GAP-10 .260 blowing primers

[verdugo60]

I am reloading for my uncle's older style DPMS Gap-10 260 that has a 26" barrel. Shoots great but at the range last week we got pressure signs and even a few blown primers with below minimum load (temp was mid 50's). Running 142 grain SMK, 41.3 grains H4350 in new Rem 260 brass loaded at COAL 2.78". With 41.3 grains, velocity is 2710 FPS. Do I just have a hot batch of H4350?

With different lot of H4350 in other buddy's .260 41.7 grains was getting exact same velocity in his bolt gun, same bullet, Lapua brass being the only difference. That's half a grain difference and getting same velocities in a semi-auto. I'm thinking powder lot.

Hodgdon starting min load is 41.5 grains, any ideas? Should I be sending this to GAP to check for headspace issues, etc?

 

[Assault116]

I'd back off the load. I'm loading 42gr in a bolt gun and it's right at the top end of acceptable pressure signs and accuracy.

Typically Gas guns simply can't comfortably run the pressures a bolt gun can before signs start showing up on the brass.

However a headspace check isn't a bad idea. Also may want to do some measuring on fired brass vs what you're sizing it to and make sure you aren't inducing some headspace that's not needed.

 

[Kiba]

Had lots of cratered and pierced primers with my new 6.5CM GAP10 last year trying to launch 140s even with very moderate loads of H4350 that I knew were very moderate based on loads with the same powder lot out of my bolt gun. After some messages and phone calls I shipped it back to GAP. Their solution was to swap the DPMS BCG which uses a .080" firing pin to an Armalite BCG which uses an .068" firing pin. With the Armalite BCG and .068" firing pin all primer issues went away.

George mentioned that going forward they were switching to the Armalite BCG in all non-308 GAP-10s to help prevent primer issues like I was experiencing.

If the cases are sized properly, it's a safe load in regards to powder, and you're still having primer issues you might want to think about a BCG swap. However, 2710fps with a 142SMK sounds a little warm for a gas gun even with a 26" barrel.

George and the gang also strongly suggested leaving the 140gr bullets for the bolt guns and running 123s or 130s in the gas guns as you can get better performance with a load that shows far less pressure signs with the 123s and 130s. The 140gr bullets seem to exacerbate pressure signs in the gas guns. I now run 123 Scenars or 130 VLDs in my 6.5CM gassers and they're flatter to 1000 than the safest 140gr loads I could work up.

With H4350 and that 26" barrel you should be able to get 2930fps+ easy out of a 123 Scenar without any pressure signs and probably 2825fps+ or so out of a 130VLD. I get 2895fps with H4350 and 123 Scenars from my 22" 6.5CM JP LRP07 with no pressure signs. Very accurate and flatter to 1000y than a 140 Amax load that showed some moderate primer flow.

If you've already pierced a few primers look at the firing pin nose carefully, they're rather easily distorted by pierced primers. If anything looks odd change it before it sticks and slam fires an entire mag through the rifle... been there, done that.

 

[BLDTYLRY]

^^^^ +1

 

[verdugo60]

Thanks guys, very helpful.

Between the 123 Scenars, the Berger 130 VLD's and the 120 AMAX (or another bullet in that weight range with decent BC) whats going to be the easiest to load for .260 Rem in mag length? I loved the Sierra's because they are not sensitive to seating depth.

Which is the cheapest and easiest to find right now? I see Scenars for about .40 cents shipped, VLD's for .43 and have seen the 120 AMAX for about .27 cents per.

 

[shootist2004]

why make the switch just for non 308 guns? if the Armalite BCG is better than why not switch for 308's too?

 

[Kiba]

Thanks guys, very helpful.

Between the 123 Scenars, the Berger 130 VLD's and the 120 AMAX (or another bullet in that weight range with decent BC) whats going to be the easiest to load for .260 Rem in mag length? I loved the Sierra's because they are not sensitive to seating depth.

Which is the cheapest and easiest to find right now? I see Scenars for about .40 cents shipped, VLD's for .43 and have seen the 120 AMAX for about .27 cents per.

I really like the 123 Scenars. They grouped better than the 123 Amax in my rifle and have a better BC (.545 vs .510.) They aren't really depth sensitive either. The 130 VLD shoots very well in my rifle too, a little better than the 123 Scenar, but are a little more seating depth sensitive but not horribly so.

At the time I was testing I could find 123 Scenars in quantity but everyone was out of 130 VLDs so I went with the Scenars. I think I paid $300 delivered for a 1000 box of the Scenars.

The upcoming Berger 130 Hybrids should be great, supposed to have a .292 G7 / .570 G1 BC.

why make the switch just for non 308 guns? if the Armalite BCG is better than why not switch for 308's too?

George told me that the .080" firing pin is sometimes needed for guys shooting surplus or mil-spec 308 with tougher NATO primers as the larger firing pin promotes more reliable ignition with the NATO primers. Also, the larger diameter isn't an issue with the typical pressure curve of 308. However, that .080 firing pin becomes a liability with the pressure curve of the typical H4350 in .260 and 6.5 CM; the larger diameter of unsupported primer can more easily flow back into the firing pin hole and lead to cratering and pierced primers.

