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M1A and Odd Headspace Question

5K views 34 replies 15 participants last post by  canmancan 
#1 ·
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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#2 ·
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.
 
#4 ·
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.
 
#5 ·
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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#9 ·
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
 
#6 · (Edited)
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.
 
#8 ·
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?
 
#10 ·
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?
 
#12 ·
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?
 
#13 ·
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.
 
#17 ·
+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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#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.
 
#19 ·
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
 
#21 ·
Not exactly...
The RCBS X-Die does limit case growth, but you must still trim your brass and RCBS suggests that it should be trimmed short, after which you can reload several times before trimming is required again. However, the M14 is very hard on brass. No matter what you do, you are only going to get 3 to 4 loadings with brass fired in an M14. Brass seperation at the base is common and comes early.
 
#22 ·
I'm curious, Welding Rod. How much fore and aft slop is there in the bolt body to receiver when the extractor and ejector is removed? If the hammer is able to drive the bolt and cartridge forward an unusual amount upon ignition, perhaps it would account for some of the overworking of your brass, resulting in unusually "blown forward" shoulders. (It would actually be a case of "blown rearward" case heads.)
 
#24 ·
The cartridge tears about 1/2-3/4" from the bolt, leaving the front part in the chamber. Best tool to remove the remnant is the chamber cleaning brush. on a section of cleaning rod. Just jam it in and pull it straight back. The wires will act like barbs and grab it good.

I wish I didn't know this. But there is a reason I've been posting to this thread.

Other idea, from working on a M94 Win 30-30. The make a 307, basically a 308 with an added rim. Shells are heavy walled, heavier even than LCM. Making them into custom 308 wouldn't be too difficult. Chuck in a lathe, turn off the rim, deepen the groove, touch it with a file to round off the 'rim' of the rimless case. More brass = longer case life?
 
#25 ·
I emailed RCBS. They got right back to me.

RCBS X mandrel won't retrofit into the usual die, I asked them. Of which I already have two sets. So I may turn out my own mandrel that will retrofit, just for the sake of a test drive. I'll simplify it, probably need one pass to do the X die thing, then a trip through the usual die de- and re- cap plus expand the neck. But it takes 3 operations to trim: trim, chamfer ID, chamfer OD, so I would be two steps ahead, and get much longer case life.

They do sell the X sizing die separately. but it cost only about $2 less the the set. Plus the mandrel is a much heavier de-capping pin shaft, better for use in crimped primer cases. Too bad it wasn't an option 25 years ago when I bought the gun and dies.
 
#28 ·
No, but can lead to a really difficult to clear malfunction as the next case tries to feed. I suppose you could etch a ring in the chamber, but I've never heard of such.

There have been broken shell extracting tools made to clear brass remnants (including just case necks) that have been made over the years in all sorts of military calibers, from 45-70 on up.
 
#27 ·
I don't know the limit, but you can burn a ring into your chamber. Not beneficial.

Usually, a case it good for umm 2-3 trimmings, then discard next time it grows to max, listed in every reloading manual.

I did make up my own mandrel to retrofit my old sizing die. Sure does work to shorten cases. I could feel somehtin happen in the last bit of stroke, must be the excess 020. getting pushed back to where it started. A quick trip to the range, with the twice-trimmed, once X'd, cases did give me a separation. So maybe two trimmings, plus fire one more time, is the limit?
 
#29 ·
Success! Longer bullet jump = lower pressure.

I do believe I've got my problem solved.

I loaded 43.5 gr IMR 4895 in Civvie Win brass, Nosler 168. To two different seating depths. Ten each. COAL of 2.760 and 2.740. Brass was fired two or three times previous to resizing with my home made X-die.

Had three case separations in the long ones, and stretching to 2.015 - 2.020 in those that didn't break. Flattened primers too.

Shorter loads, No separations, stretched to only 2.008-2.010 in the shorter loads. Primers still slightly rounded. I'm thinking that gives me a bullet jump of about .040".

I guess now I need to work up the load for accuracy again.
 
#31 ·
I've always liked the Lyman book. It gives an asterisked load, which is the load that gave the most constant internal ballistics. Accuracy starts in the chamber, if you are not consistent internally, you can't be consistent down range.

In the case of 168gr HPBT, 13 powders, two show the asterisk. IMR 4064 and 4895.

But OOOPs, the max. * they show for 4895 is 42.5. Maybe I ought to keep my glasses closer to my library. Still, other manuals say up to 47. So I guess my 43.5 load may have gone too hot for accuracy?

Now, back to my regularly scheduled bedding experiment.
 
#32 ·
I think you guys are on the right track myself. I have never used the short COAL that casebro is using but I ran it through my QuickLOAD software and I was surprised at what I saw. Personally I would test the load range that tonyben recommended, I might extend it a bit (43.0 - 44.1 grains) but I think that you guys are going to get some really nice groups in there somewhere.

As for pressure, QuickLOAD is no substitute for an actual pressure measurement but I've been pretty successful at estimating chamber pressures so far and the range of powder charge weights I mentioned should produce a chamber pressure range of about 50400 - 54400 psi, and that is equivalent to what military ammo produces regardless of what most people will tell you (check out this link for actual pressure test run by Larry Gibson http://m14forum.com/ammunition/100445-7-62-x-51-cartridge-vs-308-winchester-differences.html#post706954)
 
#33 ·
Another trip to the range today.

Setting the bullets in deeper seemed to lower the pressure. .060 shorter COAL. In civvie Win and GI WCC '67 brass. 41-42.5 grains Looks like my gun ought to like about 40gr of H4895 in either civvie or commercial brass. 41 was tighter than 42 in either case today.So something is making sense, finally...

30 rounds, slight rounded edge on all primers still, no case seps. Though flattest was GI with my high load of H-4895.

I haven't sized them yet, but some of the GI cases are actually mic shorter than before firing. I guess they must have stretched in the diameter, pulling the length back? The civvie Win stuff mostly stayed at 2.000, with a couple stretching to 2.010.

24 years with this gun, looks like my problem is solved. Hallelujah !

eta: Makes me wonder of the lore about how "GI guns CAN'T have long case life" is malarky. Many may have short bullet jump, where the bolt guns in the civvie world all have long jump to preclude high pressures?
 
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