Gunsmithing 308AR POI Shift

[hdbiker1]

I've assembled a 308AR using Mega receivers, CBI 20" hb and f/a bcg. It took me awhile to get a load that shoots well. My problem is poi shift from cold to very hot. What's unusual to me is that cold, the poi is >3moa vertical and patterns instead of groups. As it gets hot, the poi drops down and groups are very uniform and about .6moa. I've seen the opposite with most barrels, where as they get hot and the poi shifts up and to the right or left and the groups open up. Gunsmith even suggested bedding the barrel extension, which I did, with no change. Barrel chamber looks slightly off (but shoots good when hot) and currently have approx 500 rnds down it. Anyone have any insight?

Btw, the load I came up with is 41.8gr IMR 4064, Rem 9 1/2 primer, LCLR case and 168 SMK.

 

[chipsfan]

I'll take a shot at this- but may be off.

Typically any shift in POI is attributed to some sort of stress, contact point, uneven surfaces, you name it. That said, is the barrel nut seated against the upper? You can lap that front surface (I have the 223 tool- which wouldn't help you on a 308).

 

[hdbiker1]

Good shot chipsfan. After spending all day trimming a bunch of brass, I decided to take down the MaTen. After barrel removal, I tried something different. Heated the upper receiver enough so that the barrel slid in without having to beat it in, then I noticed a shiny spot inside the Troy TRX. Apparently when I switched from a mid-gas to a rifle barrel I forgot to relocate the bi-pod rail backing plate and the gas block was just sitting on an edge. Anyway, my theory is the rail plate was applying pressure to the barrel and as the barrel got warm, the length grew ever so slightly that the gas block slowly slid off the plate. When cooled, the barrel shrank enough to climb back on the ledge to put pressure back on the barrel. That's my story and I'm sticking to it. Boy do I feel like an idiot. Ok guys, kick me now.

 

[chipsfan]

Good shot chipsfan. After spending all day trimming a bunch of brass, I decided to take down the MaTen. After barrel removal, I tried something different. Heated the upper receiver enough so that the barrel slid in without having to beat it in, then I noticed a shiny spot inside the Troy TRX. Apparently when I switched from a mid-gas to a rifle barrel I forgot to relocate the bi-pod rail backing plate and the gas block was just sitting on an edge. Anyway, my theory is the rail plate was applying pressure to the barrel and as the barrel got warm, the length grew ever so slightly that the gas block slowly slid off the plate. When cooled, the barrel shrank enough to climb back on the ledge to put pressure back on the barrel. That's my story and I'm sticking to it. Boy do I feel like an idiot. Ok guys, kick me now.

That'll do it. Glad it was an easy find

 

[hdbiker1]

It's amazing, sometimes the simplest or stupidest things can really make you pull you hair out.