I will say this; with the .080 firing pin in the 6.5CM GAP-10 I was having badly cratered and pierced primers even down at a mild 39.5-40gr of H4350 behind a 140. The edges of the primer were perfect and radiused but the firing pin impact was horrible looking. After I got the rifle back with the Armalite BCG I loaded a couple of rounds with 43gr of H4350 behind a 140 to see what would happen... the primer was totally flat and the brass had a loose primer pocket-- but the firing pin impact on the primer looked perfect. That same load would have been a guaranteed pierced primer with the .080" firing pin.

 

[shootist2004]

I have one of the new gap-10s in 6 creed. I had a problem with primers falling out which cleared up after I removed the can......

 

[nuclear_shooter]

Here's a question based on ignorance... why is the pressure so much higher in a gasser than a bolt 260?

 

[verdugo60]

As I understand it the pressure is not higher, it's the ability of the "system" to tolerate the pressure that is the difference.

With a bolt gun, it locks up a like a vault, no moving parts until you work the bolt. This is why you can push max loads a bit more with a bolt gun usually.

A semi-auto has a gas system dependent on pressure, lots more moving parts without you doing anything. Too much pressure and you can break bolts, and worse.

The "fat" firing pin issue is also a problem in Accuracy International bolt guns, they were engineered to reliably ignite hard milspec primers in 308's. 6.5's use a different powder, different pressure curve so that hard firing pin smacking primers causes problems in them that it doesn't in 308's.

Again, this is not unique to GAP's or semi-auto's, its the switch from 308 to 6.5. Zak Smith discussed this in his AI AW years ago when working up loads for his beloved 260.

Here's a question based on ignorance... why is the pressure so much higher in a gasser than a bolt 260?

 

[verdugo60]

Looks like we will be switching to the 123 Scenars for this gun.

 

[bigedp51]

Try switching to a faster burning powder and lower your gas port pressure, if you have any 4895 or Varget give that a try as an experiment.

The majority of AR type rifles are over gassed to insure they cycle lower pressure ammunition. When you use a slower burning powder the gas port pressure increases and the bolt can start moving to the rear before the case releases its grip from the chamber walls. On top of this there is still pressure in the barrel which is forcing the primer out of the primer pocket.

There is a reason why the military crimps its primers and uses powders in a given burning rate. Another solution would be a adjustable gas block to control port pressure.

The first thing I would try is a faster burning powder to see if port pressure is causing the problem.

Remember if you use 4350 in a M1 Garand you will bend the op rod and you can have similar high port pressure problems in AR type gas systems.

Question, are you piercing primers or are the primers falling out of the primer pocket?

If your head clearance is excessive meaning the "air space" between the rear of the case and the bolt face, the firing pin can punch the center out of the primer as it is forced back over the firing pin.

Below if the head clearance is excessive (shoulder bump) the firing pin will act like a cookie cutter and punch the center out of the primer as the primer is forced back over the firing pin.

Below an animated image of the primer moving back to contact the bolt face when the cartridge is fired. In a gas operated system with high port pressure and slow burning powders the gas port pressure can force the primer to fall out of the primer pocket. It is also possible for the fired case to be longer than the chamber because the bolt is moving to the rear and pressure is still in the barrel. Meaning you can get false cartridge headspace readings when setting your dies for the correct shoulder bump.

Again switch to a faster powder and double check your resizing shoulder bump. I have a set of dies that will push the shoulder back .009 shorter than the GO gauge and end up with .012 head clearance.

 

[Greg Langelius *]

Other choices are the 120gr Nosler Ballistic Tip, and the 120gr Sierra MatchKing.

 

[verdugo60]

Ya, not getting pierced primers, getting primers falling out the primer pocket, plus bad ejector marks.

We have 1000 123 Scenars coming, will try those with H4350 and update. May try Varget too as it seems ok for these lighter 260 bullets in a gas gun. From what I can tell the H4350 will still get us more velocity than Varget at same pressure levels with these.

Not shooting a can, non-adjustable gas block. Thanks for the advice on here. All this has been new brass so far so the headspace issue is not on my end. I do have a hornady headspace gauge that I'll be using for the once fired brass. Gotta figure out which pieces to chuck of this once fired stuff. Obviously the ones that blew primers will probably be loose and will be getting trashed.

 

[FCS]

Easy fix is to add adjustable gas block, heavy up buffer and increase buffer spring constant.

26" barrel you have more dwell time then is "ideal" if running a rifle length gas system.

Load isn't hot, bolt is unlocking too soon. Keep bolt locked up longer, your primer issue goes away.

 

[verdugo60]

Well needless to say I think this GAP 10 likes the lapua scenars. This is 6 shots in 1.1 inch...at 300 yards! And from a ladder test, no charge is the same, .2 grain increments. Think I will settle with 42.5 h4350, which I'm hoping gets me near 2850 fps. Saw pressure at 43 even so .5 under should keep me ok in 90 degrees

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