Picky BCG

[2006Blackpearl931]

Alright, I need some help here. I just finished my 3gun build and got to run it through the paces yesterday. Here is the problem. I put a NiB BCG in it. It does not like steel case ammo, AT ALL! It won't pick up next next round from the magazine. It strips the first round fine.And one time it loaded a round in the chamber, but did not go into battery.  I shot about 75 rounds of brass case with the NiB and ran like a well oiled sewing machine. Now, the rifle will shoot steel case with with a phosphate BCG, but not with thte nickle boron. What's up with that?

Edited February 15, 2013 by 2006Blackpearl931

[TripleDigitRide]

Nickel Boron is far too high class to shoot that crappy steel case ammo.

[Dolomite_supafly]

It is a gas issue.

 

Your problem is likely because you have a M16 carrier and shooting steel cased ammo. The steel cased stuff runs at a lower pressure and lower velocity than quality brass cased ammo. Are you running a heavyweight buffer? If so toss it and go back to a standard buffer. This will likely fix the issue. If it doesn't then the only way to fix it is to drill out the gas port until it runs the steel cased ammo. But when you do that you might run into a problem of over gassing with quality ammo.

 

How used is the phosphate BCG? A used bolt is a lot looser and cycles better than a brand new BCG.

Also, are both BCG the same? Is the Bn version an M16 carrier? The extra weight of a M16 carrier, along with the underpowered ammo, could be causing it to short stroke. (I actually suspect it) It sounds like the gun is under gassed with the Bn bolt carrier which tells me it is probably a M16 carrier.

 

What I would do is drill out the gas port until your guns cycles the steel cased. Then I would install an adjustable gas block. I would adjust it so the steel cased cycles. Then when you go to brass cased adjust it again. You want just enough gas to cycle, no more. And when you get a gas block adjusted correctly the felt recoil will be reduced as well.

 

Also, make sure to really clean the chamber. On more than one occassion a cycling issue has been resolved by cleaning a dirty chamber, especially after shooting steel cased ammo.

 

Dolomite

[Guest dont_tread_on_me]

What buffer?

[2006Blackpearl931]

Dolomite you are a wealth of information. I would say you are 100% correct. The NiB carrier is a m16 style, and the phosphate carrier I tried was a "enhanced" style carrier. There is quite a bit of weight difference. I'm not sure exactly what buffer weight I have. I used a PSA milspec buffer assembly. Says it's a CAR buffer on the website.

 

I might open the gas port, or I could always take a little weight off the carrier itself. Or like Triple digit said, just shoot good ammo in it.

[Dolomite_supafly]

Honestly I would get an adjustable gas block and open the port up. Having a properly tuned gas system keeps the gun cleaner, reduces wear and reduces felt recoil.

I recommend it for any carbine length gas system even if they are not having issues.

Dolomite

[2006Blackpearl931]

Well, I have a rifle length gas system. To make matters worse, the barrel started life at 20", and I have cut it back to 18", recrowned and rethreaded. I didn't open the port any figuring the original setup would be slightly overgassed as most factory built guns are, and hoped it would balance out. I think I am right on the edge of sufficient gas supply right now.

[Dolomite_supafly]

Yeah, you probably need to go up a few sizes. It should be .093"-.096". If it is .093" then I would try .096", that is the max for a 20" barrel and a rifle length system. If the port already measures .096" then I would try .1 and try it again. Do not increase by more than .005" at a time but if you do go too big all you need is an adjustable gas block.

 

You are actually in a pretty good position. You can open the port just enough to be reliable with steel cased. And because you only have 2" past the block you will probably not over gas the gun when shooting brass cased even if you get it reliable with steel cased

 

That 2" of barrel loss is the problem.

 

Dolomite

[2006Blackpearl931]

Alright, so I weighed my buffer and it is right at 3 oz. I also removed the gas block and the current gas port is .099, however. I have a DPMS stainless 1-8 twist barrel. I've read somewhere they drill the port between the rifling. After studying it for a few minutes, I realized I had aligned the gas block square with the upper reciever. It actually needs to be canted ever so slightly to align with the port. I marked the barrel on the larger diameter where the port is, and then aligned the block to that. It isn't much diffence, but there is enough I believe it will affect how much is transferred to the bolt. I will try to get out in the next day or two and see if it helped and report back here....

[LagerHead]

I hope this works for you. I always feel dumb after reading Dolomite's advice on ARs. That man has forgotten more than I'll ever know on the subject.

[will_reed]

Let us know if that makes a difference,  but I think you're on the right track.

[2006Blackpearl931]

Got to shoot the AR today. Runs great. Shot brass, steel and a mix of both hiccup free. Thanks the help people.

[50 cal]

Dolomite you are a wealth of information. I would say you are 100% correct. The NiB carrier is a m16 style, and the phosphate carrier I tried was a "enhanced" style carrier. There is quite a bit of weight difference. I'm not sure exactly what buffer weight I have. I used a PSA milspec buffer assembly. Says it's a CAR buffer on the website.

 

I might open the gas port, or I could always take a little weight off the carrier itself. Or like Triple digit said, just shoot good ammo in it.

Do not, I repeat, do not open your gas port! Overgassing an AR is the worst thing you can do.

You just have a weight problem. If running a heavier carrier, lighten your buffer.

