AR15.Com Archives
 300 blackout won't cycle
Codyboy  [Member]
1/27/2012 6:37:55 PM
EA shaw bbl. , pistol length gas tube, non adjustable gas block, not suppressed.

Bolt not traveling far enough back to strip a round from mag.

It fires and ejects though.

Have had a suggestion to drill the gas port to 3/32"

What do ya'll think?

Thanks

ETA
This is with subsonic ammo. Haven't tried supersonic yet.
Paid Advertisement
--
TUBBY  [Team Member]
1/27/2012 6:50:27 PM
I would check the alignment of the gas tube and gas block first. Drill a bigger hole in the gas tube.
Codyboy  [Member]
1/27/2012 6:55:55 PM
OK, just tried a 125gr supersonic round and it will cycle.

So just with the subsonic is where the trouble lies.
VT_K9  [Team Member]
1/27/2012 7:01:29 PM
What is your barrel length? Some of the shorter barrels won't run Subs without modifications. You should reserve the drilling of the gas port to a last resort. You should also measure your hole and make sure you are only stepping up the absolute smallest amount (ie change to a different type of bit sizing).

In the meantime I am assuming this is a new upper? How is the gas key staked on and is it tight. How are your gas rings? What is the weight of your buffer. I believe the subs (or even all 300 blk) prefer the H2 weighted buffer. How old is your buffer spring?

The springs and buffer are good mods to have either updated or upgraded anyway and they are inexpensive. Start there before drilling.

Mike
DVANNCVANN  [Member]
1/27/2012 7:06:50 PM
Try taking the weight out of the buffer. This should allow it to cycle if it ejecting now. You can also open the gas port a bit but you may be overgassed with supersonic. Adjustable gas block is also an option with an enlarged gas port.
Codyboy  [Member]
1/27/2012 7:10:37 PM
Originally Posted By VT_K9:
What is your barrel length? Some of the shorter barrels won't run Subs without modifications. You should reserve the drilling of the gas port to a last resort. You should also measure your hole and make sure you are only stepping up the absolute smallest amount (ie change to a different type of bit sizing).

In the meantime I am assuming this is a new upper? How is the gas key staked on and is it tight. How are your gas rings? What is the weight of your buffer. I believe the subs (or even all 300 blk) prefer the H2 weighted buffer. How old is your buffer spring?

The springs and buffer are good mods to have either updated or upgraded anyway and they are inexpensive. Start there before drilling.

Mike


16" bbl, all new, upper, lower everything. Not sure about gas block. Its my cousins rifle. I know it is a low pro block but don't know anymore about it.
Not sure which buffer either.

Was going to drill the next size larger and retest.

Will find out more info on buffer weight.

Thanks

Codyboy  [Member]
1/27/2012 7:12:43 PM
Originally Posted By DVANNCVANN:
Try taking the weight out of the buffer. This should allow it to cycle if it ejecting now. You can also open the gas port a bit but you may be overgassed with supersonic. Adjustable gas block is also an option with an enlarged gas port.


So make the buffer lighter?

I'm thinkning its a standard buffer and spring.

DisplayName  [Member]
1/27/2012 7:19:05 PM
"not suppressed" that's your problem.
Codyboy  [Member]
1/27/2012 7:29:29 PM
Originally Posted By DisplayName:
"not suppressed" that's your problem.

Seriously?

or is that sarcasm?

Being honest because I don't know.

He will probably later just not now.
agBQ08  [Member]
1/27/2012 7:30:10 PM
Originally Posted By DisplayName:
"not suppressed" that's your problem.


+1

300 BLK rides a very very fine line of a lot of different physics principles that allow both subs and supers to cycle. However, certain guns have a gas port that is a fraction too small to cycle subs without the additional back pressure of a suppressor. This isn't a bad thing as it increases reliability with supers. As long as you gas block is lined up, carrier key is secure, and gas rings are in place I wouldn't worry about it.

Shoot supers to your heart's content until you have a can to throw on there and I bet you will find that it cycles subs just fine then.
Codyboy  [Member]
1/27/2012 7:35:55 PM
Yea we discussed it(being suppressed)

Assumed more pressure if suppressed so would probably cycle fine.

If he drills it now he can always get an adjustable gas block
agBQ08  [Member]
1/27/2012 7:37:50 PM
Originally Posted By Codyboy:
Yea we discussed it(being suppressed)

Assumed more pressure if suppressed so would probably cycle fine.

