Is this load too hot for my AR?

[longrangefool]

Got my 260 GAP-10 a few weeks back and have finally had some time to do some OCW tests today. I found a good accuracy node, but there's some good ejector marks on the case heads. It just seems like this gun shows pressure signs with pretty light loads, or maybe I need to use a faster different powder?

I tested 139gn Scenars and H4350 at 40.2, 40.5, 40.8, 41.1, and 41.4 grains. My best load was 41.1gn at .42" at 108 yards (repeatable). You can see that that load starts to leave some brass sticking up, and its even more pronounced in the 41.4gn load. I don't see any major primer flattening. There is a little primer cratering, but it seems to do that even on very light loads anyways. I also tried a load with H4831 at 43.5gn and there are swipe marks, but no metal shavings like the H4350.

Is this just too hot? I'm pretty well of loading for bolt rifles, but this is my first big AR. I wasn't able to get chrono numbers today as the chrony wasn't working with the overcast light, but I hope to get some tomorrow.

 

[longrangefool]

I read somewhere that someone suggested the brass neck was too thick....

I sized the brass with a .292 bushing, then the expanded ball opened up the necks on the lapua before I loaded them. Measurements were .293OD and .263ID before loading.

I measured my hand loaded lapua brass and some federal 120gn factory loads (WAY too high of pressure, flattened primer / blown out primer) post firing: Both necks measure .296 outside and .264 inside.

 

[Assault116]

I read somewhere that someone suggested the brass neck was too thick....

I sized the brass with a .292 bushing, then the expanded ball opened up the necks on the lapua before I loaded them. Measurements were .293OD and .263ID before loading.

I measured my hand loaded lapua brass and some federal 120gn factory loads (WAY too high of pressure, flattened primer / blown out primer) post firing: Both necks measure .296 outside and .264 inside.

Sounds like the necks are a touch thick for your chamber, You should be looking for at least .002 bigger than cal ID on the fired case necks. That can cause some pretty significant pressure spikes. Might try turning the necks down a bit (.002-.004) and give the same accuracy load a test again and see what it looks like. I'd bet the pressure signs go away or are at least reduced.

 

[Thumper580]

Give Winchester brass a try.....not quite as thick as the Lapua......

 

[Greg Langelius *]

My experience with 139 Scenars was that they tended to generate unexpectedly higher pressures at loads I would have expected to be relatively mild. I switched to 140SMK and A-Max, and 142SMK. I've only ever used Rem and (.243/7-08) Win brass for the .260 and all my chambers are SAAMI (except my .30BR, and it uses a wider .344" 'no turn' neck).

One other crucial variable is the length of the case necks. If they are excessively long, overpressure can come at you real fast and hard, and apparently right out of nowhere. If everything about the loaded round mic's OK, and you still have a tendency toward harder than expected bolt closure, you could easily be experiencing overlong necks

Greg

 

[918v]

Looks like you have a bench rest chamber. Have you measured the shoulder expansion and base expansion as well? You might not be able to use book data which is designed for SAAMI spec chambers.

 

[milo 2.0]

Looks like you have a bench rest chamber. Have you measured the shoulder expansion and base expansion as well? You might not be able to use book data which is designed for SAAMI spec chambers.

I'd bet money that no one at GAP would chamber an ar type rifle with a benchrest chamber.

 

[918v]

So it is normal to chamber a rifle for .001" of expansion at the neck? I'm used to .010".

 

[longrangefool]

My experience with 139 Scenars was that they tended to generate unexpectedly higher pressures at loads I would have expected to be relatively mild. I switched to 140SMK and A-Max, and 142SMK. I've only ever used Rem and (.243/7-08) Win brass for the .260 and all my chambers are SAAMI (except my .30BR, and it uses a wider .344" 'no turn' neck).

