Perfect head space and separating case heads

[Nbenz]

I recently bought a savage 10T in 308 from Cabelas as an intro long range rifle. I also just started reloading and have been having relatively good luck until just recently. I started working up a load with 168 gr Amax's and Varget when I started getting case head separations out of the blue (no other pressure signs). The Hornady manual states a max load of Varget as 44 gr and the case head separations occured at 40 gr in my testing. After looking at the chronograph the velocity was 2750 fps which didn't seem too dangerous in my limited experience (I've had 175 SMK's at this velocity in the same brass with no pressure signs). However, separating case heads and relativly high velocities told me to back the hell off on the powder charge. Unfortunately I wasn't getting the velocity I wanted at the accurate powder charges with varget so I tried reloader 15. Again the case head separations were occuring at around 40.0 gr of reloader 15. This time though the velocity was right at 2600 fps and even the 38 gr loads I tested were stretching above the case head. Since I'm new to reloading and I was thinking I may just be ignorant as hell and bumping the shoulder too far back during sizing, I bought some factory hornady ammo to test and even the factory ammo was stretching the brass! I then assumed the head space was the most likely culprit and took it to a smith to get checked out and the head space was absolutely perfect. At this point I have absolutely no clue why this gun is tearing up all my brass. Anyone have any theories?


FYI the brass that is pictured is from the factory hornady ammo. I seem to have lost the cases that fully seperated
Also the stretch mark on the brass only goes 3/4 of the way around the case if that matters at all.

 

[Mordamer]

Can you stick the end of a paper clip down inside the case and feel a crack on the inside at the same height as those marks? It looks like stretch marks, but maybe it isn't.

 

[Nbenz]

Stuck a paper clip down and couldn't feel a crack. I have cut one of the cases from the Varget test open with a band saw and could see a definite thinning of the brass by the mark.

 

[Supersubes]

You haven’t mentioned any headspace measurements, how do you know the factory rounds are stretching? Are you FL sizing for the subsequent loadings?

There is no correlation between velocity and separations.

Tracking the case headspace dimension pre firing, post firing, and post sizing will probably show you the issue. Use a tool like the Hornady headspace comparator for this.

 

[Nbenz]

I'm assuming the marks aren't superficial since as I went up on the charge they actually started separating. I use a lyman case gauge to check every round after full length sizing and the rounds fired with Varget didn't fit in the case gauge. I didn't check the factory brass but it wasn't much of a logical leap to assume it wouldn't fit either

 

[padom]

So you have no idea how much your sizing your case? Does this happen with factory ammo? How did you setup your sizing die? How much is it bumping from fired case headspace?

 

[Nbenz]

I set up the sizing die by running the ram up and screwing the die down until it touched the ram and then backed it off a half turn. I sized a piece of brass and checked it in the case gauge, and screwed the die down until the case fit in the gauge. I'm using the lyman single caliber ammo checker in 308. This is how i was taught to size brass but I would not be surprised in the slightest if this was a bad method (using a comparator seems like a slightly better method). This didn't happen with factory ammo until I used Varget while working up a load and had a case separation. Now different reloads as well as hornady factory ammo have had the same stretching above the case head.

 

[Supersubes]

Get yourself a comparator. These threads always go the same way. There are quite a few reasons this can happen. Bottom line, your loaded ammo has too much headspace, which is the only way a case can grow to the point of separation.

 

[Deleted member 10043]

It is stretching on once fired factory ammo so not a reloading issue. Check the headspace. A fired case is your go gauge and the same case with electrical tape is your field gauge.

 

[padom]

Too much shoulder bump. Get a case comparator and measure your 1xfired case. Subtract .002 and screw your die down till it touches then back off 1/4 turn. Now size and measure with comparator, screwing die down till that sized cases measures your 1x case headspace measurement minus .002. (Fired case headspace 1.750 - .002 = 1.748

 

[Nbenz]

I'll definitely pick one up!

Check the headspace. A fired case is your go gauge and the same case with electrical tape is your field gauge.

I had a smith check the headspace with both go and no go gauges and the bolt closed smooth on the go gauge and wouldn't close at all on the no go gauge. I'm relatively sure the headspace on the rifle itself isn't the issue. I could try that method anyway though. Could you explain that method a little more? What do I use the electrical tape for?

 

[supercorndogs]

That is pretty surprising to hear. Is the factory brass way undersized?

 

[Nbenz]

Too much shoulder bump. Get a case comparator and measure your 1xfired case. Subtract .002 and screw your die down till it touches then back off 1/4 turn. Now size and measure with comparator, screwing die down till that sized cases measures your 1x case headspace measurement minus .002. (Fired case headspace 1.750 - .002 = 1.748

First of all thanks for the info I'll definitely get a comparator and use that method in the future. But that still doesn't explain why factory ammo would be stretching. Factory ammo stretching points to the head space being a little out of spec and the go, no go gauges definitely point to the ammo being incorrectly sized. every theory thus far has touched on the rifle head space or the ammo being sized too short but haven't really addressed that both have been checked. Is it possible the go, no go gauges work correctly but the head space is still a little generous? or on the flip side are the hornady rounds slightly too short from the factory and my case gauge just isn't accurate enough for me to tell? Sorry if someone actually addressed this and I just missed it

 

[Nbenz]

That is pretty surprising to hear. Is the factory brass way undersized?

The unfired factory rounds seems to be perfectly in spec in my case gauge

 

[buffybuster]

Factory ammo and brass are usually sized to the minimum or even slightly below minimum headspace, to insure it will chamber in every rifle.

At it's been mentioned you need to get a cartridge headspace comparator to be able to measure your current headspace and how much you are pushing the shoulder back. Also keep in mind once fired factory ammo may required a second firing to achieve full headspace.

Without the ability to measure the headspace of your fired cases you're flying blind.

 

[Nbenz]

Factory ammo and brass are usually sized to the minimum or even slightly below minimum headspace, to insure it will chamber in every rifle.

At it's been mentioned you need to get a cartridge headspace comparator to be able to measure your current headspace and how much you are pushing the shoulder back. Also keep in mind once fired factory ammo may required a second firing to achieve full headspace.

Without the ability to measure the headspace of your fired cases you're flying blind.[/QUOTE
Oh Ok that makes sense. Just ordered a comparator and i'll update when I can figure out whats going on.

 

[Deleted member 10043]

Cool

 

[918v]

Must be some kind if rifle to get 2750 FPS out of 40grs if Varget.

 

[spife7980]

Could you explain that method a little more? What do I use the electrical tape for?

A go gauge is a certain length, a no go gauge is typically that length + .004"

By using scotch tape (not electrical, its too thick) on the back of the go gauge you can increase the length of the go gauge to that of a no go gauge. The scotch tape is .002" thick or there abouts so two layers will make it a nogo but if you add one layer at a time you can work your way up to finding your actual headspace.
A comparator is helpful to put actual numbers on it when you get it in.

A drop gauge tells you when everything is sized small enough, it doesnt tell you when something is over sized. You could be drastically moving the shoulder back to get the die in deep enough for the bottom diameter to fit in the drop gauge. A caliper can measure the base diameter and the comparators can measure the shoulder in separate operations. Gives you more insight.

This video addresses the shoulders


This addresses the base of the case

 

[Deleted member 10043]

That is why I was careful to say a field gauge and not a no go.

 

[supercorndogs]

CHS on the first firing, I would expect it to close on a field gauge.