If he drills it now he can always get an adjustable gas block


I hate adjustable gas blocks as it is just more parts to break. However, do what suits you and shoot the crap out of it. Good luck!
DVANNCVANN  [Member]
1/27/2012 8:28:39 PM
Take the weight out of the buffer and make it lighter. Less mass to move and an easy fix to put back. You can even have 2 buffers 1 weighted and 1 without and just swap them out according to what ammo you are shooting.
QuietRiot11  [Member]
1/27/2012 8:56:29 PM
Originally Posted By agBQ08:
Originally Posted By DisplayName:
"not suppressed" that's your problem.


+1

300 BLK rides a very very fine line of a lot of different physics principles that allow both subs and supers to cycle. However, certain guns have a gas port that is a fraction too small to cycle subs without the additional back pressure of a suppressor. This isn't a bad thing as it increases reliability with supers. As long as you gas block is lined up, carrier key is secure, and gas rings are in place I wouldn't worry about it.

Shoot supers to your heart's content until you have a can to throw on there and I bet you will find that it cycles subs just fine then.


+2

My Noveske 8" barrel won't cycle subs unsuppressed, but they warn you of that on their website. This is not a problem because I have no practical reason to shoot subs without my SDN-6 attached. It cycles supers suppressed and unsuppressed as well as subs suppressed great.
titleiiredneck  [Team Member]
1/27/2012 10:03:00 PM
Gas port should be .125, check alignment before you drill it out.
exhogflyer  [Team Member]
1/27/2012 10:46:36 PM
No suppressor is the problem......a can adds "backpressure" to the system so a subsonic load puts more pressure back thru the tube and then the bolt.Why does everyone want to shoot subsonic loads without a can? Very little point to this....
VT_K9  [Team Member]
1/27/2012 11:16:36 PM
The heavier buffer will change the unlock time to allow more pressure to build, causing a little bit longer push with more gas energy stored.

My experience was with a 7.3" 5.56 upper. It ran great with a Vortex flashhider. Put a KX3 on and then had too much pressure. I used a new buffer spring and the Endine buffer. I had it running as much of 100% as you can an AR.

I may have missed it, but does your bolt lock back when fired using an empty magazine? If not I would also check the extractor and ejector springs. They are usually the first parts to go.

Good Luck,

Mike
Codyboy  [Member]
1/28/2012 1:50:27 PM
Well the bbl was only drilled to a 1/16" (.0625).

We drilled it twice and ended up stopping at 3/32" (.09375)

It cycles fine now.

Thanks for the replies.

rezqme  [Member]
1/28/2012 10:51:25 PM
Originally Posted By VT_K9:
The heavier buffer will change the unlock time to allow more pressure to build, causing a little bit longer push with more gas energy stored.

My experience was with a 7.3" 5.56 upper. It ran great with a Vortex flashhider. Put a KX3 on and then had too much pressure. I used a new buffer spring and the Endine buffer. I had it running as much of 100% as you can an AR.

I may have missed it, but does your bolt lock back when fired using an empty magazine? If not I would also check the extractor and ejector springs. They are usually the first parts to go.

Good Luck,

Mike


This. At first thought, short stroking blackouts being easily remedied with a heavier buffer doesn't sound correct, but this is a great explanation of why.
AJK07734  [Team Member]
3/15/2012 7:25:27 PM
Someone pointed me to this thread, I had the exact same problem with Factory Supersonic ammo last weekend (SI Defense bbl), I put a Spike's ST-T2 buffer in thinking the heavier buffer would be preferred..looks like i screwed up by doing so, I'm going to give it a shot with a Std buffer before I break the whole damned thing down to check the Gas port size.
LoneStar1847  [Member]
3/15/2012 11:10:29 PM
The guys saying "no suppressor" are 100% right.

I have 2 300 blk's that will not cycle subs without a can. When I put the can on it... Flawless operation!
Both are carbine gas with .110 gas port on adjustable blocks so I can tune my supersonics. I run sub suppressed with gas block fully open.

As mentioned by somebody else, there is really no point in shooting subsonic rounds unsuppressed...

Sounds like the rifle is working how it should.
AJK07734  [Team Member]
3/15/2012 11:43:11 PM
Originally Posted By LoneStar1847:
The guys saying "no suppressor" are 100% right.

I have 2 300 blk's that will not cycle subs without a can. When I put the can on it... Flawless operation!
Both are carbine gas with .110 gas port on adjustable blocks so I can tune my supersonics. I run sub suppressed with gas block fully open.

As mentioned by somebody else, there is really no point in shooting subsonic rounds unsuppressed...

Sounds like the rifle is working how it should.

Supressor isnt an option here....which is why i havent bothered to try playing with Subsonic yet....Mine FTF'ed with supers though...i figure i'll try the buffer, if that doesnt work, then i'll break it down and see if the alignment is off, and also check the port size while i'm at it
Paid Advertisement
--