243winxb]

extra setback during chambering

First, the head to datum measurement must be longer then the chamber, to start. Then the slamming of the bolt can size the brass, making the head to datum shorter.

The angle of the dies shoulder & the chambers shoulder may be sligntly different. False reading.

The mechanical method of setting bump, is always better then measuring with tools. Imo.

 

[Rtheurer]

to the OP. You do realize it can take up to 3 firings before the brass will be long enough to HAVE to have the shoulder pushed back in order to chamber, I see a lot of folks set the shoulder back from once fired brass and do not realize that they are pushing the snoulder back to over.005 in some cases and are keeping it there. Brass wont last long if you keep doing that. It will keep stretching the Pee out of the case that ends in Case head separation. Very seldom have a witnessed a case reach Maximuim length after its first firing. You cant set your FL sizing die correctly until it does.

 

[243winxb]

. Is there something special about a .204 that you can size cases .002 under and still have reliable function?

The rounds head to datum measurement needs to be shorter then the chamber. About .003" is good and .005" will not hurt a thing.

The chambered round needs head clearance as it sits in the chamber.