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This is a follow up to an earlier post. My M1A is stretching brass like crazy and I wanted to know why. Now I am really stumped:

My M1A's bolt won't quite close on a 1.638" headspace gauge (Forster .308 Win field gauge). The gauge goes easily into the chamber.

I used my RCBS Precision Mic (.308 Win) to measure both the headspace gauge and the brass that I fired in the rifle, with the gas valve shut.

The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.

Couple of details: Like new rifle probably has less than 500 rounds through it. New Black Hills Match ammo was used when shooting with the gas valve shut. The bolt was stripped, and all parts, chamber, and Precision Mic thoroughly cleaned before measuring. Headspace gauge is brand new and there are no burrs on the brass.

With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.

Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?
 

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I would think that you need to find out exactly what your headspace is. I invested in a complete set of match gages several years ago and am glad for it.

The cartridge headspace of that ammo you are shooting is prob. 1.630-maximum. Your chamber headspace with component bolt could be 1.635, 1.636, or 1.637, fact is you just don't know.

I am not surprised the brass is stretching that much given that the probable cartridge headspace is @ 1.630 or less and your chamber headspace is almost .308 field reject.

What does the rifle's headspace tag say it is? If you bought a new SAI rifle, it should have had one.
 

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It's hard to answer your question because of the variations that are found with the tools, the chambers, and the cases.

There are two issues that complicate headspace measurements; the true diameter that the tool uses as a datum and the angle of the shoulder of the case. Both add errors in the actual measurement of headspace.

I have a friend who is a machinist/gunsmith and he made a HS gauge for me. His gauge is styled after the Hornady gauges but he used better materials (stainless steel) and milled the diameter to a true 0.400" with no taper to the gauge's contact point. A lot of gauges use a taper where they touch the shoulder of the case but I didn't like that idea, if there is any difference between the angle of the gauge's contact point and the case's shoulder angle it will give you a false reading. With no angle to the gauge it will always give a headspace reading at a 0.400" datum point, regardless of the angle.

SAAMI specs specify a shoulder angle of 20 degrees for a Winchester .308 and the shoulder angle spec for a 7.62x51 case is 40 degrees. Depending on chamber and case shoulder angles you will get varying degrees of accuracy when you measure the HS.

My guess is that the case's shoulder angle is less than the chamber's shoulder angle. If that were true then the case can grow a bit more before it's shoulder contacts the chamber's shoulder. That could be due to wear in the chamber or how it was originally cut. If the barrel has had hot enough loads and with enough time, flame cutting could have enlargened the front end of the chamber, including the shoulder. Considering how long your HS seems to be I think that this is a strong possibility and that basically you are looking at a shot out barrel.




This is a follow up to an earlier post. My M1A is stretching brass like crazy and I wanted to know why. Now I am really stumped:

My M1A's bolt won't quite close on a 1.638" headspace gauge (Forster .308 Win field gauge). The gauge goes easily into the chamber.

I used my RCBS Precision Mic (.308 Win) to measure both the headspace gauge and the brass that I fired in the rifle, with the gas valve shut.

The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.

Couple of details: Like new rifle probably has less than 500 rounds through it. New Black Hills Match ammo was used when shooting with the gas valve shut. The bolt was stripped, and all parts, chamber, and Precision Mic thoroughly cleaned before measuring. Headspace gauge is brand new and there are no burrs on the brass.

With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.

Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?
 

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SAAMI specs specify a shoulder angle of 20 degrees for a Winchester .308 and the shoulder angle spec for a 7.62x51 case is 40 degrees.
Not trying to snipe here, just a little clarification: SAAMI spec for .308 Win shoulder angle is 20 degrees from centerline. 7.62 NATO spec is 40 degrees overall, from shoulder to shoulder. It's the same angle...GI1
 
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Case head Space after Firing

The cases will come out longer even with the valve off sometimes, I do not know why, it happens in my rifles to some limited extent.

My best M1A has a HS of 1.638", this is the most accurate M1A of the two using Hand loads. The other M1A has a HS of 1.624", this rifle will not accept any ammunition other than Hand loads.

Your situation may be similar, in that, all "Out of the Box" ammo is going to grow longer than you desire, limiting the reloading of such cases. I have excepted the use of once fired case as a standard procedure in accuracy shooting. In both riles I set the shoulder back .005/6" under chamber Head Space. You may have to consider doing something like this to get the full potential of your rifle.
 

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I stand corrected, that's what happens when you look at something without really seeing it.

But even so, I still think that there is a chance that the chamber has been worn to the point of flaring out the shoulder. That would explain the numbers that he is getting.



