AR not chambering next round

[calebp]

Hey guys,

Recently picked up a sbr upper. Was shooting it today and it wouldnt load next round from the mag reliably. If I shouldered the **** out of it, it would run but let the wifey try a few and it was stripping the fired round but not chambering the next. Almost like “limp wristing” it but for a rifle. Guessing buffer is too heavy and I should drop the weight? Lower is a factory LMT Estonia rifle and upper is an mk18 with adjustable bcg. Cycled mostly fine for me with a couple of rounds not chambered from a mag but the wifey, with admittedly poor form, was only able to get maybe 1-2 rounds off before having to cycle charging handle to get the next to load. Thoughts, ideas ect? Shooting suppressed but with a flow through can so it basically made no difference, turned bcg from unsuppressed to the first of three levels of suppressed and it didn’t strip round and it was clear was not getting enough gas at all.

Thanks!
Caleb

 

[rockyfatcat]

cmshoot

 

[cmshoot]

Could be a lot of things.

-gas port too small

-gas block misaligned / not getting all the gas that it should

-adjustable gas block needs opened up more

-buffer too heavy

-buffer spring too heavy

-ammo too weak (some AR's don't run well with soft stuff like PMC Bronze 55grn .223)

-magazine issue

-feed ramps issue


When you fire the last round in a magazine, does the bolt lock to the rear 100% of the time?

 

[calebp]

Could be a lot of things.

-gas port too small

-gas block misaligned / not getting all the gas that it should

-adjustable gas block needs opened up more

-buffer too heavy

-buffer spring too heavy

-ammo too weak (some AR's don't run well with soft stuff like PMC Bronze 55grn .223)

-magazine issue

-feed ramps issue


When you fire the last round in a magazine, does the bolt lock to the rear 100% of the time?

Hated to bother you with free tech support lol. Holds open when I’m firing and it’s cycling reliablish, does not with the wife. Figure the buffer spring/weight would be the cheapest place to start trouble shooting

ETA: no issues with actual chambering once round is stripped from mag, issue occurred with multiple mags/ammo brands

 

[cmshoot]

Caliber, barrel length, and gas system length?

I read "Mk18" and I automatically go to 10.3" 5.56 with a carbine-length gas system, but I don't want to assume.

 

[calebp]

Caliber, barrel length, and gas system length?

I read "Mk18" and I automatically go to 10.3" 5.56 with a carbine-length gas system, but I don't want to assume.

Yes sir, factory mk18 with 10.3” barrel, assuming carbine di gas system, 5.56, bootleg adjustable bcg. It’s the one you installed the huxwrx flash hider on a week ago.

Hadn’t shot it before today. Guessing factory buffer from Lmt isn’t sprung to cycle shorter barrel length. Hoping at least lol. Should have switched it to my other rifle I had out there running flawlessly but too dumb to think about doing it in the moment.

 

[cmshoot]

Gas port size could be an issue.

The milspec Mk18 uses a .070" gas port. Works great in that application, since the US military is only using quality, full power 5.56 ammo, such as M855.

Civilians, on the other hand, tend to use a hodgepodge of ammo; 5.56, .223, whatever is on sale, etc.

This is why companies like SOLGW use .073" on their "Mk18" gas ports. It'll still cycle the hot stuff, but will run with weaker stuff, too.

DD had issues with their first Mk18's, because folks were having issues getting them to cycle properly with whatever ammo they shoved in it. DD later went with a larger gas port, which fixed the issue.

If your gas port is .070", it could be an issue, depending on what ammo you were using.

 

[Mythical Beast Forge]

Load one round in the mag and see if it locks all the way back. It’s probably the easiest way to check for being under gassed.

 

[cmshoot]

Load one round in the mag and see if it locks all the way back. It’s probably the easiest way to check for being under gassed.

That's part of the method that I recommend for all new guns, new builds, or for diagnosing problems.

Get a minimum of 12 magazines. Mark each mag so you can tell them apart (a Sharpie and numbers are perfect).

Lock the bolt to the rear, insert the first mag, hit the bolt release, and fire. Look for:
-vigorous ejection
-bolt locks to the rear

If it does both, go to the next mag. If it goes through all 12 mags without a hitch, you're good to go.

If it fails to lock back on one of the mags, sit that mag aside. If the other 11 mags work lock the bolt to the rear, then take the suspect mag and run it about 8 more times, 1 round at a time. If it doesn't fail again, it could have been a fluke. If it fails to lock back at least 1 more time, you have a bad mag. Replace it, repair it, or use it for a dedicated training mag.

If more than one magazine fails to lock the bolt to the rear, you have an issue with the firearm. Undergassed usually, but it could be too much buffer/spring, bad bolt catch/spring/plunger, issues with the BCG, etc.

 

[cmshoot]

Undergassed could be one or more of the following:

-gas port too small

-gas block misaligned

-leaking between the gas key and the carrier

-poor "seal" between the gas tube and the inside of the gas key

-poor seal in the carrier bore......could be due to gas rings, although it could be a crappy BCG or spec issue

-weak/underpowered ammo