11.5 not cycling when unsuppressed

[Nuland]

11.5 bcm upper, that would consistently double feed when unsuppressed; however with the can attached the gun ran perfectly. i saw the thread below that seems similar, but i am not sure this an extractor issue. could this be a gas system problem instead?

[TacticalMark]

What does the entire rifle consist of parts wise? What ammo and mags? need more information please.

[Nuland]

What does the entire rifle consist of parts wise? What ammo and mags? need more information please.

Many thanks. It was an upper built for me by a dealer.

BCM BFH™ 11.5" Carbine Barrel
BCM Upper Receiver Assembly M4 Flat Top (M4 Feedramps)
KAC gas block
KAC URX II
KAC triple tap
KAC QDSS-NT4
Entire lower assembly is factory from a KAC SR15
Mags: PMAGS, same thing happened with multiple mags tried

This was at a class. The rifle repeatedly choked unsuppressed, then ran perfectly for the next 1.2K rounds after I threw the can on.

This was with PMC 223 ammo

[Iraqgunz]

Double feeds are a magazine problem. Please explain what you mean and detail the components used.

Have you contacted BCM about the issue? How long has this been happening?


11.5 bcm upper, that would consistently double feed when unsuppressed; however with the can attached the gun ran perfectly. i saw the thread below that seems similar, but i am not sure this an extractor issue. could this be a gas system problem instead?

[TacticalMark]

Was the ammo PMC bronze? PMC bronze is known and from my experience a weak low power ammo. What weight of buffer are you using?

[Nuland]

Yes, it was bronze. It was whatever factory buffer came with the SR15 which is a standard 3 oz buffer if I remember correctly. Someone mentioned that it could be a problem with the gas port size. You think it is reasonable to experiment with different buffer weights and have one for suppressed and one for unsuppressed?

[TacticalMark]

You think it is reasonable to experiment with different buffer weights and have one for suppressed and one for unsuppressed?
Yes it might be worth a try, could you explain the malfunctions in better detail, was the rifle trying to feed two live rounds? Live round into spent case in the chamber?

[Digital_Damage]

Yes, it was bronze. It was whatever factory buffer came with the SR15 which is a standard 3 oz buffer if I remember correctly. Someone mentioned that it could be a problem with the gas port size. You think it is reasonable to experiment with different buffer weights and have one for suppressed and one for unsuppressed?

Buffer weight and spring for sure. The standard sr15 lower was designed for a custom length gas system.

The SR15 11.5 SBR's use a 4.5oz buffer

[Iraqgunz]

You should be using at least an H2 buffer. In my experience with 19K rounds through my BCM 11.5 and 7.5K through my 12.5 you can use an H3 buffer if you are shooting NATO spec ammo. With PMC Bronze you probably want to go with an H2.

[markm]

By double feeds, do you mean TWO live rounds trying to chamber? Or a live round and the empty that just got fired?

[Nuland]

You should be using at least an H2 buffer. In my experience with 19K rounds through my BCM 11.5 and 7.5K through my 12.5 you can use an H3 buffer if you are shooting NATO spec ammo. With PMC Bronze you probably want to go with an H2.

Many thanks, you guys all seem extremely knowledgable. I'll try this out.

[Nuland]

By double feeds, do you mean TWO live rounds trying to chamber? Or a live round and the empty that just got fired?

To be honest, I did think it was two live rounds. But based on what you are saying this may not make sense. It has been a while since I have shot this rifle. I will try reproduce this soon.

[Iraqgunz]

So what you are actually experiencing is a failure to extract (which interrupts the cycle of operation) and then a live round feeds into the casing. This is directly related to extraction and/or possibly a tight chamber.

Have you been in contact with BCM about this issue?


To be honest, I did think it was two live rounds. But based on what you are saying this may not make sense. It has been a while since I have shot this rifle. I will try reproduce this soon.

[Nuland]

So what you are actually experiencing is a failure to extract (which interrupts the cycle of operation) and then a live round feeds into the casing. This is directly related to extraction and/or possibly a tight chamber.

