Full auto BCG issues

[Wingshooter]

One of my ARs is experiencing problems while running a heavier full-auto BCG. However, it is flawless if I run it with a lighter BCG.

One issue is the brass not ejecting very far and at about 1:00 position. The other issue is the bolt not always locking back on empty mag. It seems to me that the FA BCG is too heavy for the gas system because the lighter SA BCG throws a neat little pile at 4:00 position and always locks open on empty mag. It is a 5.56 16" bbl, mid-lenth gas, carbine buffer.

Is this common for a ML gas system, or do I have a gas problem (lol)? Or should I try a lighter buffer, or maybe I should just get a SA BCG for it since it runs so well like that. Suggestions?

[Oh Shoot]

Certainly, not locking back is generally due to undergassing, and sure, the lighter BC would tend to help that a bit, but did this just start happening all of a sudden with the M16 carrier already in there all along? If so, I'd suspect gas block or gas key has come loose.

 

Also, what weight buffer is in there now?

 

Just changing carrier to the slightly lighter AR15 type carrier, while it may allow normal function, would suggest it's still running at edge of reliability to me.

 

- OS

Edited September 15, 2015 by Oh Shoot

[mikegideon]

Make sure the BCG is properly lubed. I have had similar probles pulling a dry gun out of the safe and shooting it. Most of my AR's have M16 carriers

 

[Dolomite_supafly]

It sounds like you gun is perfectly gassed with the other BCG. I would toss the other carrier back in.

[BrasilNuts]

What weight buffer are you using?

[Oh Shoot]

What weight buffer are you using?

 

Already asked.  And the more pertinent one, did the gun run fine with the M16 carrier, then later begin to show problems?  If so, I bet on a gas leak. In which case the less than an ounce lighter AR15 carrier might be but a temp fix.

 

- OS

Edited September 15, 2015 by Oh Shoot

[Wingshooter]

Certainly, not locking back is generally due to undergassing, and sure, the lighter BC would tend to help that a bit, but did this just start happening all of a sudden with the M16 carrier already in there all along? If so, I'd suspect gas block or gas key has come loose.

Also, what weight buffer is in there now?

Just changing carrier to the slightly lighter AR15 type carrier, while it may allow normal function, would suggest it's still running at edge of reliability to me.

- OS

It's been happening since it's birth :) It's a parts gun that I put together about a year ago, and I'm just now getting around to breaking it in. I got the M16 BCG from PSA a couple months ago (the sale for $79). This gun will be my primary coyote rifle. However, I used it this last weekend at 3gun with the lighter BCG from my other rifle just to see if I could get it to fail, and it ran great.

The buffer is a standard carbine. I just put it on the scale = 2.96 oz. The spring measures 10.5" uncompressed Edited September 15, 2015 by Wingshooter

[Wingshooter]

Make sure the BCG is properly lubed. I have had similar probles pulling a dry gun out of the safe and shooting it. Most of my AR's have M16 carriers
 

That's a very good point. I lubed it, but it was still on the drier side and not according to that diagram. I'll lube it up better

[Wingshooter]

It sounds like you gun is perfectly gassed with the other BCG. I would toss the other carrier back in.

That's what I'm thinking, too. I have another rifle that I wanted to keep the SA BCG with, but I could just buy another one. I just don't like it that this rifle is finiky.

I put a really cheap low profile gas block on it. I have an adjustable laying around. Maybe I should try the other gas block

[Wingshooter]

This is the troublemaker :)

[mikegideon]

This is the troublemaker :)

 

NICE. I just orded a Larue mount (with less offset) for my 1 - 4. 

[mikegideon]

That's what I'm thinking, too. I have another rifle that I wanted to keep the SA BCG with, but I could just buy another one. I just don't like it that this rifle is finiky.

I put a really cheap low profile gas block on it. I have an adjustable laying around. Maybe I should try the other gas block

 

Maybe the gas block is just misaligned?

 

I have a new A2 doing the same thing. But, it's a BCM barrel with the gas block installed by Adco. That stuff was designed to run with a properly lubed M16 carrier. The carrier group was dry as a bone. I haven't shot it again, but will as soon as I can lube it up and get it back to the range. I have a real hard time believing Adco screwed up the installation

Edited September 15, 2015 by mikegideon

[Wingshooter]

Adding an adjustable gas block will not fix this because it is a gas issue, a low gas issue.
 