[Dolomite_supafly]

Do not, I repeat, do not open your gas port! Overgassing an AR is the worst thing you can do.

You just have a weight problem. If running a heavier carrier, lighten your buffer.

You do realize this is not a standard AR barrel?

 

It is a rifle length system that has been cut back 2". It may need a larger port because of the shortened barrel beyond the gas port. When you start having to use non standard lower parts to make a specific upper run then you cannot swap uppers and that takes away the usefulness of the AR. A lower should work with any upper out there and if the lower doesn't then it is the upper that has the problem and not the lower. Having to swap out buffers and carriers when swapping uppers is silly.

 

I agree over gassing can be an issue but I do not believe that is the case here. And if it was over gassed then an adjustable gas block fixes it without affecting the lower's usefulness.

 

There should be no reason to use non standard buffers but I see it all the time. Before the first shot is fired people put in an exotic buffer or other exotic parts without trying milspec parts then complain their guns do not run right.

 

Dolomite

[50 cal]

That makes it a dissipator length then. That will ahve a rifle size gas port. The pressure curve will have dropped off considerably. The port on dissipators and 20" barrels both are right about .068" I believe. I don't have my spec book. It's at home.

He still shouldn't have to drill out the gas port. Just a change in buffer will suffice.

 

The problem s still the weaker steel cased ammo. If it eats the good brass cased ammo and the only problem he has is one particular brand of steel case, avoid the steel case stuff. Cheaper doesn't neccesarily a good thing. I tried it in 2 of my rifles. It's inaccurate as hell. Q3131A and M193 are accurate. Well, my handloads are much more so. I save money and get better accuracy.

[Dolomite_supafly]

There are two different setups for dissipators. One uses a shorter length gas system with the front sight well ahead of the port and one uses the rifle length gas system with the gas port at the front sight.

 

With the rifle length gas system, which is what the OP has, the port needs to generally be above .100". While the dissipator with the short gas system, like you describe, needs a much smaller port. The dissipator with the shorter gas system does not have the port at the front sight but under the handguard.

 

Ideally you want a lower that you can swap between all uppers without having to swap to exotic parts. And in order to do that the upper is what needs to be fixed, not the lower. There is nothing wrong with opening up the port a few thousandths at a time until the gun functions with the ammo you are going to shoot the most. Barrels are way cheaper than ammo right now so I would have zero issues making a barrel work with steel cased. The savings in the end will be less even if you have to buy another barrel. Also, you can artificially lengthen a gas system but you cannot artificially shorten a gas system so opening up a port isn't going to ruin the barrel. Worst case scenario you add ab adjustable gas block and actually tune the gun to the ammo.

 

Dolomite

[orientalexpress]

I didn't know anyone was making a dissipator with an actual rifle length gas system. All the ones I've ever seen have been carbine length. Who is making them? Or are they custom builds?

[Dolomite_supafly]

DPMS makes one

http://www.midwayusa.com/product/724394/dpms-dissipator-barrel-ar-15-556x45mm-nato-heavy-contour-1-in-9-twist-16-chrome-moly-matte-with-front-sight-base-rifle-length-sight-radius-pre-ban

 

But most are carbine length gas systems.

 

Dolomite

[Guest dont_tread_on_me]

My first AR was a bushy dissipator with two gas blocks and heavy profile, and I think it weighed as much as a SIG 556. If it were me I'd rather play with buffer weights before opening a gas port that will eventually erode itself open anyway, but I can see how it could be currently under-gassed.

[Dolomite_supafly]

Non standard buffers cost $40+ each. And if you have to buy just one to get a few different uppers running right you are already over the cost of an adjustable gas block. You can buy micro adjustables and railed adjustables for under $50 shipped. And with an adjustable gas block you can tune the gun to reduce felt recoil, minimize wear and tear and keep crud in the receiver to a minimum.

 

An eroding gas port is one more reason for an adjustable gas block. It doesn't matter how big the gas port gets the gas impulse will be the same.

 

Another advantage to tuning the upper to the lower is you don't have to remember which upper needs which buffer to run right. Sure you can use buffers to fix a problem but if you ever have to swap an upper in a hurry, or if you are in a SHTF situation and need to swap the upper, and you don't have the correct buffer you are out of luck.

 

Dolomite

[Magiccarpetrides]

What brand of BCG?

[2006Blackpearl931]

It's a WMD NIB-X. I have not had any more problems. I've shot in two matches with it so far. I ran low on brass ammo last weekend, so I mixed in some steel case stuff. Shot at the close range stuff with the steel case and shot the longer range steel plates with brass. I honestly couldn't tell a difference between the two. Dolomite is absolutely correct. I would have installed an adjustable gas block had I not realized I had an alignment issue. Even though the upper and lower are dedicated, its just as cost effective to get the adjustable block and be done. I'm a machinist by trade, so I wanted everything perfectly aligned when I was building it. End result is, the gas block had to be rotated slightly to align with the port. Does it bother me? On the clock, if it is putting holes on paper when I pull the trigger, I don't care if the block is on upside down, sideways or there at all!

[Magiccarpetrides]

Yeah the WMD ones are excellent, glad you got it fixed if it was an rguns I was gonna suggest a new extractor spring there's are notoriously weak