One other crucial variable is the length of the case necks. If they are excessively long, overpressure can come at you real fast and hard, and apparently right out of nowhere. If everything about the loaded round mic's OK, and you still have a tendency toward harder than expected bolt closure, you could easily be experiencing overlong necks

Greg

Length is within spec. Pretty much exactly 1.775 to neck/shoulder, overall trim length of 2.025. Also, I tried these same load with 130 vlds. That should have been even milder at that weight class, but same brass damage. So I dont think its a 139 Scenar issue.

 

[longrangefool]

Looks like you have a bench rest chamber. Have you measured the shoulder expansion and base expansion as well? You might not be able to use book data which is designed for SAAMI spec chambers.

Base: .4685 -> .4700

Shoulder: .4500 -> .4550

 

[FLIGHT762]

I don't know if you have read up on reloading for gas guns. They will not take a load that a bolt gun will. I've owned a first production Knight's SR-25 since 1994 and have been reloading for it since I purchased it. It will blow primers on certain factory loaded ammo. When I first got the rifle, it would blow primers with Federal 168 GMM ammo. I also own a recently built (late last year) GAP 10. Both rifles are in 308 Win.

If I use Federal brass (thick) with loads that will shoot fine with thinner brass (Hornady / Winchester) I will get pressure signs, stretched primer pockets and galled (ejector swipes) and in some cases, blown primers.

I installed a Tubb's Carrier Weight System (CWS) and it helps a bit on the ejector swipes.

Tubb Precision AR-15 Carrier Weight System (CWS?) - YouTube

You can buy the CWS here or Midway has them for a few dollars more.
Tubb Precision AR-15 Carrier Weight System



If you could find a few pieces of Winchester brass (reformed 243 or 7-08), I would try the 41.1 load and see what you get. I believe you will find the Lapua brass is thicker / heavier and will show pressure before thinner brass does. I also don't believe GAP would chamber an autoloader with a benchrest chamber. I know they do make the firing pin hole in the bolt a bit smaller on the 260 guns than they do on the 308 guns. A 41.1 grain load just may be the upper limit with your brass. Like I said, I had good luck with the Tubb's CWS (using the heavier Tungsten insert) it might be worth a try for $60.00.

One thing with the CWS. When I use it in my GAP 10, when the insert is put into the back of the carrier, it sticks out a bit and will not let the receiver close when you have it hinged open. All you do is pop open both lower receiver pins and assemble the upper and lower together in a straight line and the end of the CWS will push into the buffer. You then snap both receiver pins in place. When you want to disassemble the rifle, you pop both receiver pins and separate the upper from the lower. Tubbs explains this in the video

 

[milo 2.0]

So it is normal to chamber a rifle for .001" of expansion at the neck? I'm used to .010".

The only way for the op to find the diameter of his chamber neck may be to call GAP and get reamer dimensions. Brass doesn't have to grow to max when fired? I load for 2 7mm saums, loaded rounds measure .315", fired brass measures .317-.3175. My chamber neck is reamed at .319, and a bullet will not slip into a fired case by hand.

At .010", if you're using a Redding bushing die you've got yourself a donut factory! If nothing else you're over working the necks hard, if the brass is indeed growing .010", which I bet it's not.

 

[918v]

.010" is common. Have you ever measured fireformed necks and compared them to unfired necks? Some rifles expand the necks by .015"!

 

[MitchAlsup]

.010" is common. Have you ever measured fireformed necks and compared them to unfired necks? Some rifles expand the necks by .015"!

Ain't it the truth, my necks grow by 0.013 and sometimes 0.014. (Factory Remmy.)

 

[longrangefool]

Well, nobody local has any brass in stock, even sinclair 7mm08 is out of stock. Found some hornady 7mm08 loaded rounds that Im going to dump a couple and try the brass. I have a friend who has a 7mm08 on order so he can have them if it doesnt work out for me. I also ordered a neck turning kit from sinclair, so i can try that out also. Will post back results later today.

