Update:
Just got back from the range. The last round hold open issue on the 10.5" 5.56 is solved. I had installed a Forward Controls Design bolt catch initially when I test fired. I swapped for a USGI bolt stop and it worked flawlessly.
On the .300 Black build I did two things: 1) I slightly filed the very top edge of the bolt stop...just enough to round off the sharp edge at the top and 2) used the standard VLTOR A5 spring in place of the green Sprinco. This seems to have solved the bolt lock issue. Unfortunately two new issues cropped up:
1) the empty cases are just dribbling out of the ejection port. I may be getting 1 foot of ejection or so at 2-3:00 and occasionally a spent case would get hung up. This was surprising since the cases were getting kicked out a good distance at about 4:00 last time.
2) I put about 10-15 rounds downrange with no issue and made a quick adjustment to the optic (PA 3X micro prism) as I had to get the bullet strikes about an inch to the right. I put another 3-5 rounds downrange and noticed that I wasn't hitting the target anymore. I could see the dirt strikes but I wasn't even near the paper. I checked the suppressor and didn't see any signs of a baffle strike. This is a Dead Air Sandman S on a KeyMo brake. I made sure the suppressor was tight on the mount (it was) and when I got home made sure the keymo brake hadn't started to work loose (it hadn't).
Either my micro prism crapped the bed and completely lost its zero, or my suppressor is now throwing rounds about at least two feet to the left of aim now.
Ive got the suppressor recessed under the handguard a few inches so I couldn't shoot with it dismounted I'm going to grab a spare optic and take the hand guards off so I can shoot with and without the suppressor to see what is going on.
Ive got two new builds that I test fired today that I need some help with:
Build 1:
.300 Black
8.5" Rainier barrel (pistol length gas)
non-adjustable gas block
Colt BCG
DeadAir Sandman-S
Silencerco ambi receiver (bolt hold open both sides)
Vltor A5 w/Sprinco Green Spring and A5H2 buffer
.300 Black specific PMAGs.
Suppressor stayed mounted the whole time.
Shooting factory 124 grain supers SIG and S&B) and 220 grain subs (Federal).
Problem: On the supers the bolt would lock back with one or two rounds left in the magazine. It wasn't consistent doing this. Shooting subs I got last round lock back with no malfunctions.
The bolt face was past the stop (not hung up on the mag or anything).
Ejection was at 3-4 o'clock. Tried 2 different mags.
Tore down the gun for cleaning and the top surface of the bolt hold open seemed slightly beat up with very slight peening where the bolt lugs would ride over
the top edge when the lever is depressed to run the bolt into battery.
I've tried to post a photo of this...hopefully it works.
I was thinking maybe this was a BCG velocity problem on the supers but not sure. I probably should have swapped out the buffer for the H3 but I ran out of supers.
Build 2:
5.56 10.5" Criterion Core (carbine length gas)
Superlative adjustable gas block
Colt BCG, Aero standard lower
Vltor A5 with Sprinco Green (tried both A5H2 and A5H3 buffers)
Gen 3 PMAG
Shooting IMI 77 grain and 55 grain Federal
No suppressor
Problem: loading one round at at time in the mag I would consistently get last round hold open. Loading more than one round in the mag (2, 6, 8, etc)
I would get consistent cycling with ejection at 4 o'clock but the bolt would not lock back on the last round.
I tried both the H2 and H3 buffers. I also confirmed the superlative was 18 clicks from the closed position and also tried opening the bleed a few clicks to see if that was the issue.
Nothing seemed to help.
Would lock back when loading one round in mag but anymore than 1 round it wouldn't hold open. I stupidly only brought one mag with me.
I appreciate any input on either problem.
Pics of the hold bolt stop from the .300 Black build:
Attached File Attached File