First post , and first bb build quest.

[Redtail]

Hi all.
Found my way here on a recommendation from another board.
Put together a thumper that won’t cycle ..so far. Cycles and ejects by hand fine.

Relevant's below.

Davidson/aero XL upper 16” barrel and gas system. Aero lower /mil spec lpk ,Lancer mag with Tromix follower and half moon mod. A2 stock/ spring 5 oz. buffer.
Using hornady black 250.

Edit to add, BCA barrel and Toolcraft BCG.

Where I am so far with troubleshooting.

First thing tried was to add a BCM extractor spring.
Eliminated stove pipes but still no cycle or lock back.

Went through gas system, block port is aligned, no blockages, gas key on carrier is tight, gas rings on bolt are correct.
Barrel port measured .066 from factory ;8.375” from barrel crown. I opened port up to .078.

Third range trip with larger port size gave 1 bolt lock back on half dozen or so tries. Sometimes she will chamber second rd, sometimes will not.

I believe the barrel port may still need to be larger.
All suggestions or input appreciated .

RT

[Bmt85]

Do you have another weight buffer to try?
Are you running the upper soaked in oil?
Did you clean it before the last range session?
Did you check the extractor to make sure it's not biting into the cases?

I'm not trying to treat you like you don't know anything. I don't know you, and just want to cover some basics.

Work on bolt hold open first. Double check to make sure that when the bcg is all the way rearward, that there is some room between the bolt and bolt catch. Also check that the bolt hold open comes up high enough to catch a good amount of the bolt.

For me, I don't like to mess with new builds until I put at least 100rds through them, unless there is a significant issue. Run it super wet with oil, and do a good cleaning after every range trip until it's running 100%. After it's running, go light on the oil. Some builds can be gassed right, but tolerances can be tight on everything, so it ends up choking.

If your having trouble feeding, test to make sure the extractor or ejector aren't causing issues. Pull the bolt, hold it with the bolt face straight up, grab a piece of brass, slip the case under the extractor, and push back against the ejector until the case is inline with the bolt body. This should take a bit of force. Slowly let off the ejector, see if you feel anything hanging up, if so, somethings wrong (debris in the ejector channel, bad spring, something). Keep letting off the ejector, when your not putting any pressure against it, let go of the case. It should fall freely from the bolt. If not, that means the extractor is biting into the case. Lightly file the corners so they are slightly rounded. That should fix it.

I'd start there.

[Redtail]

Thank you for the fast reply.
There were 40 rds. through it first trip, only 8 on the second .

Do you have another weight buffer to try? Yes,I could remove the weights to bring it down , or use a carbine spring/buffer and spacer in tube. Also going to swap the lower with known good onenext trip.
Are you running the upper soaked in oil? First trip, yes really soaked it.
Did you clean it before the last range session? Yes took everything apart and cleaned excess lube.
Did you check the extractor to make sure it's not biting into the cases? I did not, I will . I appreciate your suggestions. The brass looks normal.

Edit to add. This is a "fixed "mag build , due to being in a 2A oppressed location. The action must be opened to load.
Although I don't see how the mag release being locked would have any bearing on function , but thought it worthy of mention.

RT

[Bmt85]

I would try lowering the weight to see if it cycles. If it does, leave it like that for a bit, after a range trip or 2 try upping the weight back up. If it still doesn't function you can go back to the lighter weight buffer and try again. If all else fails, try opening the port a little more. I'd still run it wet until you get it functioning correctly.

I don't think the fixed mag is affecting anything.

[dantheman]

I guess I'm no longer the only New Yorker running a fixed magazine 450 Busmaster...

Welcome aboard, Redtail.

Dan

[gunner1]

Thank you for the fast reply.
There were 40 rds. through it first trip, only 8 on the second .

Do you have another weight buffer to try? Yes,I could remove the weights to bring it down , or use a carbine spring/buffer and spacer in tube. Also going to swap the lower with known good onenext trip.
Are you running the upper soaked in oil? First trip, yes really soaked it.
Did you clean it before the last range session? Yes took everything apart and cleaned excess lube.
Did you check the extractor to make sure it's not biting into the cases? I did not, I will . I appreciate your suggestions. The brass looks normal.

Edit to add. This is a "fixed "mag build , due to being in a 2A oppressed location. The action must be opened to load.
Although I don't see how the mag release being locked would have any bearing on function , but thought it worthy of mention.

RT

As a New Yorker with a fixed mag 450 Bushmaster as well, let me give you a tip: it's ALOT easier to load that fixed mag if you actually separate the upper from the lower.

[gunner1]

I guess I'm no longer the only New Yorker running a fixed magazine 450 Busmaster...

Welcome aboard, Redtail.

Dan

Me too!

[Redtail]

Update thread time.
Range time is hard to come by , especially with holidays and such.
Tried the carbine buffer/ spring. No change.
Back to A2.
Opened up gas port one more size, measures .092 now.
Give her another go ASAP.
Still under 50 rds. through her, as I am an impatient bastard and when she malfunctions I hate to waste ammo.
Will Post after next trip.
Run her wet and hard is the plan.

May be an adjustable gas block in my future.

[Redtail]

pepe feels.JPG (55.14 KiB) Viewed 10116 times

RESOLVED.
.092 Port size did the trick.