Build Question - Buffer / Rifle not cycling

[jk2401]

Hey everyone - I attempted to build my own AR15 with a bunch of parts. I got it all assembled, and upon firing a test round (both .223 practice ammo and 5.56 NATO ammo), the bolt carrier group wouldn't lock fully back, it stopped around 75-80%. Original specs were as follows -

Criterion .223 Hybrid Wylde Barrel 18"
BCM Discount Blemished Upper
Aero M4E1 Lower
Superlative Arms Adjustable Gas Block
Expo Arms Nitride Bolt Carrier Group
Armaspec Stealth Recoil Spring Gen 4 H Buffer 3.8 oz.

Fiddling with the adjustable gas block didn't prove to be successful, so I switched out the Superlative adjustable gas block for an Aero fixed gas block and had a gunsmith set it properly. Still no luck, the same problem occurred - bolt carrier group wouldn't lock fully back and got stuck in the same position. On my next step, I'm changing out the Armaspec Stealth Recoil Spring H buffer and ordered a Sprinco White buffer spring and a 3 oz. Luth-AR Carbine buffer weight. As luck would have it, I received a white/red Sprinco buffer spring and not the basic white one. So, if you've had experience with Sprinco recoil buffer springs, should I give the white/red buffer spring a try, or should I just exchange it for the white one I originally ordered? Any other suggestions as to what might not be working properly?

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[Pepe-lepew]

I think you are going the right direction in going to a standard buffer weight of 3 oz and standard buffer spring. That should make it a little over gassed, as designed.

If it still malfunctions do a test to see if it is fully stroking the bolt. Fire a round while pressing on the bolt catch. If the bolt locks back, it is fully stroking. If not, it is short stroking and you need to look for the gas leak or restriction to gas flow.

The likely culprit to gas issues is the BCG. If you have another known good one, swap them out. Also check to make sure the end of the gas tube is not worn. Other areas are gas port size, gas block alignment, gas leakage at the gas block at the barrel or tube. If you do find the BCG is the problem, it could be leakage at the key, out of spec key bore, out of spec carrier bolt bore or bolt tail bore, out of spec bolt tail or worn bolt rings.

My personal experience with cycle issues, I have seen out of spec bolt carriers bores, broken gas key bolts and worn gas tubes cause problems. I also have seen an over gassed system cause problems picking up the next round that required a heavy spring and buffer. I also have found an out of spec rifle buffer tube that was nearly a 1/4" shorter than it should be.

 

[powermad]

That barrel is a rifle gas system, most 18" barrels have a 0.995" gas port and generally use a rifle action with a buffer around 5.4 oz.
I use an A5 action with the A5H2 buffer (5.3 oz) on my 18" and 14.7" mid gas.

If it's not cycling with the carbine action and you've ruled out the gas block then as noted I would try a different BCG.

 

[P7M13]

Where do you test fire?

 

[solv3nt]

Where do you test fire?

Downtown Potland

 

[Yarome]

I run a criterion 20" wylde on one of mine. .750 port rifle length gas, JP adjustable block, white spring, H2 buffer in an M4E1 upper. I tend to agree with @Pepe-lepew that it's likely a gas or BCG issue. Some good tips in his post.

Going to basics, did anyone verify the gas port was drilled the correct size and is free of any obstructions? From there it's just backtracking the block, tube, key, bolt and gas rings. IE., running a wire through the tube to ensure there are no obstruction (IE., random packing material found it's way inside), the key throat meets spec, etc etc.

Swapping the BCG is a quick way to determine any issue there. Checking the upper channels for any abnormality is a good idea too. Also checking the bolt lugs and extension lock for proper seating and free of burrs or such. I'm assuming headspace was checked...

The components you listed certainly don't throw up any red flags... all quite decent and known to typically be better quality... but there may still be an out of spec anomaly present. Never assume. It's likely a small issue easily rectifiable, but it "can" be a pain tracking it down and really just a process of elimination.

I've found that having a decent caliper and checking specs on all parts before assembly is time well spent on any build. At the very least it gives you a good idea of potential culprits if any issues arise. IE., still in spec but an upper on the slightly tight side and a BCG on the slightly larger side of spec. KWIM?

