Need help with new SBR upper

[spartigus]

It's my 2nd post and I already need some help guys. I took a new SBR upper out today and started having problems from the get-go.

It's parts are: factory Sabre Defense lower w/ H3 buffer, Sabre Defense A3 upper, BCM Gunfighter, BCM BCG, Centurion Arms 12.5" barrel, Larue 11.0 rail, PWS QuickComp for a Gemtech G5.

The problem: Almost every time I fired, the BCG wouldn't strip the next round off the mag. It would extract the round fired, but wouldn't grab the next. I'd have to rack it each time to get a new round in the chamber. I also had several double feeds. I used a variety of USGI mags, PMags, and EMags and shot 55gr Wolf, 62gr Wolf, 55gr PMC, and 55gr Winchester putting a total of about 200 rounds downrange. I switched buffers to an H2 and even tried my friend's SBRed LMT lower and the problems still occurred. The only time none of this happened was when I used my G5 with the upper, during which it had no problems at all.

Any ideas what could be happening? I'm good at building rifles, but definitely need more experience diagnosing problems.

[Eric]

After firing the last round in the mag, does the bolt catch reliability engage and lock the action open?
Describe what you are experiencing when you get your "double feed." This term is often misused and should be describing two live rounds trying to feed into the chamber at the same time.

Most feed issues are due to bad mags, but it sounds like you examined that angle already. Hopefully you're running it properly lubricated - better to have too much than too little. Feed problems can also related to problems with the gas system, either too little or too much gas is a bad thing. If there is too little, the BCG fails to retract far enough into the receiver extension to strip off a fresh round. If there is too much gas, the BCG can cycle too fast and the mag isn't able to push up the next round before the action is closed. Just to muck up the equation even more, when you use a suppressor, the gas pressure will be increased.

[spartigus]

The bolt locked back every time on an empty mag without fail. And when I say doublefeed, that's exactly whats happening, 2 live rounds are somehow trying to be chambered at once. I thought it might have been lube at first, but after adding quite a bit more, it happened less, but still happened.

It sounds like from what youre saying that there isn't enough pressure without the suppressor. Would going to a lighter buffer help?

[rebelEMPIRE]

I would personally try swapping out the Bolt Carrier for a known good one. Perhaps the upper receiver's geometry is off (highly unlikely but worth a check). But most likely, I think there is a lack of pressure during cycling. Is your gas block dead on? If the port is blocked ever so slightly, that's enough for problems to compound, especially on a new un-broken-in rifle. Use a light weight buffer to see if it helps until things smooth out. Also, keep reporting your findings as it would help others with similar issues.

-rebelEMPIRE

[spartigus]

The bolt carrier group is from Bravo Company, so it is a known good one.

[rebelEMPIRE]

The bolt carrier group is from Bravo Company, so it is a known good one.

Apologies for not being clear in my original post. I meant a known fully functioning one. Regardless of where or whom you acquired an item from, there is always the possibility of a bad product slipping through quality control--which was my intention to eliminate the possibility of. I agree that BCM makes top quality parts, but even they can make mistakes sometimes.

-rebelEMPIRE

[Eric]

Apologies for not being clear in my original post. I meant a known fully functioning one. Regardless of where or whom you acquired an item from, there is always the possibility of a bad product slipping through quality control--which was my intention to eliminate the possibility of. I agree that BCM makes top quality parts, but even they can make mistakes sometimes.

-rebelEMPIRE

Good call. I recently received a bad carrier from Colt, in a new 6920. Stuff happens.

Parts swapping is a legitimate diagnostic method and makes sense given the type of malfunction.

A standard USGI or Pmag should function fine. However, in this case, it would be worthwhile to try out an extra power magazine spring. Specialized Armament part #SA09002 is for the single Extreme Duty spring for the USGI 30 round mag. SAW also has complete mags available with their spring already installed. Wolff Gun Springs part #60905 also has extra power springs. With either of the XP springs installed, be sure to only load 28 rounds in the mag.

[Stickman]

Have you figured this one out and fixed it yet?

If not, drop the H3 bufer, and go with a standard, which should help with your gas issues. That might be a quick cure right there.

Additional things to check would be the BCG as already stated, the gas block, and rail alignment with the gas tube.

