Assistance with under-gassed ar15 build?

[edzz]

Before free float hand guards were the norm I received a quality (not saying good quality) free floated 6x45mm upper from oLIMPIC arms. With no factory ammo available in 6x45mm I loaded a ladder from the book from bottom to top listed load. Nothing cycled, not even close.

So... back at home I disassembled it and found the barrel gas port/ gas block port were not lined up (the carbon on the barrel showed this). Repositioning the gas block forward to compensate for the "missing" front hand guard retainer that would have spaced the gas block properly solved the lack of gas.

So... My long way of saying, look at the carbon marking on the barrel for indication of bad port alignment.

 

[helloyatches]

For what? This is a new BCG.

UGhhh Alignment...

The rings openings are supposed to be staggered.

 

[kmk1012]

Before free float hand guards were the norm I received a quality (not saying good quality) free floated 6x45mm upper from oLIMPIC arms. With no factory ammo available in 6x45mm I loaded a ladder from the book from bottom to top listed load. Nothing cycled, not even close.

So... back at home I disassembled it and found the barrel gas port/ gas block port were not lined up (the carbon on the barrel showed this). Repositioning the gas block forward to compensate for the "missing" front hand guard retainer that would have spaced the gas block properly solved the lack of gas.

So... My long way of saying, look at the carbon marking on the barrel for indication of bad port alignment.

It initially was misaligned (my fault) but was corrected last night. It does have an end cap due to the wooden furniture on it. I like the idea of just shooting it to loosen it up a bit, we have plenty of ammo to do so, might just have to fire it several hundred times this weekend if the weather holds out. I'll post an update on any progress. Thank you all who helped!

 

[titsonritz]

UGhhh Alignment...

The rings openings are supposed to be staggered.

Again this is NOT true, see post #44.

 

[Dillon1000]

EASY WAY take a 5/64 drill and open up the gas port on the barrel
that's one size bigger than stock. 3/32 is two sizes bigger. If it still does not work you have problems some place else. When you drill be careful
not to put much pressure on the drill, you don't want to break through hard and ding the bottom inside of the barrel. Wire brush it to get metal
chips out. One size is not going to hurt anything.

 

[titsonritz]

EASY WAY take a 5/64 drill and open up the gas port on the barrel
that's one size bigger than stock. 3/32 is two sizes bigger. If it still does not work you have problems some place else. When you drill be careful
not to put much pressure on the drill, you don't want to break through hard and ding the bottom inside of the barrel. Wire brush it to get metal
chips out. One size is not going to hurt anything.

Stock what?

5/64" = 0.078125"
3/32 = .09375"

Both of which are too small for an 18" barrel with a rifle length gas system. As mentioned above, Crane MK12 SPR specs their GP size at .0995".

 

[CharlesAFerg]

Update: I fired the other (known working) rifle with my new bolt in it and it ran fine. I also swapped buffer systems between the two, even swapped entire lowers with each buffer separately. I suspect this is either a gas block issue or the ammo isn't powerful enough. I was told by Centurion arms that they always start with a standard spring with an H2 buffer, and the barrels are tuned for full power ammo and shooting suppressed.

However, isn't 62gr Aguila full power? Wouldn't an H2 make my problem worse?

 

[DirectDrive]

Update: I fired the other (known working) rifle with my new bolt in it and it ran fine. I also swapped buffer systems between the two, even swapped entire lowers with each buffer separately. I suspect this is either a gas block issue or the ammo isn't powerful enough. I was told by Centurion arms that they always start with a standard spring with an H2 buffer, and the barrels are tuned for full power ammo and shooting suppressed.

However, isn't 62gr Aguila full power? Wouldn't an H2 make my problem worse?

I would start with standard carbine spring and standard carbine buffer.
Get it running and then increase buffer weight until I like the recoil impulse/ejection pattern.

Don't know about the Aguila....does it match M855 specs ?

 

[CharlesAFerg]

I would start with standard carbine spring and standard carbine buffer.
Get it running and then increase buffer weight until I like the recoil impulse/ejection pattern.

Don't know about the Aguila....does it match M855 specs ?

It doesn't run any better with the standard carbine spring from my other rifle.

 

[DirectDrive]

It doesn't run any better with the standard carbine spring from my other rifle.

Do you have a Standard Carbine buffer to try ?
Full power ammo
Carrier and carrier tunnel wet with CLP

 

[CharlesAFerg]

Do you have a Standard Carbine buffer to try ?
Full power ammo
Carrier and carrier tunnel wet with CLP

I've done both of these things. I'll email centurion arms this weekend. I've ruled out everything else, so it really sounds like there's a misalignment with the gas block on the barrel. The gas block doesn't look like it sits at a perfect 12 oclock, anyway. It may be pinned, but it's possible they pinned it offset. I'm just so tired of switching every possible part and wasting my ammo testing this..

 

[bellarum]

I've done both of these things. I'll email centurion arms this weekend. I've ruled out everything else, so it really sounds like there's a misalignment with the gas block on the barrel. The gas block doesn't look like it sits at a perfect 12 oclock, anyway. It may be pinned, but it's possible they pinned it offset. I'm just so tired of switching every possible part and wasting my ammo testing this..

Any update? Did you get it to run?