AR Gas Enigma

[giggitygiggity]

I have an AR that I recently put together. It has been giving me issues which I believe are due to it being undergassed. It has a 20” barrel from Classic Firearms (https://www.classicfirearms.com/arbarrel20parkerizedstraightfluted1in8heavybarrel), a Daniel Defense clamp-on style low profile gas block, an Anderson rifle gas tube, a Fail Zero NiBoron BCG, a UTG/Leapers fixed rifle stock with rifle spring and buffer, and a Midwest Industries 2-chamber muzzle brake. The upper receiver is a no-name generic receiver as is the charging handle.

I have tested it with Federal Tactical 55gr FMJ ammo and my reloads (55gr FMJ pulls with 24.5g H335 and CCI SR primers).

The rifle does better with the Federal loads. However, regardless of the ammo, the rifle fails to consistently lock open on the last round. Additionally, it has difficulty reliably firing and the chambering a new round; it usually does not jam; instead it will fire a round and then either have a dead trigger or will not cycle enough to even reset the trigger, let alone pick up a new round.

I have swapped out the lower for one with a carbine buffer tube system. I have inserted a carbine buffer in place of the rifle buffer. I have swapped out the Fail Zero BCG for an AIM NiBoron BCG. I had the same issues with a pinned generic gas tube and block which I replace with the current clamp-on Daniel Defense and Anderson combo hoping that the clamp on gas block would produce a tighter seal. I confirmed proper alignment of the gas block and barrel gas port. There is no binding as I manually cycle the action.

I left the bolt locked back for a week thinking maybe the rifle buffer spring was too strong. However, after troubleshooting, swapping parts, etc, the issues still persist.

The only other thing that I can think of is that the barrel gas port is too smakl, but I do not want to bore out a bigger gas port if I can avoid it.

If anyone has any suggestions or insight, please let me know either on this thread or via PM. Thanks!

 

[MistWolf]

When you say you verified gas port alignment, I assume you mean the alignment left and right. Did you also verify alignment fore and aft?

Most low profile gas blocks are made to be used with free float handguards. When used with a handguard cap, it causes the gas block to be misaligned with the gas port. I had this problem once. I had to shave the gas block to get it to work.

If the barrel gas port is mislocated, it will cause a misalignment. I had this problem with one barrel. I had to shave the gas block to get it to work.

Another problem could be the gas key or the carrier. Test the upper using a carrier you know for a fact works. Test the Fail Zero and the AIM carriers in an upper that you know for a fact work. Use a bolt that you know for a fact works for these tests. Don't take any short cuts. I once found a carrier that leaked even with a good gas key and good gas rings. It made things complicated because the problem was originally a bad gas block. I didn't realize the short stroking was due to two different sources.

Another source of short stroking is the gas tube. If it's out of limits, it will leak either at the gas block or the gas key. I think it's unlikely, but not impossible.

Check for partial blockages of the gas tube and gas key. Do not poke anything into the gas tube you don't want taking up permanent residence in there. Blow air or spray solvent through the tube to test it for blockages.

 

[someguy2800]

I had the same issue with my UTG fixed A2 stock and buffer. I've ran several 223 barrels on it from 16-20" and all of them were unreliable to some extent. I would get occasional failures to lock back and occasional misfeeds from short stroking. The barrel I finally settled on is a 20" ballistic advantage and it was the worst one yet for short stroking. It had no hope of locking back on am empty mag and the brass would sometimes not even make it out of the chamber. I took it apart to check the alignment and seal of everything twice and it was all fine.

Finally I cut an inch off the buffer spring and started replacing weights out of the buffer with wood dowel and eventually worked down to a completely empty buffer. I think it was originally 5 oz and now it's less than 2 I think. It's been 100% reliable since then and always locks back.

I use the same lower with a 300 blackout and 7.62x39 upper with an adjustable gas block so I can tune that down so both are reliable with the same lower.

 

[HankC]

The 0.085" gas port diameter in the spec is at the under size side. Should be 0.093"-0.098" for 20" rifle length per
http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html
Even less gas if any gas leak, which is common when new but will seal off after maybe 50-100 rnds. Keep shooting may improve, If not, open the gas port. Of course check your gas block alignment first, it should be obvious while gas hole in the block is much larger than needed. Also check if any severe gas leakage at gas block, gas tube and gas key. Check gas rings also.

 

[Demi-human]

I am doing this same dance with a four fifty upper. I only have twenty, single shot, rounds through it. The last time I fired it was the only time it actually chambred the next round from the magazine. I then pulled the handle and popped it out! I had gotten used to checking for a round and ejected the only cartridge that self loaded!