Have you been in contact with BCM about this issue?

Interesting, thanks.

I did not contact BCM but only because I did not purchase the complete upper from them but rather purchased parts and had a dealer assemble the upper.

[PatrioticDisorder]

So what you are actually experiencing is a failure to extract (which interrupts the cycle of operation) and then a live round feeds into the casing. This is directly related to extraction and/or possibly a tight chamber.

Have you been in contact with BCM about this issue?

Could it not be an ammo issue with over pressured ammo? OP did you examine the brass by any chance?

I mention this because I had some "mystery" malfunctions with Independence XM193 a while back with a DD mid length (spent casing not ejecting, live round trying to feed into it) and I didn't figure it out until I took my new 11.5 SR-15 upper out for the first time (using the old independence ammo, I was trying to shoot it to get rid of it) and had to mortar the new build to get a casing of that crap ammo unstuck (BCG was 1/4 from being closed and completely stuck), this was within the first 10 rounds out of that upper, I also had 1 malfunction prior to that (2 in 10 rounds). Examining the brass ALL were bulged around the entire ass end of the casing and at least 1 had the primer blown out. Same build ran like a charm after that with Federal XM193.

[Nuland]

Iraqgunz et al: I was mistaken, I actually was running an H2 buffer at the time; however, I do believe the buffer spring was indeed the stock SR15 spring. Just in case I have switched the BCM bolt with a brand new BCM bolt (without O-ring installed). Additionally, I have ordered an H and H3 buffer as well as sprinco red and blue springs and standard mil-spec buffer spring. My question is: do you have a recommended spring and buffer combination you would recommend starting out with for suppressed and unsuppressed, respectively. I plan to be shoot XM193. I will try various combinations at a carbine class in a couple weeks, just wondering what you recommend starting with.

[TheChunkNorris]

You might want to check out a Vltor A5 Kit. It helped me sort out an issue I was having with my H&K MR556 with a 10.4" set up.

[Iraqgunz]

If you look at my posts above I specifically mention the buffer/spring combos.


Iraqgunz et al: I was mistaken, I actually was running an H2 buffer at the time; however, I do believe the buffer spring was indeed the stock SR15 spring. Just in case I have switched the BCM bolt with a brand new BCM bolt (without O-ring installed). Additionally, I have ordered an H and H3 buffer as well as sprinco red and blue springs and standard mil-spec buffer spring. My question is: do you have a recommended spring and buffer combination you would recommend starting out with for suppressed and unsuppressed, respectively. I plan to be shoot XM193. I will try various combinations at a carbine class in a couple weeks, just wondering what you recommend starting with.

[Nuland]

If you look at my posts above I specifically mention the buffer/spring combos.

Thanks. Unless I am missing something, I think this is all you mentioned: "You should be using at least an H2 buffer. In my experience with 19K rounds through my BCM 11.5 and 7.5K through my 12.5 you can use an H3 buffer if you are shooting NATO spec ammo. With PMC Bronze you probably want to go with an H2."

My main question is regarding buffer and spring combinations including the springco options. Are you saying that my first trial combination should be an H3 with mil spec ammo and spring?

[TinyCrumb]

Buffer weight and spring for sure. The standard sr15 lower was designed for a custom length gas system.

The SR15 11.5 SBR's use a 4.5oz buffer

This. I was also having function issues with a build that I used a SR-15 lower for. When I pulled it apart I noticed the buffer is extremely light (around carbine weight) and the spring was much shorter than my mil-spec springs.

Grab a heavier buffer (start with H2 or so) and get a standard mil-spec spring and you'll probably be GTG.

Also, if the gas block is slightly misaligned it would explain the proper function suppressed because of the increased pressure.

[Thump_rrr]

You may also want to check your gas tube for wear or misalignment.
http://i24.photobucket.com/albums/c40/Thump_rrr/E230891C-EAAB-456D-985B-76A86713470B_zpsdywqfftx.jpg (http://s24.photobucket.com/user/Thump_rrr/media/E230891C-EAAB-456D-985B-76A86713470B_zpsdywqfftx.jpg.html)