.450 BM wont cycle.

[redtail]

Fixed mag Thumper wont cycle.
Davidson/Aero upper, Tromix follower in a Lancer mag. A2 trinity stock/spring. Toolcraft BCG. Aero lower- milspec lpk.

1st trip to range, 35 rds. stovepipes fail to cycle.
Started out with a light lube , after having the no cycle issue I soaked it good. Problem got worse with the wet.
I also had a few ftf with light strikes on primer.

Installed BCM extractor sping. Cleaned her good.

2nd trip, 5 rds. BCM extracter spring solved the stovepipe issue.

Rifle will still not cycle , fails the 1 rd. bolt hold open test every time.
Loaded 2 rds. fired one, after ejection, pulled trigger got the click..opened bolt to an empty chamber
On one rd. of 5 I tried , firing single shot, the bolt was partially locked back...not completelythough; had to manually charge it the last inch or so.
Ideas before I go further, Under gassed and not cycling bolt is my next guess ?

Spec sheet says .750 micro gas block carbine length ss tube

 

[dsdmmat]

1) Check the gas block alignment in relation to the gas port.

2) check serviceability and orientation of bolt gas rings.

3) drill out gas port a few thousands of an inch and try again. Repeat as necessary.

4) drill out gas port a lot and install an adjustable gas block

 

[redtail]

Gas rings are good on bolt, made sure when I put it together.
I'm a rook when it comes to upper assembly,I can't see how to get the tube off let alone check the port to block hole...lol
?

 

[Phauxtoe]

My first thought own a new malfunctioning build
correct springs?
trusted ammo?

 

[redtail]

My first thought own a new malfunctioning build
correct springs?
trusted ammo?

Stock A2 buffer & spring and milpec trigger group passes function test.
Hornady black 250g ftx

 

[redtail]

Stock A2 buffer & spring and milpec trigger group passes function test.
Hornady black 250g ftx

I guess I could swap out the lower.

 

[Edwardteach72]

What size is the gas hole?

 

[Edwardteach72]

Ok sorry just saw it is .750

 

[Edwardteach72]

You may want to bump it up to .875 hole

 

[Edwardteach72]

Never mind looks like tromix for their barrels the 18 and 16" barrels are the .750 and the 12.5 and 10" are .875

 

[ManrisFrack]

First, what does your brass look like? If it's frosted on the body and the head is all chewed up it may be an indication that the rifle is way OVER gassed. This means the bolt is trying to unlock and pull the case out before the pressure has dropped enough to let the case spring back off of the chamber walls. If your brass looks normal, you're probably under gassed.

Check your gas system for alignment and leaks before you go drilling on your barrel.

You checked the rings, that's good.

Check the gas key on the carrier, make sure it's tight and nothing is obstructed.

Check that the end of the gas tube is aligned to the key and mates properly as the carrier goes forward. The tube also needs a proper flare to seal with the gas key, so take a look at that.

By taking the handguard off, you'll be able to see the gas block. Make sure there are no soot marks coming out from under or around the gas block. Those are indication of leakage.

Take the gas block off (or just loosen and slide toward the muzzle). You should see a soot ring where the hole in the block sits on the barrel. The ring must fully encompass the hole in the barrel.

ETA:
{Also make sure the hole in the gas tube is drilled and aligned so that it matches the hole in the gas block. Blow air through the gas tube to make sure it doesn't have any obstructions.

Check the gas port in the barrel for metal chips or other contamination.}


If all this stuff checks out, then think about drilling the barrel's gas port a few thou larger.

 

[Edwardteach72]

^check what @ManrisFrack is saying, I have read a lot that the .450bm is pretty over gassed

 

[redtail]

I'll follow up.

 

[dsdmmat]

Ok sorry just saw it is .750

The gas hole is not 3/4 of an inch. The gas block is probably 3/4 inch and the gas port should be somewhere between 0.070” and 0.099”

 

[Acer-m14]

unpin the mag and try .. maybe Thumper is telling you it hates being in NY LOL ..

 

[Eddie Gunks]

Shit upper. Send it back.

Most likely gas hole/ orientation issue. But may have other issues as well.

 

[redtail]

I can't find a set screw anywhere on this handguard, could it be threaded onto the barrel nut only?
Handguard listed as Rem/DPMS M-111. Can't find any helpful info with web search.

