Correct way to determine M1A Headspace?

[Fatelvis]

In the past, I've been FL sizing, as shown in the instructions, w/ the die touching the shellplate. I've gotten head seps after 2 firings, so I determined I'm sizing them too much. (I'm using a milder load of 40.5 grns. of 4895 under a 168 SMK in LC cases). I've read Zediker's "Once fried" and many other things concerning loading for the M14, but have one question...... If I shut off the gas valve to make the rifle a single shot, can I use THAT fired case as a reference as to the chamber dimensions,(unlike the semi-auto case that is not accurate) and in turn, adjust my FL sizer accordingly? Thanx alot-

 

[USSR]

Fatelvis,

Get yourself a RCBS Precision Mic in .308. Measure a factory loaded round, then measure one of your fired cases. Set your FL sizing die to move your shoulder back so that it measures the same as the factory loaded round.

Don

 

[Fatelvis]

USSR, normally I would use that same approach, but the M1A extracts very quickly and vigirously, (before the pressure in the chamber fully drops). This tends to give a false and erratic HS measurement of the fired cases. Zediker calls it "fire DEforming", instead of fireforming. Thanks for the imput-

 

[Mark whiz]

USSR may have the right idea - at least to do it in a cheap way. Measure an unfired mil-surplus cartridge......that should get you close. Springfield enclosed the number of the actual headspace of my rifle within the box and I use the RCBS Precision Mic religiously on my sized cases to make sure I'm good. Seems like it never fails that I can size 50 cases and at least a couple have to be sized again cause the shoulder wasn't set back quite enough - even though I never touched the die. If you're the original owner (or if you know the chamber hasn't been messed with) you might can call Springfield and see if they have the headspace number for that particular rifle on file.

Outside of that, you either need to get the correct tools to acurately measure your headspace or find a gunsmith that can do it for you.

Locking down the gas valve probably won't be of much help. Everytime I've left that valve closed on my M1A, I had a hell of a time getting the bolt to pull back to extract the case - that HAS to be deforming the case some.

 

[ReloaderFred]

I've got two M1A's, a standard and a National Match. The RCBS Precision Mic die will give you an accurate reading on the headspace for these rifles. On my standard M1A, I've measured fired cases with both the gas valve open and closed, and they read exactly the same both ways. You can't close the gas valve on a National Match rifle, since they're made to stay open all the time.

You've properly diagnosed the head separation problem. The shoulders are being set back too far and the brass is stretching to the point of separating.

I wouldn't use a factory or military round for a standard. Almost every one I've measured was undersized, which makes sense, as they have to fit in a huge variety of chambers, and are only designed to be fired once without separating. Use fired brass from your rifle and then adjust the die to set the shoulder back about .002", or .003" maximum. You'll find that even though you'll get a round once in awhile that will measure a little longer headspace, it will still chamber and fire. (That's the magic part about reloading, and everyone knows there's no explaining magic.) Once you do this, your brass will last longer, but probably not the brass that has already been oversized.

Hope this helps.

Fred

 

[Fatelvis]

Thanks guys. The barrel on my rifle has been changed to a heavy Douglas 1:10. I'm sure it was chambered correctly, (done by John Holliger, years ago). I tried measuring the fired brass, and it was all over the place, as far as numbers go. Do I assume the longest measured case is the most accurate depiction of my chamber?

 

[USSR]

...but the M1A extracts very quickly and vigirously, (before the pressure in the chamber fully drops). This tends to give a false and erratic HS measurement of the fired cases.

Doesn't really matter what the reading is on the fired cases, it only gives you an idea as to your starting point. You will probably be okay if you resize your brass so they are one or two thou longer than the reading you get on the factory ammo (and it will work your brass less), but you will have to play with it to be sure.

Don

 

[30Cal]

Take a fired case. Measure. Size. Measure. Repeat. On average, you want to see the shoulder being pushed back about 0.002-0.004".

Ty