Measure the size of the port and go up by .003"-.005" and it should fix the issue without the need to use an adjustable gas block.
 
Personally I would use the lightest setup you can because it DOES reduce felt recoil, especially when it is gassed perfectly. And running a lightweight setup is no less reliable, actually more.
 
Most people think that a heavy buffer will help the carrier muscle through the crud in the receiver but that is just not the case. The BCG and buffer contact the bottom of the buffer tube and come to a complete stop. Then the spring exerts pressure on the BCG and buffer, forcing it forward and back into battery. And because it essentially starts from a stop the lightweight setup will gain more velocity than the heavy buffer. And velocity increases energy faster than increasing weight. So the extra energy from the faster velocity will have more energy to power through the crud than a slower, heavier setup.
 
And when the bolt is unlocking it is the gas in the barrel that exerts energy on the BCG and buffer. Because of that if you use a heavy buffer it will impact the bottom of the buffer at a much higher velocity than when returning to battery. And if you are running a heavy buffer or heavy BCG that extra weight translates into the felt recoil.
 
A heavy buffer is used to slow the BCG and buffer speed but the same can be done with an adjustable gas block.
 
I run lightweight setups in my ARs. And by lightweight I mean a buffer than weighs well under 1.5 ounces. I basically cut the buffer in half, remove weights and replace it with copper dust then build a spacer for the other end of the spring to make up for the material removed from the buffer.

Great info. Thanks. What's the best way to measure port size... shank of a drill bit?

[Wingshooter]

Maybe the gas block is just misaligned?
 
I have a new A2 doing the same thing. But, it's a BCM barrel with the gas block installed by Adco. That stuff was designed to run with a properly lubed M16 carrier. The carrier group was dry as a bone. I haven't shot it again, but will as soon as I can lube it up and get it back to the range. I have a real hard time believing Adco screwed up the installation

I checked the gas block allignment. It seed to be proper, but it's worth another look. I've always questioned the block itself because it was a no-name cheapy.

I'm anxious to do a proper lube on the BCG and give it another try. It was way too dry.

[mikegideon]

I checked the gas block allignment. It seed to be proper, but it's worth another look. I've always questioned the block itself because it was a no-name cheapy.

I'm anxious to do a proper lube on the BCG and give it another try. It was way too dry.

 

Not much to go wrong with a gas block if it doesn't leak.

[Wingshooter]

I have the BCG all lubed up now. If I can, I'll take it back out for another spin tomorrow to see if there is any improvement.

[Oh Shoot]

I put a really cheap low profile gas block on it. I have an adjustable laying around. Maybe I should try the other gas block

 

You know the block has to offset a skosh from barrel shoulder (width of handguard retainer that's not there), right?

 

- OS

Edited September 15, 2015 by Oh Shoot

[Ted S.]

Make sure the BCG is properly lubed. I have had similar probles pulling a dry gun out of the safe and shooting it. Most of my AR's have M16 carriers

 

 

 

Didn't know a BCG had a lube schedule. I just cover the whole thing in lube and pray for the best.

[Oh Shoot]

Didn't know a BCG had a lube schedule. I just cover the whole thing in lube and pray for the best.

 

Yeah, never heard of an AR failing from bolt or carrier being too wet.

 

- OS

[Wingshooter]

You know the block has to offset a skosh from barrel shoulder (width of handguard retainer that's not there), right?
 
- OS

Good thought. I do know that, but I can't remember if I built it that way or not. It's been a while since I put it together, and it's just been sitting around unfired until now. I want to say that I did factor in the offset because I've done that on other rifles, but I need to confirm it on this one. I'll have to check it out. Thanks

[Wingshooter]

Didn't know a BCG had a lube schedule. I just cover the whole thing in lube and pray for the best.

I usually just spray them down with CLP or something similar. This particular BCG is new, and I tested it pretty dry. The lighter BCG that functioned was pretty wet from my other AR. Maybe it'll be that simple of a fix