Fired cases- can't fit bullet in mouth

[jccinohio]

Shooting a .308 with a match barrel. When I shoot my reloads with 110gr varmint bullets, the case mouth is open and I can easily drop a bullet in the fired case: I get good precision with these. However, when I shoot 175gr smk reloads or factory FGMM I notice that the very end of the case mouth is sloped in enough so that I have to push to get a bullet in a fired case: I can't seem to get good precision with these. The pic below is not my pic, but same issue (they never resolved anything in the thread it came from).

My reloads are once fired Lapua cases from the same rifle. Annealed. Chamfered and deburred, but not extremely. I tried turning them down a little, to 13.5 thou wall thickness (measured with ball caliper) and not any discernable help. The 110gr loads were not turned. There is no carbon ring as checked with bore scope. Cases are trimmed to 2.008" before loading, and not too long after firing as checked with borescope. Using varget for both loads. The 110gr loads are about 3100fps, the 175gr reloads are just shy of 2600 the FGMM are 2645fps.

I'm guessing that since the 110gr open fine that it is related to interior ballistics? The pressure with the 110gr must be different..

Is this an issue or is it not related to precision? Should I try trimming a little more to say 12 thou wall thickeness, or is that too far? Maybe the case mouth turns in after the bullet has left as the case cools?

Discuss..

 

[Mark M.]

So If I understand you, and I think I do, the only difference with the two loads is in the weight of the bullets, 110 and 175, and then when fired, you end up with two different brass conditions, is that correct

 

[Kracken McGraw]

Show us some pics of your fired cases. You might try deburring the outside first, then do inside chamfer as the last step.

 

[okie]

check the case length??

 

[Dan-Schenk]

I was having the same issue with a tight chamber on a 284, I found out I needed to trim about 0.002 under the typical trim to length.

 

[Kracken McGraw]

Good catch guys^^^^^^.

 

[dellet]

Check headspace and neck length fit with the borescope.

Headspace determines how long the neck is, less body, more neck. Case length can be fine, and still pinch the bullet if the base to shoulder length is short.

It may be that the pressure is just enough higher with the heavier bullet load to allow the neck to stretch enough to crimp the bullet.

Either way the only way to round off the end of the neck is for it to be to long. Reducing diameter may help, but only because your moving the case neck slightly back out of the angle in the chamber.

 

[AlNyhus]

Three things you need to know:
1. How long is your chamber
2. What's the chamber neck diameter
3. What does a neck measure with a seated bullet

 

[okie]

YEP ^^^^
I would trim a couple back 3 or 4 thou. and try. I'd rather my cases be too short than too long.

 

[SKWERLZ]

Yep typical causes for that are long case neck, short chamber neck, tapered chamber neck and tight chamber neck.

 

[jccinohio]

There's plenty of headspace on the factory ammo, and it still does it. I run mine pretty tight ~.001-.002 bump.

Neck trimmed and loaded 175gr SMK round is .335, 175gr SMK fired cases are coming out at .341 everywhere except the last say .030" of the mouth, and there it tapers down to .333. The 110gr fired cases (non neck trimmed) are .339" at the end of the mouth.

This is a fired case that is 2.011" case length deprimed.. The longest they come out is about 2.015". I'd say that gap is at least .020".

 

[Bill K]

Trim the brass you shoot the 175's in just a shade shorter than you do with the 110's, due to increase in pressure with the 175's, bet your issue will solve itself.

 

[jccinohio]

Failed to mention that I am using a suppressor.. so increased back pressure. Next time I'll try without it, but need to resolve the issue WITH it as that's why I have it..

So I turned the necks down to 12 thou, and shortened the case up to 2.005". Only did 3 (with suppressor).. one still held the bullet to where I couldn't shake it out easily.. one could be shook out easily, and one the bullet was free.. So a little better I guess.

 

[DanCarey]

Be careful when trimming a case to less than excepted length. When you shoot a lot of cases trimmed to the minimum length and then shoot one at maximum length, you will have that condition you are experiencing. Carbon buildup at the end of the shorter case will be the cause.

 

[Dusty Stevens]

Pressure is your issue. Case length has nothing to do with it or youd see it with the lighter bullets too. Show us the fired primers in the heavy bullet load

 

[dellet]

There's plenty of headspace on the factory ammo, and it still does it. I run mine pretty tight ~.001-.002 bump.

Plenty of headspace will compound the problem. In a piece of brass with the same overall length…
The less headspace you have, bolt face to shoulder contact in the chamber.
The more neck clearance, shoulder contact to end chamber you will have.
Moving the shoulder forward, shortens the neck.

To keep the open end of the neck from closing when fired by reducing the diameter, you’ll will probably need to turn it down to .330-31 loaded, based on a fired diameter of .333”

I would trim a piece of fired brass until the bullet can be inserted freely, set the die up for .001” bump and check results.

A close measurement and comparison of the light bullet fired cases vs the heavy bullet fired cases will likely show more stretch, longer neck, and or base to shoulder, and explain why on one and not the other. You’re likely bumping one case back farther than the other.

A chamber cast to look for a taper at the end of the neck would not hurt.

 

[jccinohio]

Top 2 for reference, mid two are my reloads, bottom are factory FGMM.. Primers may be a smidge flat, but more so on the factory than mine, and I don't think it's overpressure by any means.

 

[jccinohio]

What's the minimum wall thickness you'd want a piece of .308 Win brass? I imagine the more you take it down the shorter the brass life?

 

[DanCarey]

Its not about wall thickness. The accepted distance between loaded neck OD and the chamber ID is .002 to .005 depending on the intended use of the firearm. That will dictate the wall thickness you want.

 

[jccinohio]

Ok.. so I took a fired case and sized it with the Redding S die with no bushing to get the headspace out of the equation. The case was 2.012" case length, so longer by far than my reloads.. I took that case and opened it up even more toward the end by rounding it out by wigglin' a pin gauge around inside. Got to .341" before it showed resistance going in the chamber (firing pin and ejector removed). So the interior neck diameter must be at least .340". So at .335 I was around .005" difference between loaded neck OD and the chamber ID. That was before I turned to .012 wall thickness. Going to .012 helped some, so maybe I just need to go a little further.. to .0115.