[spartigus]

I haven't had a chance to get to the range yet, but hopefully I will this weekend. I checked the gasblock and tube alignment and that seems okay (the gasblock is pinned and set screw type) and I've thrown some carbine, H, H2, H3, and 9mm buffers into the range bag along with another buffer spring and BCG. Hopefully I'll get it figured out. I'll definitely let you guys know how it goes.

[jettavr6]

Hi, new to the forum, but I am having a very similar issue. Went to the range for the first time with a new build today. Mega lower and upper, BCM barrel, BCM bcg. My Bcg would extract a shell and then Jam, with the extracted shell and a new round sandwiched in the chamber. Through a process of elimination I discovered it was the BCG. after switching to a buddies BCG, just a standard CMT, it ran like butter...no problems. every time I put the BCM back in it jammed again. But with the exception of one jam, the BCM cycled fine in my buddies rifle. I read too late, that installing the o-ring they come with can fix extractor issues. apparently they have a special spring in the extractor that doesn't require the o-ring. Maybe that will make a difference.

[rebelEMPIRE]

Through a process of elimination I discovered it was the BCG.

Not necessarily. It might be a combination of your upper, bcg, bolt rings, and gas port. Your rifle is new which leads me to suspect the following...

after switching to a buddies BCG, just a standard CMT, it ran like butter...no problems. every time I put the BCM back in it jammed again

This is most likely because your buddy's rifle is broken in. Broken in parts will always run more smoothly than new parts, guaranteed. Your buddy's gas port is sealed to with carbon build up to reduce blow by, therefore increasing pressure. The mating surface of the upper receiver where the bolt carrier rides is also smoother in your friend's rifle than in yours. And because your friend's bolt carrier is smoother, with broken in bolt rings, it's guaranteed to run better than your new BCM BCG.

I read too late, that installing the o-ring they come with can fix extractor issues. apparently they have a special spring in the extractor that doesn't require the o-ring. Maybe that will make a difference.

I don't believe it to be an extractor issue, but rather a combination of new parts issue. Try running your rifle with extra lubrication the next go around. It's not a bad thing to have too much lube since you can always clean your rifle when you want, but running without enough lubrication is definitely not a good idea. If your problem persists, then the next culprit is probably your ejector. Also a cheap fix, but more difficult to tackle. You'll need some special tools for that job. I recommend just bringing it in to get it professionally inspected and repaired if that were the case.

-rebelEMPIRE

[jettavr6]

I was originally thinking it could just need to be broken in as well. I am going to try to put a couple hundred more rounds through it later this week and see what happens. One other thing it was doing; it would often eject a case and then fail to load a new round. The bolt would go forward all the way, pull the trigger and click..nothing. I pulled the charging handle and there would be no round in the chamber, it wouldnt strip a new round from the mag. This happened in my gun and in my buddie's gun. Any thoughts on that issue being related? And thanks for the advice so far.

[rebelEMPIRE]

Any thoughts on that issue being related? And thanks for the advice so far.

No problem. I enjoy being helpful.

A round failing to strip out of the magazine by the bolt is a result of insufficient pressure during bolt carrier cycling and/or too high of a friction coefficient between moving surfaces. That said, we can remedy the problem by either reducing the friction coefficient in the affected area, or decreasing the necessary pressure to cycle the BCG aft enough for it to allow the next round in the magazine to be raised into position for chambering. We can also hypothesize that the magazine spring does not react quickly enough (weak spring) for new round chambering; but in the case of a brand new rifle, this is unlikely the case.

I wouldn't recommend increasing the pressure at this point (more powder charge) as it may lead to damage and/or injury to your equipment and yourself. However, reducing the friction coefficient between the moving surfaces will allow the bolt carrier to cycle more easily (read: add more lubrication). Also, reducing the total mass of moving objects will reduce the overall energy necessary to induce a change in motion (read: use a lighter weight buffer). However, the easiest and most surefire way to increase reliability in your case is to lower the k-factor of your buffer spring (energy required to compress the spring).

In short, use a weaker buffer spring, lighter weight buffer, or more lubrication until the parts are well broken in. I hope this fixes your issue.

-rebelEMPIRE

[jettavr6]

Thanks for the advice. I will be going to the range tomorrow to troubleshoot, I will let you know how I make out.

[jettavr6]

Well, for all interested, it turned out to be a simple problem. Two of the gas rings had their gaps aligned, allowing some gas to blow by. I staggered them and now it runs flawlessly. I know there is some debate as to whether or not the rings really need to be staggered, but in this case it made the difference.