Frankenstein AR-15 troubleshooting question

[JEVapa]

It's short stroking...either undergassed, or your limp wristing, it sounds.

Since it locks to the rear when you're in the prone with a solid backstop (you on the ground), and not when you are standing, I'm betting it's the latter. Working on your stance should help.

However, Get rid of this thing first:

Super 42 H3 buffer/spring.

Get a normal carbine buffer and regular spring. You're not running a heavy enough barrel for an H3 or a heavy duty spring. We don't run those in 308s.

You don't need an H or H2 either.

Adjustable gas blocks are good if you're switching between suppressed and unsuppressed...that's pretty much it.

Bet it runs like a champ after.

 

[JEVapa]

I forgot to add, if you built it yourself, a common mistake is to bump the gas block up against the shoulder on the seat. There should be a gap that's the thickness of the handguard retaining cap. All in spec gas blocks have this dimension. Some people use shims or cut up an old end cap. Some custom blocks do not have this dimension. Per Zipty's comment

It could also be as simple as a misaligned gas block.

Get rid of that heavy buffer and spring for sure though.

 

[undeg01]

I forgot to add, if you built it yourself, a common mistake is to bump the gas block up against the shoulder on the seat. There should be a gap that's the thickness of the handguard retaining cap. All in spec gas blocks have this dimension. Some people use shims or cut up an old end cap. Some custom blocks do not have this dimension. Per Zipty's comment


Get rid of that heavy buffer and spring for sure though.


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This is very much correct. I have a 300 blk that wouldn’t cycle, even though I had what appeared to be an appropriate gap. Ended up, I had to gap it nearly double of normal to properly align the gas port and gas block. If you pull your gas block, you should see a carbon ring around the port in the barrel. If that ring is not uniform around the hole, your gas block is misaligned.

 

[diggler1833]

I use calipers to determine how much gap is needed at the shoulder between barrel and block. It isn't a perfect solution, but darn if it won't get you where you need to be the first time around.

[Distance from shoulder to center of gas port in barrel - Distance from gas block edge to center of port in gas block = amount of gap needed]. Of course I've head that sliding a credit card in there usually works and is faster too.

However in this case, I'm going to bet my $5 of Taco Bell lunch money that it is the extra heavy buffer as has already been stated.

Best of luck.

 

[JEVapa]

I use calipers to determine how much gap is needed at the shoulder between barrel and block. It isn't a perfect solution, but darn if it won't get you where you need to be the first time around.

[Distance from shoulder to center of gas port in barrel - Distance from gas block edge to center of port in gas block = amount of gap needed]. Of course I've head that sliding a credit card in there usually works and is faster too.

However in this case, I'm going to bet my $5 of Taco Bell lunch money that it is the extra heavy buffer as has already been stated.

Best of luck.

I cut an end cap so it makes a U-shaped gage and slip it in there, tighten the GB, voila, done.

 

[sparkstable]

My personal feeling was essentially "limp wristing." I am absorbing some energy from the recoil that isn't happening when rested. I try to lean into it a bit so I can keep my weight forward into the gun rather than "shelfing" it on my chest. No issues with my MP15.

I have a stock milspec buffer and spring I can try. Perhaps that will allow the bolt to keep traveling even if I am taking some of the shock into my body.

I did pull the charging handle/bolt back with an empty mag. It locks fine.

When assembled my friend (the more experienced of us and who had done this before, just not an 11.5") used a bore scope to check gas block alignment.

Have also been told that the gas key may be too large allowing too much gas out into the chamber (this was from a rather grumpy smith at the range/store I was at). He took a quick look and said the key was large and I should toss it and get a new one.

I can try stock bcg from the mp15 as well to see if that makes a difference.

Thanks for the input everyone. It is nice to have some options to go down before getting to total reassembly on the gas system. Once I get a chance to test the buffer and bcg I will report back what I find.

 

[JEVapa]

Have also been told that the gas key may be too large allowing too much gas out into the chamber (this was from a rather grumpy smith at the range/store I was at). He took a quick look and said the key was large and I should toss it and get a new one.

That sounds like gibberish to me and a poor recommendation. Did the gas key come attached and staked to the carrier? That's to make you spend $$. Don't do that.

I'm 99% sure your gas system is fine.
I'm 99% sure your recoil system is too much.

 

[Foxfire5]

It sounds like you're under gassed. When I set up an adjustable gas block, (that you don't have) its adjusted down until the BCG won't lock open on the last round, then opened up 1/2 round.
It appears to me that your right at that breaking point where the gas is just enough to lock open when resting the rifle on the mag but not when firing offhand. There is a tiny bit of movement in that mag when firing that is the issue as when resting on that mag, it's pushing further up vs offhand when it's hanging.
My suggestion is to go to a variable gas block so you can tune it.

My take also.

 

[Seadog]

My personal feeling was essentially "limp wristing." I am absorbing some energy from the recoil that isn't happening when rested. I try to lean into it a bit so I can keep my weight forward into the gun rather than "shelfing" it on my chest. No issues with my MP15.

I have a stock milspec buffer and spring I can try. Perhaps that will allow the bolt to keep traveling even if I am taking some of the shock into my body.

I did pull the charging handle/bolt back with an empty mag. It locks fine.

When assembled my friend (the more experienced of us and who had done this before, just not an 11.5") used a bore scope to check gas block alignment.

Have also been told that the gas key may be too large allowing too much gas out into the chamber (this was from a rather grumpy smith at the range/store I was at). He took a quick look and said the key was large and I should toss it and get a new one.

I can try stock bcg from the mp15 as well to see if that makes a difference.

Thanks for the input everyone. It is nice to have some options to go down before getting to total reassembly on the gas system. Once I get a chance to test the buffer and bcg I will report back what I find.

Unless this is a super old bolt carrier assembly from a NAM parts kit there is no way to tell just looking at a gas key. That smith was full of shite. You can’t tell looking at the gas key if it is to big. There are no, go gauges for that type of thing. If this is a new assembly he was just guessing.

 

[sparkstable]

Unless this is a super old bolt carrier assembly from a NAM parts kit there is no way to tell just looking at a gas key. That smith was full of shite. You can’t tell looking at the gas key if it is to big. There are no, go gauges for that type of thing. If this is a new assembly he was just guessing.

It was a new ToolCraft NiB. The gun was assembled last Sat, first fired Sunday after (only rest shots, no issues), then again just the other day with a few more rested shots to tune the zero and then standing shots when the issue revealed itself.

The smith took the bcg back into the work room for a bit, came out, and said it was too large and gave a measurement (can't remember exactly what it was... 0.8something something and that it should have been 0.7something something I think... been a few days and I've slept since then, lol).

The plan is to go Monday and try new buffer weight, old milspec buffer and spring combo, and milspec bcg. Hopefully between those a working combo will reveal itself.