Long case length after firing

[Spencercoc74]

hi. Yes, I'm new here. I have a question that is not necessarily related to reloading but the folks here are probably the best equipped to answer intelligently.

I have two semi-auto 308 rifles. I recently shot a box of factory ammunition 10 rounds through each rifle. The brass length measures around 2.022 from one and 2.005 or less from the other. Both rifles are new, less than 200 rounds through each. I’m starting to believe that my first rifle is not properly chambered but I’m not certain what exactly is incorrect. Yes I understand that these are two separate rifles from different manufacturers and the chambers will be different. There aren’t any signs of excessive pressure on the brass or primers. I am reloading and I am not looking forward to trimming brass after each firing. Has anyone had a similar issue or know what may wrong with my first rifle?

 

[RegionRat]

The first step is to ask if you own and use a shoulder datum comparator?
Have you measured unfired and then the fired dimension?
Here are the SAAMI specs for your ammo/chamber.

You can see there is generous tolerance given to the ammo and chambers.

What we want to know is the two states of those dimensions I circled for the unfired and then fired ammo, versus the hypothetical chambers which can have some variability in those two lengths.

If we first check the unfired ammo, and then compare the fired ammo, we still don't know the reference dimensions since a comparator dimension is not absolutely anchored unless you do so with a reference tool like a GoGage with a known shoulder datum length.

By itself, a shoulder datum length measurement of your ammo is a good starting point. Cases can grow there and we hope the chambers are properly headspaced.

A GoGage is a good investment if you are going to stick with that design for very long. It can also be used to take a fairly good measurement of the chamber headspace by using a layer of tape until we hit interference.

So lets take the first steps by inspecting ammo before and after and then you can decide if you want to invest in the GoGage.

There is also the chamber overall length, which I also circled above. The easy ways are with a tool called an over-all-length gage, but we can also get a fair estimate by inserting a known length empty case and sending a borescope in for a look at the gaps.

Here is what a chamber length gage looks like. You have to sacrifice a case by cutting it down and then inserting it with this tool.

So, if you are willing to borrow or buy a GoGage and headspace comparator tool, then the shoulder datum length question can be put to bed for each chamber. Then, one of either the borescope estimate or the use of the above tooling will give you the chamber length.

If you still have questions, circle back.

 

[Edsel]

1. If available, use chamber gauges to determine actual headspacing.

2. If unavailable, get your longest piece of fired brass, get a baseline measurement, and gradually FLSize it decrementally by -0.001” in a stepwise manner, and assess the headspace measurement at which the brass chambers for both rifles, by pushing on the bolt carrier with your thumb. Ejectors will have to be removed prior, for best results.

3. Compare dimensions of fired brass relative to chamber headspacing. One would think that the brass measuring 2.022 comes from the rifle headspaced long, but not necessarily.

4. If not, consider the possibility of overgassing and early extraction leading to “bloated” brass (these are the rifles whose brass benefit from carbide dies sized slightly small). My LWRCi R.E.P.R. is a repeat offender as far as this finding is concerned - aggressive extraction on top of generous chamber headspacing.

For reference, one of my SIG 716 16” chambers is spaced at -0.002” below SAAMI maximum, while my LWRCi R.E.P.R. 20” is spaced at +0.002” above SAAMI maximum. That’s a difference of 0.004” between the two rifles. The latter rifle has a lousy recoil impulse despite being much heavier and having the longer barrel, so you can guess from that as to how it’s treating spent brass.

In short, attempt to measure your chamber’s headspacing and compare dimensions to fired brass. If it doesn’t yield what you would typically expect, consider how the rifle is tuned. Or, it may be an issue concerning both. There may be more minutae involved, but these are probably the two more common culprits.

Hope this helps.

 

[supercorndogs]

Any pictures of the cases? Some cases are long after firing due to the bullet crimp.

 

[Spencercoc74]

Thanks for all of the help so far.

Headspace is a difference of 0.003 between the two rifles. I had discounted headspace once I understood what it was and the symptoms of excess headspace. I believe the problem is between where the headspace measurement ends and the throat begins, if there is in fact an issue with the chamber. The overgassing/agressive extraction is an interesting theory.
I just measure neck thickness for fun. A half to a full thousandth thinner. Not a huge surprise.
I’ll do some more checking this afternoon.