Just today I made sure that the gas block was lined up with the port by using an air compressor blow nozzle. I plugged the chamber with a rag and blew air in the tube from the upper reciever. Saw dust blew away from the muzzle, so it is very free flowing. I am at the upper end for load so I am about to put the original buffer back in to see if it will function. I have a Spike's.

If not, I too will use @MistWolf 's excellent suggestions. (And others too.)

 

[FL-NC]

Sounds like an issue in the gas block.

 

[giggitygiggity]

Thanks for the input. I think it is a gas port issue because most of the troubleshooting items have been attempted. I’ve used two different gas blocks and tubes and ensured that each were aligned properly (forward, aft, right, and left).

What is the best way to increase the gas port size to a larger (within spec) size? What precautions or items do I need to be aware of when increasing the gas port size? Thanks.

 

[giggitygiggity]

Thanks for the input. I think it is a gas port issue because most of the troubleshooting items have been attempted. I’ve used two different gas blocks and tubes and ensured that each were aligned properly (forward, aft, right, and left).

What is the best way to increase the gas port size to a larger (within spec) size? What precautions or items do I need to be aware of when increasing the gas port size? Thanks.

 

[giggitygiggity]

Thanks for the input. I think it is a gas port issue because most of the troubleshooting items have been attempted. I’ve used two different gas blocks and tubes and ensured that each were aligned properly (forward, aft, right, and left).

What is the best way to increase the gas port size to a larger (within spec) size? What precautions or items do I need to be aware of when increasing the gas port size? Thanks.

 

[someguy2800]

I would play with the buffer and spring before I started messing with the gas port myself, but to each his own. I know mine would be fixed with a bigger gas port too, but why mess with drilling a $300 barrel when you can cut a $5 spring and take out buffer weight for free?

 

[zaitcev]

A 20" barrel isn't going to see that much use, but I know for sure that gas ports open up with use. You can make it larger, but you can't make it smaller (well, you can install an adjustable gas block). That's why I, too, would empty out the buffer first and only tinker with the port after that.

 

[MistWolf]

With all due respect, cutting the recoil spring and reducing the buffer weight is not the way to fix the problem. If the gas port is too small, it needs to be opened up.

Thanks for the input. I think it is a gas port issue because most of the troubleshooting items have been attempted. I’ve used two different gas blocks and tubes and ensured that each were aligned properly (forward, aft, right, and left).

What is the best way to increase the gas port size to a larger (within spec) size? What precautions or items do I need to be aware of when increasing the gas port size? Thanks.

Giggity, go back to the rifle RE and buffer. Get a drill index and find out what size the gas port is. Use the next size drill up and carefully open the gas port. You want to keep the drill straight and be careful to not break the drill in the hole. You also need to be careful you don't tag the barrel with the drill when it breaks through. One way to protect the bore is to put an aluminum, brass or wood rod inside to stop the drill bit. Another is to use a drill stop. You can use several layers of masking tape as a makeshift drill stop. After drilling, there will probably be burrs. Knock away what burrs you can with a rod. You can finish deburring the gas port by running a few rounds through the barrel.

Be patient with the project. Open the port one bit size and do a lock back check. Once the gas port is large enough to affect a lock back check STOP!

However, the easiest way to get your rifle to work is to increase back pressure with a suppressor.

 

[giggitygiggity]

What is a rifle RE?

 

[MistWolf]

Rifle length receiver extension

 

[luzyfuerza]

I had a 300 blackout upper that exhibited the same symptoms as your upper. I drilled out the port as recommended above...problem solved. Get a variety of drill bits, use a drill press and put a protective rod inside the barrel while you drill, take your time, and you'll likely end up with an upper/barrel combo that works well on all your lowers.

Manfacturers don't always get everything right...

 

[zaitcev]

Last time I had to drill a barrel, it was one of those 5.56 barrels that H&M Gunworks contracted somewhere to replace the Spainish ones. That thing was absolutely insane. No cobalt could dent it, I ruined a whole bunch of bits. I was close enough to try a masonry bit The only thing that stopped me was that the hole had to be very precisely made. Eventually I used a carbide mill bit very carefully in my drill press. It's not a drill bit, strictly speaking, but it was hard enough and did the job. It may not come as a surprise to the experienced people out here, but I was completely taken aback by the hardness of the steel in that barrel.

 

[boom boom]

Last time I had to drill a barrel, it was one of those 5.56 barrels that H&M Gunworks contracted somewhere to replace the Spainish ones. That thing was absolutely insane. No cobalt could dent it, I ruined a whole bunch of bits. I was close enough to try a masonry bit The only thing that stopped me was that the hole had to be very precisely made. Eventually I used a carbide mill bit very carefully in my drill press. It's not a drill bit, strictly speaking, but it was hard enough and did the job. It may not come as a surprise to the experienced people out here, but I was completely taken aback by the hardness of the steel in that barrel.

