why lighter bullets in 6.5 gas guns??

[sch2046]

I just build a 6.5 creedmore on the DPMS platform. Everyone seems to be shooting lighter bullets in their gas guns. Is there a reason? I shoot 136 up to 140's in my 260 bolt gun.
Thanks for the clarity.

 

[BLDTYLRY]

Probably due to mag length limitations.

 

[Kiba]

In my experience (and advice given by both JP & GAP) the 140gr bullets are hard on the brass & primers in a 6.5 gas gun when used with the "usual" powders / burn rates (i.e. H4350).

With my GAP-10 in 6.5CM (and to a lesser extent my LRP-07 in 6.5CM) I had primer issues when using 140gr Amax & H4350 even with moderate charges of H4350; the GAP10 would pierce primers and the LRP-07 showed some cratering. At the suggestion of both JP & GAP I tried lighter bullets, specifically 130 VLDs and 123 Scenars. Primer condition improved substantially with the lighter bullets and with the velocities I was able to push the 130s & 123s to (without seeing any pressure signs) they're better in drop than the safest load/velocity I could run with the 140 Amax and about identical for windage.

Besides the primer issues, both rifles also had occasional extraction/ejection issues with 140s (even with moderate powder charges) and since switching to the 130/123 loads those have gone away-- probably because of the pressure curve & timing with H4350 and the 140s. Both rifles are much happier and more reliable sending the 130s at around 2800 and the 123s at about 2920 compared to 140s in the 2700 range-- performance is great and the lighter bullets don't show the pressure signs than the 140s did.

For now I'll keep the 140's for my 6.5CM bolt rifles and feed the gassers 123s or 130s.

 

[Here2Learn]

Pressure over speed

 

[sch2046]

Pierced primers is what I am battling now. I will try the 123 scenars. Thanks for the heads up. Much appreciated.

 

[Kiba]

Pierced primers is what I am battling now. I will try the 123 scenars. Thanks for the heads up. Much appreciated.

My GAP10 went back to GAP for the piercing primer issue. GAP's fix was to swap the DPMS bolt & barrel extension for an Armalite bolt & barrel extension. The DPMS bolt has a .080" firing pin and the Armalite has a smaller .068" firing pin, the smaller firing pin diameter was a HUGE improvement in primer condition. Switching to the 123s or 130s made things even better and took care of the occasional extraction/ejection issues.

The .080 firing pin isn't an issue for 308 (and is helpful for igniting tough mil spec primers) but for 6.5CM with higher pressures and a different powder with a longer pressure curve the larger diameter firing pin can cause problems such as cratering & pierced primers.

My JP runs a .080 firing pin and switching to 123/130s made all the difference in primer condition.

 

[sch2046]

I just put in a heavy buffer, buffer to see if it helps with the primers, I will also test some lighter bullets. More to come......

 

[pkt1199]

Thanks for the info in the thread. I was planning on loading 140s for a GAP 10 but now having second thoughts!

 

[Unknown]

I have never liked having to keep two kinds of ammo for bolt/gas guns, but when looking for the best performance, vs "acceptable" performance, that is what seems to be the norm.

I think this is the same reason that USGI 175 grain match ammo shoots pretty well in many guns, but when people want to squeeze the very best in performance out of an individual rifle, they have to keep more than one load on hand.

I finally quit trying to do two different loads when I have gas/bolt guns in the same cartridge, and I accept that my performance won't be quite as good as it possibly can be. When I need that "untimate" performance, I'll switch to another rifle that is the only one in that cartridge, and it has had loads tailored to it's needs.

But that is just me. Many other people don't mind having different loads for gas guns and bolt guns around.