Fired cases- can't fit bullet in mouth

[dellet]

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.

No.

What that shows is that your neck is too long for the chamber.

Look at the SAMMI drawing. What is happening is the neck of the case is hitting the 45* angle in the chamber where the transition from neck to bullet diameter happens.

By reducing the diameter, it moves the contact point forward allowing the same result as shortening the neck.

So now the advantages of tightening the tolerances to a “match chamber” are lost. To keep from pinching the bullet, you need to reduce the loaded neck diameter to the same or more clearance than a standard chamber.

Or just cut the neck back a few thousandths.

 

[jccinohio]

No.

What that shows is that your neck is too long for the chamber.

...

I'm not sure I understand.. The fired case was 2.012". I had to go through 30 or so to find a looong one. I was trimming to 2.008", in the test I did earlier today I went to 2.005". Also the pic I showed of the borescope, it seems (again with a real long fired case) there's .030" or more free space from the end of the case mouth. So even with these looong fired cases I don't think I'm hitting that 45 degree slope, much less with a trimmed & loaded round. Maybe I need to just get some cerrosafe or what ever they call it today..

 

[Dusty Stevens]

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.

View attachment 1428305

Its not about overpressure its under pressure, same reason if you go even lighter itll put a big dent in the shoulder

 

[Dusty Stevens]

No.

What that shows is that your neck is too long for the chamber.

Look at the SAMMI drawing. What is happening is the neck of the case is hitting the 45* angle in the chamber where the transition from neck to bullet diameter happens.

By reducing the diameter, it moves the contact point forward allowing the same result as shortening the neck.

So now the advantages of tightening the tolerances to a “match chamber” are lost. To keep from pinching the bullet, you need to reduce the loaded neck diameter to the same or more clearance than a standard chamber.

Or just cut the neck back a few thousandths.

View attachment 1428397

Why are the cases with the lighter bullet not doing it?

 

[dellet]

Why are the cases with the lighter bullet not doing it?

Neck isn’t stretching enough to pinch the bullet or hit the taper.

 

[DanCarey]

Lets talk about that posted picture. What is the gray stripe between the end of the chamber and the barrel?

Even if it is a match barrel and a match chamber, it won't change the length of the cases.
You can do a chamber cast as you mentioned, I don't think you will learn much from it and its a pain in the ^*^ to do.

The stated case maximum length for the 308 is still less than the actual chamber. You could run all your brass at max and still not touch the 35 degree taper at the end of the neck.

The bore scope is your best bet. I have a Hawkeye and a Sony camera that I use. I set the camera for the best resolution, so I can blow up the size of the picture on the computer.

Edit, I enlarged your picture up 500% and that gray band looks like carbon to me.

"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:"

 

[PhilC]

Due to magnification effect of the borescope, you cannot "estimate" there's .030" or more clearance.

Remove the carbon completely and use a Sinclair chamber length gage or equivalent to know the exact chamber length.

 

[dellet]

Based on the numbers in various posts,
brass was trimmed to 2.008”.
Shoulders bumped back .001-.002” headspace.
Fired cases measure as long as 2.015”

That’s a lot of stretch in a case that has already been fired and sized, on a single firing if those numbers are correct. It would also be almost all in the neck.

For what it’s worth, here’s a before and after fire forming photo of a case that shrank .005”. Please excuse the rough machining, that chamber is about 130 years old.

 

[jccinohio]

Lets talk about that posted picture. What is the gray stripe between the end of the chamber and the barrel?
..

That dark band is the shadow from my cheap borescope light.. Here's another pic from the other side of the wall.. so no drop off to cast a shadow..

I'm going to try trimming a little more.. maybe 2.003".. and not chamfering as much (since it's a boat tail)

 

[DanCarey]

OK, here is what I think. When looking at the (metal) you see tool marks and fire cutting. That gray band is smooth.

You can drill a 1/4 inch hole thru the primer pocket of a 308 case, put the case in the chamber then the bore scope and see where the case neck is.

Here is another thought, You can remove the decapping assy, lube up a 308 case and tap it into the die with a plastic hammer until the neck is much longer than normal. using a knife tap the mouth edge slightly creating a saw tooth effect. Drill the pocket out with a number 7 drill and tap the hole with a 1/4 X 20 thread. install a screw in the case and put it into the chamber and turn it a bit then look at the chamber neck with your bore scope. If its carbon, you will see where the case marked it. Don't over do the test. The barrel steel is harder than the brass so if you are careful you'll be ok.