 

[longrangefool]

Well, doing better! I dumped 2 of the hornady rounds out and ran it through my redding die. Neck was so much thinner that it didnt even resize. Since I dont have smaller bushings, I ran it through my rcbs 260 fl sizer, and ended up with .287OD and .261ID. Loaded up a 139gn scenar with 41.1gn of h4350 and a cci 200 primer. One round no visible head contact, one round with very light visible swipe mark (I can only just feel it with my nail). Post firing size is .294OD, .264ID. So I guess I'll just wait for my neck turner and give the lapua another chance.

Can anyone measure me a winchester 7mm08 case neck? I can get one box of 50 hornady 7mm08 locally for 49.99 8| winchester sounds cheaper, but only if its thin enough...

 

[Assault116]

Winchester is thin enough and Cabela's has Win 243 brass in stock so it'd be an easy neck up. I'd give that a try if I were you. But you can accomplish the same thing by turning your Lapua necks a bit and I'd bet the pressure signs go away by doing that as well. Just depends on what brass you want to run in the long run. If you've got a good stock of the Lapua, I'd turn the necks and then just keep an eye out for doughnuts and re-turn them as necessary. YMMV.

 

[hessian]

Try a faster powder. Your picks are OK for a bolt gun , but maybe too slow for a gas gun. If it was a 308,I'd say 4895 or thereabouts. I know nothing about the 260. Chamber pressure needs to drop pretty quickly for the bolt to extract the case. Any rim deformations?

 

[milo 2.0]

Ain't it the truth, my necks grow by 0.013 and sometimes 0.014. (Factory Remmy.)

Wow, you and 918 may be correct, I've just never seen it in my chambers. I was kind of at odds with C. Dixon using a .295 neck chamber on a couple of my 6.5x47's, that was a .007 grow, and needed neck turning to get rid of donuts. Never again!

 

[longrangefool]

Try a faster powder. Your picks are OK for a bolt gun , but maybe too slow for a gas gun. If it was a 308,I'd say 4895 or thereabouts. I know nothing about the 260. Chamber pressure needs to drop pretty quickly for the bolt to extract the case. Any rim deformations?

Looking at my Lapua cases, there is a slight extractor ding on the edge of the rim and a noticeable "pull" mark on the inner rim. The Hornady brass doesn't show any extractor marks on the edges of the rim, but you can see (lighter) pull marks on the inner ring.

 

[longrangefool]

Try a faster powder. Your picks are OK for a bolt gun , but maybe too slow for a gas gun. If it was a 308,I'd say 4895 or thereabouts. I know nothing about the 260. Chamber pressure needs to drop pretty quickly for the bolt to extract the case. Any rim deformations?

I checked in my Lyman 49th ed and my Berger 1st ed load books, and of all the powders recommended, the next fastest powder after H4350 that I have is Reloder 15. Unfortunately, it will be next weekend before I can get back out to the range, but I'll report back with results.

 

[longrangefool]

Well, I got my neck turner the other day, turned a few lapua necks down .004 and fired a few rounds into a stump at my house.... Same result as the hornady brass. One round just a very light ejector mark and no mark on the other. I have yet to try the re15, maybe tomorrow at the range if weather cooperates. Thanks guys for pointing me the right way!

 

[Clark]

post firing: Both necks measure .296 outside and .264 inside.

SAAMI says chambers should be .299" at rear and .298" at the front, +.002".
I have one factory barrel that Douglas chambered and it is at the min; .299" and .298".
If a case came fired out of a chamber and measured .296", I would not be blaming the chamber.
But a .264" bullet is barely going to fit back in that case.
So the necks are too thick.

The same thing happens to me when I neck down .308 brass to .260, 10% of the time I get horrible pressure spikes.

And ejector swipes are not acceptable on brass to be reloaded.

 

[longrangefool]

Just for closure: rounds loaded with re-15 (of varying charge) were unable to cycle correctly in my rifle. They appear to short stroke so re15 must be too fast. I ordered the CWS and the rifle works great now with 139s and the neck turned lapua brass.