308 Gas gun questions

sawgunner2001

Sergeant
Full Member
Minuteman
Dec 25, 2006
534
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Minneapolis, MN
First off, I'm familiar with loading for bolt guns and recognizing their pressure signs and knowing when to call it quits. Today I took out a "new to me" GAP built AR10 (20" barrel) with Varget and 178gr Amax bullets seated at 2.805" in Win brass. First load fired was 43.0gr at 2511fps. Primers starting to loose their rounded edge right out the gate. Next I tried 43.5gr which chronoed at 2546fps but showed ejector swipe. I stopped there despite having additional loads of 44, 44.5, and 45gr. Correct me if I'm wrong, but ejector swipe is just as much a sign of pressure in gas guns as bolt guns? I obviously can't feel sticky bolt lift, so what other pressure signs are there to look for? Are there any suggestions to get a little higher velocity? I'm not far from sea level and would like to be a little closer to 2600.
 
Re: 308 Gas gun questions

Ejector swipe is not a reliable indicator of pressure in AR10's. ESPECIALLY when using Varget. This has been talked about before here, so some searching will turn up other useful info.

http://www.snipershide.com/forum/ubbthre...927#Post2938927

When using Varget I get ejector swipes even at start loads sometimes, and at random intervals when doing load workups, gas port pressure is higher with slower powders like Varget, causing swipes. Doesnt mean the ammo is overpressure, its just the way that rifle functions.

Flat (really flat) and cratered primers are what I use to judge max loads in my 308 AR, and I haven't broken anything yet in spite of running Varget exclusively. Your 43.0 and 43.5 grain loads shouldn't be overpressure, but I would venture a guess that the 45.0 probably will prove too hot. Just a guess though, time will tell. With a gas gun I break loads approaching max down into smaller work up imcrements, like after 44.0 grains I would start increasing powder 0.2 grains at a time, just my method.
 
Re: 308 Gas gun questions

Without adjusting the gas port, Varget will be a bit slow for gas guns.

Don't add powder to a 20" .308 to try to get that last bit of velocity; 2600 fps from a 20" barrel with 180grain bullets may not be a realistic goal. Besides, it isn't necessary - find an accuracy node and stick to it.

It is hard to find reliable signs of pressure in gas guns without comparing what is to what went before in the same rifle, but watch the primers.

.308 gas guns are not like .223 AR's: You can't add powder until the primers blow and then back off a bit. Doing that with the big gun is a recipe for trouble.
 
Re: 308 Gas gun questions

That being the case, does repeated ejector swipes reduce case life significantly? Secondly, would it be wise to try H4895?

Honestly, I didn't look at the primers on the 43.5gr load. I saw the ejector swipe and called it quits. I'll unload the car and take a look at the primers...see what they say.

Thanks
 
Re: 308 Gas gun questions

I have one of the new GAP-10's with a 20" barrel

I am using Winchester cases with varget, CCI BR2 primers, and Hornady Match 178 gr BTHP. with my old lot of varget I ended up using 44.5 gr. this load was worked up in 90+ degree weather. I got a new batch of varget that is slower and ended bumping it up to 45 gr. no pressure signs and on the 5th reload with this brass. this load is getting 2620fps and is very accurate.
this is a stiff load so make sure to work up slowly.
 
Re: 308 Gas gun questions

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Graham</div><div class="ubbcode-body">Without adjusting the gas port, Varget will be a bit slow for gas guns.</div></div>

{Finishing the sentance}

This means that the pressre in the barrel will still be quite high when the bullet uncorks the gas port and the action will cycle violently.
 
Re: 308 Gas gun questions

Going to one of the 4895 powders is probably the quickest and right answer. for whatever reason 4895 works really well in .308 gas guns.
You won't be losing much if anything by using the faster powder and it should get you out shooting without much work. 41.5gr is a good place to start.
 
Re: 308 Gas gun questions

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawgunner2001</div><div class="ubbcode-body">That being the case, does repeated ejector swipes reduce case life significantly? Secondly, would it be wise to try H4895?

Honestly, I didn't look at the primers on the 43.5gr load. I saw the ejector swipe and called it quits. I'll unload the car and take a look at the primers...see what they say.

Thanks </div></div>

I am using IMR 4895 (not H4895, but somewhat similar), in my 308 LMT MWSE, and am currently testing 40.0, 40.5, 41.0, 41.5, and 42.0 grains, using Lapua cases, Winchester LR primers, and Sierra 175g Matchking bullets.

ALL the loadings between 40.0 and 42.0 work fine, with no pressure signs on the primers. It is hard for me to assess accuracy accurately, as I am still elarning to shoot this rifle, so cannot yet fire groups consistent enough to positively pick the "winner" in the ladder test,but ALL the loads are shooting betetr than I can. The ones that seem best are interestingly enough both the 42.0 and the 40.0 - the heaviest and the lightest in the test.

I also ran 3 range sessions in a row, the first with Federal Gold medal and the last 2 with the handloads, without cleaning the rifle. About 120 rounds total. During the 3rd session, the groups opened up indicating the rifle wnated cleaning, and sure enough, when I opened it up to clean it, both the barrel and the bolt parts were fairly dirty. However, the residue left by the IMR4895 handloads cleaned up VERY easily, using only M-Pro7 and NO copepr solvent required.

The IMR4895 weighs well on my Chargemaster. I have no idea how well or poorly it might meter from a powder measure.

I went with the IMR4895 because Zediker recommends it as a "can't go wrong" loading in his book on handloading for competition.

Jim G
 
Re: 308 Gas gun questions

IMR 4895 is a classic powder for the M1A, a gun that can be very sensitive to excess port pressures. In that gun, the higher port pressures will bend stuff (op rods). In an AR gun, it will just slam things around harder and eventually cause more wear. I know some people that use Varget in the M1A, but I opt for what I know is safe for the gun. I use Varget in my bolt guns.

I've been thinking of trying the H4895 in the M1A and my Larue 7.62 because of its insensitivity to ambient temps. But first I have to use up a bunch of surplus military IMR4895 I got from Wideners a while back.
 
Re: 308 Gas gun questions

I had the same results as the OP, 43 gr varget with a 175 SMk is fine and very accurate in my AR10 but 44 gr and up (bolt loads) blows all the primers out of the cases.
be carefull and start low with varget or use another powder.
 
Re: 308 Gas gun questions

I've been using ramshot TAC . 43g in hornady brass for 175 SMK. I have a 22.5" krieger and it produces 2700fps with my suppressor on. I could load more powder, but have to take the can off; otherwise ill get flat primers intermittently @ 43.1 and consistently @ 43.2. The addition of a adjustable gas block has slowed the cyclic rate and dropped felt recoil significantly..

I shot 175's over 44g of varget the other day without flat primers. Don't know my fps though