Spot anneal the area where you are drilling first. Some people do it with a pinpoint flame while others use an arc. Old trick for D&T hard receivers for scopes.

 

[Demi-human]

A lighter buffer did the trick for me! Six rounds in a row. Time for more magazines!

To be clear, my gas port and tube were open. I hope your problems get resolved easily, @giggitygiggity .

 

[giggitygiggity]

I finally got around to fixing my rifle. As many of you suggested, I simply drilled out the gas port. Although the factory specs say it was .085, I found that it was closer to .093 based on drill bits being able to fall through. Regardless, I drill it out to about .096 and fired off a mag of two rounds. Both rounds fired without issue and the rifle locked back. I then loaded up a mag of 8 rounds and they all fired without issue and locked back on the last round. I feel it's safe to say that the issue has been resolved. I just wanted to thank everyone on here that recommended drilling out the gas port.

 

[Coal Dragger]

If it was .093” it was probably about perfectly ported to run actual 5.56mm NATO.

You should have tried running some M193 or M855 loaded to 5.56 NATO pressures first.

Now you’ve over gassed your rifle for 5.56 NATO to run light loaded trash .223.... not something I’d personally call an improvement.

 

[LoonWulf]

Both the handloads and the Feds are on the light side, but still i dont think .096 is overly large for a 20" rifle gas system. Over gassed with hotter ammo yes, but again nor horribly.
I tuned my newest builds gas block with 855, to lock back and 1/4 turn more. The Federal 55fmjs operate the bolt a little slugishly but do lock back.

 

[MachIVshooter]

If it was .093” it was probably about perfectly ported to run actual 5.56mm NATO.

You should have tried running some M193 or M855 loaded to 5.56 NATO pressures first.

Now you’ve over gassed your rifle for 5.56 NATO to run light loaded trash .223.... not something I’d personally call an improvement.

Since the rifle now works with the ammo he intends to use, of course it's an improvement. As for being "over-gassed", not even as much so as the hundreds of thousands of 16" guns with carbine length gas out there, and certainly not as much as any AR running suppressed with a fixed aperture gas block.

 

[MistWolf]

Since the rifle now works with the ammo he intends to use, of course it's an improvement. As for being "over-gassed", not even as much so as the hundreds of thousands of 16" guns with carbine length gas out there, and certainly not as much as any AR running suppressed with a fixed aperture gas block.

My Colt 6920 wasn't over gassed.

There are suppressed ARs with non adjustable gas blocks that aren't over gassed.

It's all about the gas port diameter.

 

[Coal Dragger]

At the end of the day you either want an AR that is made to run 5.56mm NATO optimally or one that is made to run .223 Remington optimally. Adjustable gas blocks are one solution to allow both to adjusted for, but add complexity, cost, and I’ve seen and experienced mixed results where reliability are concerned.

I’m admittedly a proponent of running 5.56mm NATO chambers, and 1/8” or 1/7” barrels with porting for full power 5.56mm NATO ammo... and then feeding the rifle full powered 5.56mm NATO factory ammo. Or loading ammo the upper end of .223 Remington or using 5.56mm data if possible. For me the AR is first and foremost a fighting rifle, albeit one that lends itself to utility and recreation. I’m all for them being fun to shoot, but first and foremost I want utter reliability in adverse conditions while utilizing military grade ammo. So I carry that bias, and freely admit it.

If the OP is only wanting a range toy to put very light .223 Remington through then that’s fine. I still maintain that trying 5.56mm in it should have been in order along with some stouter loaded .223. Most of my properly ported 5.56mm guns don’t like light 55gr remanufactured ammo, or lightly loaded reloads. I don’t hold that against them because they run like sewing machines on the ammo they were actually designed for, and also run full power .223 Remington just fine as well.

On the other hand I have one that is clearly not of good quality (looking at you CMMG) that is absurdly over gassed. Clearly ported for bubba and his love of steel cased 55gr Wolf, this upper is an absolute suckfest shooting M855, or M193. Flings brass to the 1:30 position and beats up the cases, bangs the bolt carrier and buffer violently to the rear generating far more recoil than needed. Waiting for the next panic and I’ll dump it at a gun show for cheap to some mouth breather who will enjoy it more than I.

 

[LoonWulf]

Not trying to pick a fight, by any means, but In a fighting gun i would think you would want something that literally ate ANYTHING rather than being specific to the top end of the power range?

Again its very unlikely ill ever have to use my AR in a firefight, i certainly wont for home defense as its not the rifle i keep in the house, so mine choking on low powered federal ammo dosent really bother me any...if it became an issue id open my gas block another turn or two, and be pretty confident it would cycle the weakest stuff i ran across.