Trouble with an AR build - thoughts?

[yukonjasper]

I need learned opinions about an issue that I came across this weekend while dialing in a Grendel Build from last winter – 2017-2018.  (I know, I know – finally got round to it – first time on the range had never been shot before. Lots of life events last year kept me from getting this out to shoot. Anywho…………)

-      PSA blem lower

-      MBT trigger group

-      Aero upper – 18in stainless barrel

-      Monmouth – Nickel Boron BCG

-      BCM Vlotor Charging handle

So, I finally get to the range to dial in the Nikon Monarch 3 4-16x42 scope and ran into some issues. 

-      First problem were FTF the first round off the mag – only loaded 2-3 rounds because I was sighting.  This was a 5 round mag (forget the manufacturer – C Industries? maybe), so I slap the mag to get it to seat and continue to get some issues picking up the first round.  Was using Wolf steel case, so I chalked it up to maybe those are not friendly or maybe the mag is not fitting correctly since it was very inconsistent. I brought a box of Federal Fusion, so I switched over to see if the Brass would make it better. Still some issues – (my thought – Damn mag isn’t seating correctly – should have ponied up for the more expensive mags – my bad for trying to go budget). Should also add that I had several misfire/light strikes on the Wolf ammo.

-      Second – more concerning problem.  After a couple of rounds, I had the bolt lock back, so solidly that the bolt would not move, the bolt release was not activated and could not be moved. Further, when I tried to push the takedown pins, they were really, really tight, everything was bound up.  I managed to get the front pin disengaged –bolt springs forward once the pressure is relieved.  Rear pin came out at that point normally.  I separated the two halves, inspected them for obvious signs of a problem – nothing(admittedly, I am not sure what I’m looking for anyway, but thinking aluminum shavings etc.), so I reassembled.  A few shots later, same thing.  Pins were jammed again. Was able to get them out eventually, but much more effort than is normal.  At that point, I was somewhat dialed in – about an 1.5” low and ½” right at 25 yards – could have used some tweaking, but at least close.  I ended the session since I didn’t know how to remedy it. 

I was struck when I first mated the upper and the lower that they really fit tight, no slop, very solid – TOO Solid/Tight? 

I am a neophyte in the world of ARs, so I’m asking those who have more experience if you can point me in the right direction. 

 

Thanks in advance for your thoughts. 

[Ghost]

How do the primers look on the fired cartridges?

[photogpat]

Assuming .223/556

What buffer and buffer spring are you running?

[Ironbear]

-      Second – more concerning problem.  After a couple of rounds, I had the bolt lock back, so solidly that the bolt would not move, the bolt release was not activated and could not be moved. Further, when I tried to push the takedown pins, they were really, really tight, everything was bound up.  I managed to get the front pin disengaged –bolt springs forward once the pressure is relieved.  Rear pin came out at that point normally.  I separated the two halves, inspected them for obvious signs of a problem – nothing(admittedly, I am not sure what I’m looking for anyway, but thinking aluminum shavings etc.), so I reassembled.  A few shots later, same thing.  Pins were jammed again. Was able to get them out eventually, but much more effort than is normal.  At that point, I was somewhat dialed in – about an 1.5” low and ½” right at 25 yards – could have used some tweaking, but at least close.  I ended the session since I didn’t know how to remedy it.

Well, I can't hardly be considered an expert on AR's, but your symptoms...
-Jammed open on shooting
-Pressure on takedown pins
-Pressure and bolt released on opening

seem to indicate some kind of interference between the BCG and the lower half. I would look most closely at the things hanging down from the top and the things sticking up from the bottom.

From (distant, fuzzy) memory, the most prominent thing would be the hammer. I could see that if the hammer got hung up most, but not all the way back it could wedge the BCG... but I can't image what would jam it.

[yukonjasper]

Sorry, did not look. Misfires were light strikes.

[Rip Van Winkle]

Sounds like the bolt carrier is binding on something in the lower receiver. You didn't say if you'd dropped the poorly fitting magazine, could that be the problem?

Otherwise, you'll need to isolate what's binding between the two halfs. The hammer maybe? Are you sure the trigger group is assembled correctly?

[INOR]

Check the buffer tube and buffer retaining pin. Bolt sounds like it’s hanging up there.


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[yukonjasper]

It's a 6.5 Grendel upper. Dropping the mag had no effect. Wondering what would cause the intermittent problem? If the hammer was catching, wouldn't that be every time because the lack of clearance would cause consistent return to battery issues?
I appreciate the input.

[Holland&Holland]

Did you try one round in the mag? Does it lock back correctly after that one round? Do you have an adjustable gas block? Does the mag and the bolt catch interface correctly?

[yukonjasper]

Did not do one round. No adjustable gas block. I did notice the bolt release worked initially, when jammed, it was not movable and in the depressed position.

[Holland&Holland]

Did not do one round. No adjustable gas block. I did notice the bolt release worked initially, when jammed, it was not movable and in the depressed position.

Depressed as in engaged or released?

[yukonjasper]

Depressed like I pushed it in to release the BCG and jammed in that position.

[Holland&Holland]

Depressed like I pushed it in to release the BCG and jammed in that position.

Where is the BCG at that time? It must be putting some pressure on it or the mag otherwise there is nothing to put pressure on it and keep it jammed.

[yukonjasper]

It gets jammed about a quarter to a third of the way forward. I applied some force to the charging handle but not a lot. Didn't want to bend or damage it.

[Ghost]

Do any parts show signs of wear? ie finish worn off?