AR-15 Problems with Plain Base & Checkless

[P Flados]

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.

[popper]

You did find one of the problems, there are others. I run lower pressure loads PB but all HV stuff is GCd. It will also plug up the gas key. For some reason a harder alloy with Cu alleviates the problem some. Powder burn rate seems to make a difference also.

[mehavey]

I absolutely subscribe to the finding that high[er]-pressure PB bullets get their trailing edge nicked/cut/scooped/gouged on gas-ported barrels.
That PC'ing didn't seem to alleviate the problem is a new data point to add to the pile.
< long sigh.... >

It will also plug up the gas key.

We hear a lot of that, but I've never actually seen/experienced it (and I shoot a lot of cast in gas guns)
Can anyone actually point to actual/personal/empirical data?

[Burnt Fingers]

When I got my first 300 BO upper I had all kinds of problems with cast boolits. The rings on the bolt were welded together with lead.

I never got that barrel to shoot cast at all.

My next upper, a 10.5" shot much better with cast but still had flyers. With gas checked boolits the problem went away. Same for my 350 Legelnd upper.

I will also note that my RAR in 300 Blackout shot all my cast boolit loads quite well. It's the gas port and gas cutting of the boolit base.

Next up I'll test my 7.62x39 upper. But it's going to be gas checked boolits all the way.

[P Flados]

So I downloaded the IDZ plans for a gas check maker from

https://castboolits.gunloads.com/sho...gas-checkmaker

I made adjustments as needed to accommodate my material and to make Drink Can Gas Checks (DCGCs).

I put them on top of already powder coated bullets and assembled a couple of loads.

My first trip to the range got terrible results.

I made some adjustments and one load (at a lower power level) worked pretty good. Accuracy was about as good as the gun shoots with jacketed, feeding was great and the cases edjected @ ~ 4:30 (near perfect).

At least now I have one decent load and reason to hope that I can work up something even better. For the moment I am guessing that my first DCGC loads may have been too hot for the thin drink can material on top of the powder coat. I may try checking and then doing the shake & bake and see how that works. If needed I can make a new forming punch to allow using thicker material.

[P Flados]

Well, I just finished running a test that I had been wanted to do since just before I started this thread.

I made a new gas check maker to use with aluminum flashing and create "standard" thickness checks.

I bought a Lee CTL312-160-2R mold that lets me use the checks described above (I found a better than expected price for this one at Midway compare to a higher than expected price for a similar mold at Titan). I found that I can cast, coat and then install checks with only one pass through the sizing die for the whole operation. I end up with a sequence that gives me a securely crimped in place check (hard to remove with a finger nail) installed after the coating. To me, uncoated gas checks seem to have the least potential for accuracy affecting gas cutting damage as the base of the bullet goes by the gas port. Final weight was 165 gr.

Due to the new brace rules the ATF is considering, I put a 1-7 twist 7.5" 300 BO barrel back on an upper. This barrel had been removed from service as it shot ok with jacketed, but always shot terrible with my unchecked cast boolits.

As previously discussed in this thread, my first gas check boolits for AR-15 use were drink can gas checks. To get the checks to not be prone to falling off, I have to install them before coating. They shoot good in a 10.5" 300 BO barrel. Today I found that they shot terrible in the 7.5".

I then switched to my 165 gr Lee bullets with the uncoated standard thickness checks. The gun shot about as well as it ever has with jacketed bullets.

At this point, I have more confidence than ever that my theories first discussed in this thread are valid. Gas checks are a really good idea in gas operated semi-autos. Some guns need them more than others, but do not be surprised if unchecked bullets cause lead residue accumulation / fouling in the bolt and poor accuracy. Drink can gas check installed before coating seem less effective for maintaining decent accuracy, while standard dimension aluminum gas checks installed after coating did well in the most demanding application I tried.