Not trying to snipe here, just a little clarification: SAAMI spec for .308 Win shoulder angle is 20 degrees from centerline. 7.62 NATO spec is 40 degrees overall, from shoulder to shoulder. It's the same angle...GI1
 

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I have the similar problem. My rifle has always suffered frim short case life- 2-3 firings. My brass stretches about .020" each time, necessitating trimming every time. I fire-formed new Winnie brass with a light load and bullet, then sized to the max that the head space can handle- stripped bolt closes by finger, but has no slop. (interesting, to make the brass .005 longer than just full sizing, I needed to set the die .020 from the holder.) Anyway, brass fits headspace, loads of 43.5 IMR 4895, 168gr HPBT, Win LR or Fed 210 primers. Primers are flat, brass grows .020.

Sak hmmm 67 barrel, headspace, lands, etc matches my Win spare.

I use the Win spare as a case length gauge, along with a depth micrometer. I have compared the two chambers last time the SAK was loose, various cartridges measure the same in both barrels. And sizing .005 loner than the RCBS dies full length to, and being just snug in the rifle, would mean my chamber is about right. Full length sized brass is a good match to several brand new factory loads- M80 ball, Musgrove, and FGMM. No function probs with the longer brass.

I had allowed my length to creep up to 2.835- still fit in the mag. But reading up on 'bullet jump', it seems seating to where the bullet touches the lands = an extra 7,000 psi. I seated a bullet long, inked the bullet, dropped it into the chamber, rotated to mark the lands, mic'ed the head depth, to establish land depth. Adjust COAL to suit the particular bullet ogive. 2.830 would be just touching. Last set of loads, I made the coal fersure 2.790, so a bullet jump of .040? No help, still flattening the primers, still stretching to 2.020.

Whaddya think, try a longer bullet jump? Shorten coal to 2.760? Any other reason for high pressure signs?

Next item would be chamber diameter. Both barrels, brass rattles around radially- like .010 ? Is a "Large Base" die available? I know shot shells have a 'small base' die.

Hey Art L., could shell diameter be the prob re: one of your questions about re-forming 30-05 brass to 308? How it is accurate first firing, but second + loadings are sub-accurate?
 

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It's hard to answer your question because of the variations that are found with the tools, the chambers, and the cases. is a strong possibility and that basically you are looking at a shot out barrel...
RAMMAC nailed it precisely. DI5

I made my own HS guage using a true 0.400" ring guage and have the same results with My M14 clones. The fired cases, with port closed, measure longer than I would suspect. But tolerances in chamber angles and a little elastic "give" to the bolt under 50KPSI pressure will result in cases that "appear" stretched.

Try this: After firing, with port closed, pull the bolt back about an inch and let it go forward slowly to rechamber the fired case. You will note that the case does NOT slide easily back into the chamber. It has stretched and as also stretched the chamber and bolt lockup which will rebound and trap the case.

So, Every case will APPEAR to be stretched, leading you to think that you HS is off. But it's nothing to worry about. Measure your HS with a real HS guage and let it go. A couple thou are of absolutely NO concern.

JWB
 

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Some food for thought:

Here is a link to chamber and cartridge designs: http://m14forum.com/ammunition/84411-chamber-drawings.html

The smallest chamber, and largest diameter cartridge would be .001 different.

The largest chamber diameter and smallest (most squished by resizing) cartridge difference is .011.

IIRC, my cartridge/chamber clearance is about .010.

So, our problems with case strength length wise, is really case stretch radially. (I measure my stretch after re-sizing, where it counts).

See, when fired the case gets bigger in diameter, when resized, the excess diameter is squashed into excess length, NOT merely compress back in to a thicker wall.

Adjusting final length is easy, as I've done by adjusting die 'depth'. But the extra width turns into extra length when squashed back to the original diameter.

So, I think me and Welding Rod need "wider base dies".

I wonder if I can cut down to some other dies? To get a wider base and still the correct head space?
 

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Here is another thought for you, maybe your bolt face is not square with the chamber.
Maybe your fired brass is now basically crooked, causing the headspace reading anomely.

Just a thought, maybe not a good one, but it has been a long day.
 

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I've been through the squareness question. It's a Fed Ord receiver, the barrel had been lock tited in, seemingly crooked. R&R, cleaning out the lock tite, now seems OK. Cases used to be crooked, and rip at the bottom after one or two firings. Now I'm getting 2-3 and case bottom are symmetrical to the sides. And head space is tight, with my "partial length resizing".

