P Flados
04-29-2021, 06:44 PM
For several years now I have tried to get powder coated plain base and checkless (gas check boolits with no check installed) to work in the AR-15 platform.
I started with a 16.2" 357AR Max (similar to 350L)
Since then, I have worked a bunch with different bullets and barrels in 300 BO. I also built an upper with a 7 TCU barrel to play with.
From the beginning, accuracy has been less than I hoped for and even when I got a load that seems to shoot pretty well, I seem to get too many random unexplained flyers.
Along the way, I also noted a lot of lead deposits in the back of the bolt and lead buildup at the muzzle when there is a muzzle device.
Today at the range, I think I may have inadvertently added a major piece of evidence to support what was previously just a suspicion.
Last week, I was running two different (one SS, one 4140) 10.5" 1-8 300 BO barrels through some functional / feeding testing. As I expected, both were notably overgassed with my loads (156 gr boolit and WC 680 at Quickload predicted pressures of just over 50 ksi).
To correct the "overgassed" situation I made up some spacers to position the gas block more forward on the barrel. Earlier this week I ran with a 0.095" spacer which did not even cycle the action. After coming home I pulled the gas block and looked at the powder residue at the port. I confirmed that the 0.095" spacer had choked off most of the gas flow with an opening width of only ~0.015". Although the gun did not cycle, I had two loads that shot decent (under 3 MOA at 20 yards). These loads were with a custom BO specific 156 gr plain base NLG with a profile similar to a Lee C312-155-2R.
Today, I shot the gun with an 0.065" spacer and got good function. I got positive last round lock back with a light load (150 gr boolit at a QL predicted 34 ksi). Then I shot some more of the loads that should have shot good based on the results earlier. They shot terrible (> 10 MOA).
So this is now coming together to potentially explain some of what I have been struggling with. I am thinking that my plain base and checkless boolits are subject to gas cutting damage as the base just goes past the gas port.
When this damage is smaller and more uniform, accuracy can be reasonable. I have a lower pressure load (under 40 ksi) in my 16" 7 TCU that shot better (1.5 MOA) than any high pressure load. The 16" barrel also lowers the pressure as the boolit exits the muzzle such that base defects are less significant.
On the other hand, this week I was running a 10.5" barrel with loads at over 50 ksi. When I ran the load with negligible gas port flow, it shot well. When I ran the load with normal gas port flow, I am sure I was getting more gas cutting, it seems the damage was more non-uniform, and the high gas pressure at the muzzle was pushing the base off track to a larger degree.
I have been a very strong advocate of trying powder coated plain base and checkless boolits based on my experience in other (not gas operated) guns where they seemed to work as good as a J Word. However, now I think the evidence is mounting that even with powder coat, gas checks really are needed in the AR-15 platform (and other high pressure gas operated semi auto guns).
Now that I have a mini lathe, fabbing up a gas check maker should be doable. Anyone have a link to a thread for one of the better designs for a DIY plain base gas check maker? I will want to make one each in 7mm, 30 cal & 35 cal.
I started with a 16.2" 357AR Max (similar to 350L)
Since then, I have worked a bunch with different bullets and barrels in 300 BO. I also built an upper with a 7 TCU barrel to play with.
From the beginning, accuracy has been less than I hoped for and even when I got a load that seems to shoot pretty well, I seem to get too many random unexplained flyers.
Along the way, I also noted a lot of lead deposits in the back of the bolt and lead buildup at the muzzle when there is a muzzle device.
Today at the range, I think I may have inadvertently added a major piece of evidence to support what was previously just a suspicion.
Last week, I was running two different (one SS, one 4140) 10.5" 1-8 300 BO barrels through some functional / feeding testing. As I expected, both were notably overgassed with my loads (156 gr boolit and WC 680 at Quickload predicted pressures of just over 50 ksi).
To correct the "overgassed" situation I made up some spacers to position the gas block more forward on the barrel. Earlier this week I ran with a 0.095" spacer which did not even cycle the action. After coming home I pulled the gas block and looked at the powder residue at the port. I confirmed that the 0.095" spacer had choked off most of the gas flow with an opening width of only ~0.015". Although the gun did not cycle, I had two loads that shot decent (under 3 MOA at 20 yards). These loads were with a custom BO specific 156 gr plain base NLG with a profile similar to a Lee C312-155-2R.
Today, I shot the gun with an 0.065" spacer and got good function. I got positive last round lock back with a light load (150 gr boolit at a QL predicted 34 ksi). Then I shot some more of the loads that should have shot good based on the results earlier. They shot terrible (> 10 MOA).
So this is now coming together to potentially explain some of what I have been struggling with. I am thinking that my plain base and checkless boolits are subject to gas cutting damage as the base just goes past the gas port.
When this damage is smaller and more uniform, accuracy can be reasonable. I have a lower pressure load (under 40 ksi) in my 16" 7 TCU that shot better (1.5 MOA) than any high pressure load. The 16" barrel also lowers the pressure as the boolit exits the muzzle such that base defects are less significant.
On the other hand, this week I was running a 10.5" barrel with loads at over 50 ksi. When I ran the load with negligible gas port flow, it shot well. When I ran the load with normal gas port flow, I am sure I was getting more gas cutting, it seems the damage was more non-uniform, and the high gas pressure at the muzzle was pushing the base off track to a larger degree.
I have been a very strong advocate of trying powder coated plain base and checkless boolits based on my experience in other (not gas operated) guns where they seemed to work as good as a J Word. However, now I think the evidence is mounting that even with powder coat, gas checks really are needed in the AR-15 platform (and other high pressure gas operated semi auto guns).
Now that I have a mini lathe, fabbing up a gas check maker should be doable. Anyone have a link to a thread for one of the better designs for a DIY plain base gas check maker? I will want to make one each in 7mm, 30 cal & 35 cal.