 

[edzz]

Hey everyone - I attempted to build my own AR15 with a bunch of parts. I got it all assembled, and upon firing a test round (both .223 practice ammo and 5.56 NATO ammo), the bolt carrier group wouldn't lock fully back, it stopped around 75-80%. Original specs were as follows -

Criterion .223 Hybrid Wylde Barrel 18"
BCM Discount Blemished Upper
Aero M4E1 Lower
Superlative Arms Adjustable Gas Block
Expo Arms Nitride Bolt Carrier Group
Armaspec Stealth Recoil Spring Gen 4 H Buffer 3.8 oz.

Fiddling with the adjustable gas block didn't prove to be successful, so I switched out the Superlative adjustable gas block for an Aero fixed gas block and had a gunsmith set it properly. Still no luck, the same problem occurred - bolt carrier group wouldn't lock fully back and got stuck in the same position. On my next step, I'm changing out the Armaspec Stealth Recoil Spring H buffer and ordered a Sprinco White buffer spring and a 3 oz. Luth-AR Carbine buffer weight. As luck would have it, I received a white/red Sprinco buffer spring and not the basic white one. So, if you've had experience with Sprinco recoil buffer springs, should I give the white/red buffer spring a try, or should I just exchange it for the white one I originally ordered? Any other suggestions as to what might not be working properly?

Will the BCG fully cycle and lock back on an empty magazine when manually cycled? The "it stopped around 75-80%", and "and got stuck in same position" statements imply that the bolt is encountering resistance and binding.

Will the BCG manually cycle without unusual resistance?
Do you have another AR-15 you can swap parts with?

 

[msgriff]

When the BCG cycles rearward, the Armaspec SRS guide rod travels within the bolt carrier inner diameter. Some carriers do not have an inside dimension for the guide rod to fit for the full distance required.

 

[TTSX]

You did lubricate your gun, right? I've found after the first couple magazines and a cleaning and re lube, things loosen up. There are a TON of places on an AR where friction occurs, even places you don't think about

 

[edzz]

You did lubricate your gun, right? I've found after the first couple magazines and a cleaning and re lube, things loosen up. There are a TON of places on an AR where friction occurs, even places you don't think about

What he said.

Especially for break in run it WET, CLP is our friend.

 

[drstrangelove]

I've seen more cycling issues from improperly installed buffer retention pins and buffer tube thread depth than any other issue. Start with a hard look at clearances.

 

[jk2401]

Thanks everyone! I'll keep all of these items in mind on my next attempt. I did clean and lubricate the BCG with ballistol before the test run. The current BCG is brand new, but I did buy two of them, so I have a backup to try next. Hopefully I can return the faulty one if it turns out to be the BCG. Will keep this thread posted!

 

[arrowshooter]

If you're still experiencing incomplete cycling after going over whats mentioned above, I would like to expand upon what edzz has asked:

Will the BCG fully cycle and lock back on an empty magazine when manually cycled? The "it stopped around 75-80%", and "and got stuck in same position" statements imply that the bolt is encountering resistance and binding.

Will the BCG manually cycle without unusual resistance?
Do you have another AR-15 you can swap parts with?

After you have confirmed that the bolt will fully cycle by hand, clean off the bolt and clean the lubricant out the upper receiver. Lightly oil the bolt, reassemble and fully cycle the action by hand at least 100 times. You will be able to smell it when the parts start to burnish together. Clean everything up, lube as you would normally and 'choot it. If you still have cycling issues then:

Assuming that you have now switched over to a more conventional spring and buffer (I would actually use a mil spec spring), you need to confirm that you are getting the proper amount of gas to do the job. I found that pulling the weights out of the buffer is one way to get this done. It put's the weight of the buffer around 1 oz and will be a definite indicator if your gas port needs to be opened up a bit. If it cycles you know. I did this with an 18" rifle length gas build with similar issues and I ended up needing to open the gas port two steps. I then used an Odin Works adjustable weight buffer to tune the recoil back and it's ran perfect ever since.

Once you have it all figured out you can then start switching the original parts back in, or not.

 

[jk2401]

The lighter buffer spring did the trick! I tested it out today with the hot white sprinco buffer spring. The BCG fully locked back on one test round, and the gun cycled just fine with multiple rounds. Thanks so much everyone!