 

[Edwardteach72]

I can't find a set screw anywhere on this handguard, could it be threaded onto the barrel nut only?
Handguard listed as Rem/DPMS M-111. Can't find any helpful info with web search.

So this is what your barrel nut looks like. Looks to be just threaded on? Thsts weird

 

[Edwardteach72]

And here is the locking ring?

 

[redtail]

I thought it was odd. Must be indexed and thread locked.
Thank you for that info, my duck search didn't come up with much.

 

[Edwardteach72]

I thought it was odd. Must be indexed and thread locked.
Thank you for that info, my duck search didn't come up with much.

Yea that is weird.. idk how I feel about that system especially for a thumper. But hopefully it helped. Keep us updated

 

[ICEMAN]

I would try a H3 buffet before messing with gas system.

 

[redtail]

The block port measurement is most likely off a tiny bit , tough spot to get at.

The gas key on the bcg seems good barely any air escapes when I blew into it ,fingers blocking the holes.

 

[ManrisFrack]

I'll follow up. View attachment 56821

Tough from pics but those don't look too bad. I see a few with scratches parallel to the bore but the rims look really good. No swipes or deformation on the head?

Gas system looks good so far. You can see a faint impression of the ring around the gas port in the pic where you're measuring it. As long as the barrel port is fully surrounded by the block's port, it's fine. Yours looks good. Take the rear-most set screw out of the gas block and check the tube/block junction for alignment and debris.

@ICEMAN 's suggestion to switch buffers is a good one. Just gotta figure out what's in there now and whether you need to slow the action or speed it up. It'll cost time and a little bit of money to get different weight buffers but that's reversible. If you go too far with drilling the port, you'll need an adjustable gas block or carrier key.

 

[redtail]

Thank you to all who replied and offered help for the newb on this one.

Better brass pic.
A2 buffer weighs @5 oz.
A2 springs from severel A2 kits look and feel the same.
I had an old A2 buffer in the box from a .308 R10, decided to compare.
The A2 from the .308 is shorter ,yet almost same weight 2@ just .02 oz heavier than the 5.56 A2 buffer.

?

 

[ManrisFrack]

Yep, I missed the fact that you're running an A2 stock with a carbine ported barrel. It's probably ported to run on a carbine buffer/spring/tube system.

Did you buy the rifle complete or just the upper?

The rifle buffer is about 2oz heavier than a standard carbine buffer. That may well be your problem. You could take the weights out of the buffer or replace them with plastic or aluminum weights. I know KAKIndustry.com sells aluminum weights. You could probably find other companies that do too.

Or you could drill the port out one drill size at a time until it works. You trying to get it working for Saturday?

 

[Edwardteach72]

Yup 2nd the carbine everything. The 450bm was designed around carbine/mid length and not rifle lenght system

 

[BDinPutnam]

I read the thread title and thought it said "went cyclic" at first!

 

[redtail]

Yep, I missed the fact that you're running an A2 stock with a carbine ported barrel. It's probably ported to run on a carbine buffer/spring/tube system.

Did you buy the rifle complete or just the upper?

The rifle buffer is about 2oz heavier than a standard carbine buffer. That may well be your problem. You could take the weights out of the buffer or replace them with plastic or aluminum weights. I know KAKIndustry.com sells aluminum weights. You could probably find other companies that do too.

Or you could drill the port out one drill size at a time until it works. You trying to get it working for Saturday?

I was, wanting a DRT deer gun.
Waited to long to range test her though, didn't anticipate more than sighting in. The trusty Rem 700 will start instead.

I will bring the smaller .308 rifle buffer and also a carbine 3oz. next range trip.
Any known issues running a carbine buffer with rifle length spring/tube ?

 

[Edwardteach72]

I was, wanting a DRT deer gun.
Waited to long to range test her though, didn't anticipate more than sighting in. The trusty Rem 700 will start instead.

I will bring the smaller .308 rifle buffer and also a carbine 3oz. next range trip.
Any known issues running a carbine buffer with rifle length spring/tube ?

Not sure about that specific set up but my pistol thumber has run (very low round count so far) 2 boxes flawlessly. 12.5" barrel with carbine length gas system and .875 gas block journal (their 18" barrel is mid-length gas system and .750 gas block journal)

This is paired up with what came in the PSA pistol build kit, so back half is using carbine buffer spring, 6 positioning mil-spec buffer tube, and standard carbine buffer.


Not sure if that helps you or anything. Goodluck this weekend and hopefully you will get thumber up and running