Help needed with short stroking new upper

AR/M4 AR Technical Discussion
1

Gents,

Any ideas you all may have that might help me solve this short stroking issues will be GREATLY appreciated…

Here’s a list of Specs for reference:

[b]SPECS:

*LaRue “Stealth” M4 upper receiver

*Daniel Defense 16" Middy C/L barrel

*Daniel Defense F/A BCG

*LaRue Low-Profile gas block

*PRI Rifle length carbon fiber handguard

*PRI GasBuster charging handle

*Vltor VST-1 folding front sight

*Surefire MB556K muzzle brake

*MagPul M-BUS rear sight[/b]

I took this new build to the range a week ago, and was thrilled:rolleyes: to find that it is a one shot wonder. It will fire, the bolt will eject the fired casing, but the BCG will not cycle fully to strip the next round from the magazine.

I initially had an H buffer installed, and after my first range trip with the upper, I stripped it down, I made a slight adjustment to the gasblocks position, swapped out the gastube, and swapped out the H buffer for a standard weight carbine buffer, thinking that it may have been over-buffered since it is a mid-length gas system barrel.

I took it out again yesterday, and had the same result as my first trip:mad:. I then swapped the lower, and the bolt carrier to other known properly functioning items. Same result. I gave up on it at that point, as it was wasting too much of my training time, and grabbed my other duty rifle and shot away my frustration.

One thing that I did notice, is that the chamber on the barrel seems like it may be a bit tight. Sometimes the first round from a freshly inserted mag wont chamber completely (about 1 out of 10 times), and when doing a brass check via pulling back on the charging handle, most of the time I cannot get the charging handle to budge by-hand. :confused:

When I got home I started stripping the upper down again…

I noticed an unusual amount of wear on a particular spot inside of the upper receiver shown in the center of the picture below:

Here’s a expanded view for reference :

I dont think this is a normal spot for this amount of wear to show-up. Now im thinking that this may be a symptom of what is causing me to think the chamber is tight, and making it difficult to pull the charging handle back to do a brass/chamber check:confused:

This upper was used previously on one of my other builds with zero functional issues. I recently switched to the PRI receiver block LINK: http://www.midwayusa.com/viewproduct/?productnumber=878754&utm_source=froogle&utm_medium=free&utm_campaign=649 after misplacing my old Bushmaster clamp-on style action/receiver block, and im thinking that maybe when installing the new barrel into this receiver, when torque-ing down the barrel nut, I may have flexed the receiver enough to damage it, which might be the causing the carrier to bind inside of the receiver, and be the culprit of the “short stroking”…Hmmmm.

I ordered a new upper receiver, as Im thinking that I may have twisted the receiver enough to distort it when torque-ing down the barrel nut.

Let me know what you guys think…

Thanks in advance!

2

If you have a suspicion that the upper is twisted, binding up the carrier, have you tried running a stripped carrier inside the upper to check for binding?

Also, you say you adjusted the gas block position and also swapped out the gas tube? Is the carrier gas key aligning properly with the gas tube? You could have binding there as well.

3

Also, that vice block you link to, is it designed for doing barrel work?

One thing I notice in the design of it compared to say the DPMS Panther Claw, is that the Panther Claw has internal support for the upper receiver whereas the PRI block does not. The PRI block would appear to put all the torque during barrel install, on the upper receiver lugs.

ETA: PRI’s item description does not mention barrel work. I think that block may be designed more with light maintenance in mind… cleaning, sight adj. ect.

M16-AR15 Upper Receiver Block Grip this handy block in a bench vice and hold the upper receiver by the lugs. It leaves the entire upper receiver accessible to work on sight systems or clean the bore.

http://http://www.pri-mounts.com/mm5/merchant.mvc?Screen=PROD&Store_Code=PRI&Product_Code=05_0083&Category_Code=TOOLS

4

The gas tube is properly aligned.

Yeah, I’m thinking that I may have mangled the upper during the barrel install by using the PRI vice block during the install. I gonna pick up another BM action block, or the DPMS Version posted above prior to mounting the new upper.

Anyone else see the receiver wear pictured above as unusual?

5

From what you describe, having run the upper before without problems, then having a problem with the upper on a lower that is GTG, it sounds as if there is a misalignment of the upper. I would think if there was that much binding on a “press check” and it was a gas tube / carrier key alignment issue, the tube and key would definitely show signs of the misalignment. If you are sure that the tube is properly aligned it may very well be that the upper has been flexed. The PRI block is not meant for barrel removal / replacement and it is possible that you flexed the upper out of shape.

As for the picture of the wear from the charging handle, that is normal, especially as round count goes up. Does the charging handle by itself glide in and out smoothly? Does the carrier glide smoothly onto the gas key? Is the barrel properly torqued? Any small movement in the barrel? Are the threads on the barrel and the upper in good shape?

FWIW - This is an albeit high round count Colt, but here are the same wear marks, in a great functioning rifle. Of course I had to wipe the grunge away to see the wear marks.

Sux, but let us know how it comes out.

6

I’m interested in the potential tight chamber. Do you think you twisted the lugs or distorted the entire receiver? If it’s the receiver Quib’s test should verify it. If it’s the lugs then you could still have a tight chamber, correct?

You could probably still use your PRI receiver block if you used it like this one:

http://www.adcofirearms.com/itemdetails_.cfm?inventorynumber=1626