Re: 6mmAR Turbo 40D+ more
6mmART40 revisited.
The upper has about 1900 rounds through it now. It's been an up and down experience for me. Mostly a thumbs up.
The AA2520 powder ended up being a very temperature unstable powder. Nearly 150fps difference between summer and winter. I quit using it even though it meters better than any powder I've used.
At about the 500 round mark the barrel started coming loose from the barrel extension. It took me almost 500 rounds/3-4 months to determine what the problem was do to the barrel unscrewing "very slowly" and accuracy-POA shifts getting gradually worse. Robert Whitley fixed the problem for me.
When I got the upper back I worked up a new load. 30.2 grains of H-4895/105Amax/2.245OACL shot the best producing the lowest ES. Velocity was in the 2900 fps range and was a hot load. Every once in a while I'd get a jam with a failure to feed. The round went into the chamber most of the way but the BCG would stop at about 80% of it's travel. It was a easy jam to deal with because all I'd have to do was pull back the bolt till the hammer cocked and let it go.
Another year went by and I decided to try and fix the feeding issue by altering the magazines. I took .030 off the front of the lips where the cartridge is released, widened the lips .010 and polished the edges and releaved the area of the mag in the forward part that guides the shoulder of the case into the feed ramp. The mods helped reliability but did not eliminate them.
Recently I fireformed some new Lapua brass. After fireforming I discovered the previous load of 30.2 grains H-4895/105 Amax in conjunction with a new 8lb keg of H-4895 would not work, way to hot???
So I worked up a new load. 28.3 grains of H-4895/105Amax/2.248" shot best overall. The barrel is starting to show it's age as I'm only able to get 3/4" groups at 100Y now. The velocity has decreased to 2740 fps. The good thing is I think I stumbled onto why the rifle was jamming once in a while. I'm guessing it was because of the higher pressures of the last load (maybe the case wall grabbing the chamber too hard or gas system timing???). This new load hasn't malfunctioned at all in 300 rounds.
Good news! I finally won my first long range match with the upper using the new load last weekend after all these years, YAY!
I had gotten three 2cnd place finishes and also some top 5's before with this upper with it giving a few hiccups but I'm much happier now that the upper is working reliably.
For the next barrel I'm thinking I'll order a reamer made to my cases and dies with zero freebore so the bullets won't have such a large jump as it does now. Also I want to have a muzzle brake put on along with getting the barrel Melonited.