Let me conceptualize a bit about sizing the brass:

Lets think of a cartridge slit and opened up, into a rectangular shape sheet of brass. Before sizing, it may have grown .010 in length. But, due to a .010 looseness in the chamber, it has grown .031 in width. (Pi x diameter = circumference, or width). So, before sizing, the brass is .010 x .031 greater in area. So we resize, and the area stays the same, but circumference is squashed back to size, meaning the length grows by enough to off set the same area, length grows by ummm an extra .030? Minus some, a bit of the brass is made thicker when worked, so extra .010? So stretch is not due to head space, stretch is due to "diameter space" ?

So, less diameter sizing while still re-sizing length to head space specs?
Maybe size the body in a 30-06 die, to attain max diameter, then cut off a 308 die to use only for neck and shoulder, and head space length?
 

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casebro,

That's a pretty stiff load of IMR 4895, especially in a service rifle.

"Anyway, brass fits headspace, loads of 43.5 IMR 4895, 168gr HPBT, Win LR or Fed 210 primers. Primers are flat, brass grows .020."

The Sierra manual lists 41.3 as a max load giving 2600 fps and the Hornady manual lists a "Service Rifle" max load is listed as 41.4 giving 2500 fps.
 

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casebro,

That's a pretty stiff load of IMR 4895, especially in a service rifle.

"Anyway, brass fits headspace, loads of 43.5 IMR 4895, 168gr HPBT, Win LR or Fed 210 primers. Primers are flat, brass grows .020."

The Sierra manual lists 41.3 as a max load giving 2600 fps and the Hornady manual lists a "Service Rifle" max load is listed as 41.4 giving 2500 fps.
Speer says 47, Lyman says 42.5 makes only 47K, and the chronny seems to agree. Plus a ton of on line references, right on this site.

I use 41.5 in GI/match brass, 43.5 in Civvie.
 

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casebro,

That's a pretty stiff load of IMR 4895, especially in a service rifle.

"Anyway, brass fits headspace, loads of 43.5 IMR 4895, 168gr HPBT, Win LR or Fed 210 primers. Primers are flat, brass grows .020."

The Sierra manual lists 41.3 as a max load giving 2600 fps and the Hornady manual lists a "Service Rifle" max load is listed as 41.4 giving 2500 fps.
Speer says 47, Lyman says 42.5 makes only 47K, and the chronny seems to agree. Plus a ton of on line references, right on this site.

I use 41.5 in GI/heavy match brass, LCM or FGMM. Perhaps my 43.5 was a misprint? a load of 41.5 in civvie brass gave 2405 on the chronny.
 

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Have you considered that you may be lengthening the cases when pulling them back over the expander ball of the sizing die during the downstroke after sizing?

Try sizing a case without the cecapping rod and expander ball installed as an easy check.
+1... my father ran a test on this. He has a Rem 700 with a really tight and true chamber. He would measure the case neck runout on fired cases and the necks would be almost perfectly true and straight, near zero runout. Then after resizing, he would measure again and not only were the cases stretched longer, but the expander ball would pull the necks out crooked, the necks had more runout after sizing than they did when they came out of the rem chamber after firing...
 
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Discussion Starter · #18 ·
jmoore - Thanks for tip. The stretch I am measuring and have indicated (.013" to .018") is in the head space dimension of the fired brass, not the overall case length. I am measuring the headspace dimensions of the cases before resizing.

Jbrooks - I will try that. Flex sounds logical.

Also sounds logical the shoulder area of my chamber may not be flat or true to the bolt face.

In either or both cases, bummer as it looks like there will be no way to get any life out of the brass. If it was a headspace issue at least I could have gotten it fixed by SAI.

But, this rifle does seem quite accurate, shooting 1 1/2" 5 shot groups with match ammo so on the other hand I really didn't want to replace the barrel.
 

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Welding Rod,

If you unscrew the sizing die a tad (0.01"...) you will end up with a case that is Mostly resized and will lengthen your case life.

The DOWNSIDE of this is that the case will probably NOT just simply drop into the chamber friction-free.

But, for non-combat/life-threatening situation (i.e., paper punching) this should work. The force of the bolt slamming forward will ensure proper chambering.

I have actually found there to be NO measureable difference in group size using this method, but case life will improve slightly.

Bear in mind that most ammo (even milsurp) is right around 1.630" HS to begin with and a NATO chamber minimum is specified to be 1.6355". So a round fired in even a minimum chamber will expand at least 0.005" plus probably 0.003" to make up for the flex bwtween the bolt and the chamber. Since your chamber HS is 1.638" +/-, you are loking at a minimum of about 0.01" stretch in any case.

We all have to admit that the design of the rifle is as a Combat weapon, not a match rifle. So we have to put up with some of the side effects like shorter case life and possible feeding issues. Otherwise use factor ammo ro resize to factory specs. (BTW, I have found many dies will actually OVER-resize, that is, they resize to Less than factor specs. so you have a lot of wiggle-room.)

JWB
 
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