AR gas help needed

[flashpan]

I'm not to swift on diagnosing AR problems so I appreciate any help. I have two rifle length AR's (one is 6mm the other 556)...I use the same fixed lower for both. The 6mm runs great but my new 556 appears to be under gassed because it will not keep the bolt back on a empty mag so it also will not advance rnds from the mag. I have tried different mags...no help, tried different ammo...no help. Also made sure the 556 is lubed although it has a Nickle Boron M16 BCG. I do not want to change any buffer springs because the 6mm runs perfect. Would an adjustable gas block in the 556 provide more gas? Ideas? Help!

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[jbett98]

What weight buffer do you have?

 

[oli700]

I do not want to change any buffer springs because the 6mm runs perfect. Would an adjustable gas block in the 556 provide more gas? Ideas? Help!

no, it wont because you need to add gas

You need to drill the gas hole larger on the 556 barrel

 

[Medic!]

I had a Nickle boron carrier group run slow in an AR.
I think the coating made the bolt a little tight. It felt tighter than the original that ran fine in that gun?

Maybe it's a couple thousandths thick?

In time it loosened up and ran fine.

Edit: The bolt was tight in the carrier.
You could not move it with a flick of the wrist.

And don't forget to check the gas rings.

 

[jbett98]

Wouldn't swapping the recoil buffer to a lighter one help?

 

[oli700]

Wouldn't swapping the recoil buffer to a lighter one help?

it may, he said he didn't want to change anything because the 6mm runs perfect so he has to go after the gas hole on the 556

 

[jbett98]

But won't the gas hole enlargement create another problem in which the 6mm upper is now slamming the BCG too hard?
I would think that swapping out different weight buffers would be a simple task.

 

[oli700]

But won't the gas hole enlargement create another problem in which the 6mm upper is now slamming the BCG too hard?
I would think that swapping out different weight buffers would be a simple task.

the 556 upper has its own gas system independent of the 6mm upper

basically he wants both uppers to run perfect without fussing with the one lower/buffer system that runs both uppers

 

[jbett98]

OK, I missed that somehow. Makes perfect sense now.

 

[oli700]

OK, I missed that somehow. Makes perfect sense now.

I do it all the time....I am a bad scanner lol

 

[Twodogs]

Seen this before try switching ammo in the .556.

 

[flashpan]

Gret responses folks thanks
I have tried 5 types ammo with diff bullet weights with no luck.
I think Medic may be on to something as the bolt is a bit sticky in the carrier...the BCG is brand new by the way...gas rings perfect.
I have not checked the buffer and spring but I assume its std rifle length...guess I could switch to a med length buffer/spring then put a adj gas block on the 6mm?
But first I think I will just run more ammo thru it ... the upper and BCG are brand new.
Keep em coming .. .thanks again

 

[308]

If the 6mm is the 6.8mm putting out ~1450ft/lbs at the muzzle vs 1330ft/lbs for the .223/5,56, and if the recoil spring and buffer are selected for the 6.8, then one might expect the .223/5.56 to have an issue throwing the bolt carrier to full recoil.
My $0.02

 

[Garg]

Drilling out the gas hole on the barrel, is a fairly common issue. oli nailed it.
/thread

 

[oli700]

Drilling out the gas hole on the barrel, is a fairly common issue. oli nailed it.
/thread

suppose the gas block could be out of alignment on the 556 upper ?......its almost always no unless builder was intoxicated but if the hole is slightly small and the the block is slightly off then that might be just enough

 

[Garg]

Building while intoxicated..... LOL... O wait....

 

[flashpan]

More good help ... thanks
308 good point...the 6mm WOA generates 1700 ftlbs at the muzzle (87grs @ 3000fps) so prob does have a stiff buffer/spring...I'll check...maybe just not compatible. And I will check the alignment of the gas block...it does have some carbon around the block and tube...is that normal or is their leakage? Getting it narrowed down now... much obliged.

 

[oli700]

it does have some carbon around the block and tube...is that normal or is their leakage? Getting it narrowed down now... much obliged.

well, to an extent. mine have build up but they get shot a crap ton . I just took apart one of my best running, never ever a hiccup rifle gas uppers and it had a bunch of build up

 

[erudne]

1st Make all the changes you can without spending a dime call the BCG MFG and tell them the situation, follow their advise

 

[Medic!]

I told you my bolt was tight in the carrier. I thought due to the Nickle boron's added thickness?

Is yours tight? If so remove the gas rings and see if the bolt moves freely in the carrier without them.
If so you can use a thin diamond file to file a few thousandths off the rings at the gap ends.
The rings will make a smaller circle when compressed in the tighter carrier bore and wont bind.

I remembered after my post that I used a one piece gas ring and that fixed my tight bolt to carrier fit. [I just pulled it out and looked at it]
But you can just file your gas rings down. If you test and find them to be too tight.