16” barreled 300blk upper problem.

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

Moderators: gds, bakerjw, renegade

2BearArms
Silent Operator
Posts: 55
Joined: Thu Aug 23, 2012 11:53 pm
Location: SW CT

16” barreled 300blk upper problem.

Post by 2BearArms »

16” barreled 300blk upper problem.

I have a 16” barreled 300blk upper that shoots supersonic factory ammo just fine. But with factory Sub Sonic ammo it will eject the spent brass and the bolt would close but it didn’t strip off the next round to chamber it. It is very consistent. I also tried it with only 1 round in the mag to see if the bolt would lock open and it wouldn’t. all of this behavior is the same with or without the suppressor.

I tried a heavier buffer weight after reading up on this problem. I got one that allows me to easily change the weights in it. I haven’t yet tried to make it lighter only heavier than the standard 3oz. I just don’t know how light I should try. Any suggestions?

I also tried a different buffer spring. It is a Tubb’s Flat-Wire buffer spring. It still wouldn’t strip off the next round to chamber it. I also swapped the bolt carrier, it had an M16 carrier and I swapped it for the lighter AR15 carrier. No difference in functioning.

It is a mid-length gas system. And as I said it works just fine with supersonic factory ammo. But I think it is under gassed for use with Sub Sonic factory ammo. If I were to open up the gas port (without using an adjustable gas block) I assume it would be over gassed for supers. Is there a recommended gas port size for a 16” barrel mid length gas system?

I have a muzzle brake with Rocksett keeping it on so to remove the current gas block would be difficult because it needs to slide off the front on the barrel.

So my options are –

Play around with a lighter setup on the buffer weight that I have and see if that fixes it. And then see how that weight works with supers. A

And maybe when using this lower with the 5.56 upper I can just swap out the weight for the 3oz one. If I am swapping uppers it is easy to also swap the buffer weight.

And if that doesn’t work and I can get the fixed gas block off and install a new adjustable gas block I should be all set.

Or I could basically just give up and use this upper just for supersonic ammo.

So Tomorrow I am going to a range and will play with the buffer weight and see if that makes any difference.
20X11
Silent But Deadly
Posts: 1560
Joined: Fri Aug 03, 2012 11:43 pm
Location: Aurora, CO

Re: 16” barreled 300blk upper problem.

Post by 20X11 »

Try a different magazine?..Not all work well with the heavier subs.
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: 16” barreled 300blk upper problem.

Post by dellet »

Are you sure it is Mid Length? about 9" from the receiver instead of about 7" for carbine.

If it truly is mid length, you might be in trouble for subs without a lot of work, factory ammo may just not work.

Opening the port is an option, but would want to know the current size.

Heavier is the wrong direction.

Who made the barrel?
300 Blackout, not just for sub-sonics.
2BearArms
Silent Operator
Posts: 55
Joined: Thu Aug 23, 2012 11:53 pm
Location: SW CT

Re: 16” barreled 300blk upper problem.

Post by 2BearArms »

Thank you for the replies.

I just looked up how to measure the gas tube length. I was assuming that one measured from the middle of the gas block to the end of the tube in the upper receiver. And I now see it is measured from the part of the gas block nearest the receiver to the muzzle thread shoulder. So based on that I am getting 7.3” so it looks like it is a carbine length gas system. Thank you for questioning that since it made me double check me assumptions.

The upper is a SOTA Arms upper that I bought years ago. It would never properly cycled Subs. I only about a year ago got 2 suppressors. The lower is a DPMS that came with a 5.56 upper.

To answer the other questions asked.

I don’t know the current size of the port. I thought that going with a heavier buffer weight sounded counter to what I thought but since the existing one is 3oz I thought that was the lightest one that people use. So I figured I would try that out.

I have tried different mags and it makes no difference.

I also have an Other with a 12.5” barrel and it has an adjustable gas block and it will cycle anything I feed it (suppressed and not) with the proper setting of the gas block.
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: 16” barreled 300blk upper problem.

Post by dellet »

The 12.5" is probably pistol gas length, most are.

Gas tube length can be confusing as there is is about 2" past the barrel into the receiver. A port 7" down the barrel, needs a 9" tube.

Most common failures for carbine gas are, gas leaks or poorly aligned block, too much weight, bad loads, small port. Then you get into things like tight moving parts.

With a good flash light you can generally look through the handgaurd to try and see carbon tracks from under the gas block or where the tube goes in.

You should be able to point the rifle down and push the carrier back about an inch or so, enough to see the bolt face and let it go. Is should drop freely enough to fully seat in the chamber.

Carbine buffer is a bit under 3 OZ and a full auto carrier are what the specs call for.

It's probably worth removing the gas block and checking two thing. Port size and look at the carbon ring. The port should be centered in a ring of carbon. Not uncommon for the block the be placed wrong and restricting the gas flow.

Last, for factory ammo, maybe try the Remington UMC subs. they are not the best, but they are extremely reliable. if they don't cycle unsuppressed probably nothing will.
300 Blackout, not just for sub-sonics.
2BearArms
Silent Operator
Posts: 55
Joined: Thu Aug 23, 2012 11:53 pm
Location: SW CT

Re: 16” barreled 300blk upper problem.

Post by 2BearArms »

dellet wrote: Wed Dec 02, 2020 5:53 pm The 12.5" is probably pistol gas length, most are.

Gas tube length can be confusing as there is is about 2" past the barrel into the receiver. A port 7" down the barrel, needs a 9" tube.

Most common failures for carbine gas are, gas leaks or poorly aligned block, too much weight, bad loads, small port. Then you get into things like tight moving parts.

With a good flash light you can generally look through the handgaurd to try and see carbon tracks from under the gas block or where the tube goes in.

You should be able to point the rifle down and push the carrier back about an inch or so, enough to see the bolt face and let it go. Is should drop freely enough to fully seat in the chamber.

Carbine buffer is a bit under 3 OZ and a full auto carrier are what the specs call for.

It's probably worth removing the gas block and checking two thing. Port size and look at the carbon ring. The port should be centered in a ring of carbon. Not uncommon for the block the be placed wrong and restricting the gas flow.

Last, for factory ammo, maybe try the Remington UMC subs. they are not the best, but they are extremely reliable. if they don't cycle unsuppressed probably nothing will.
I am using the Remington UMC subs. And it won't cycle them properly with or without the Suppressor.The Remington UMC supersonic 120gr and 115gr work great.
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: 16” barreled 300blk upper problem.

Post by dellet »

2BearArms wrote: Wed Dec 02, 2020 6:36 pm
dellet wrote: Wed Dec 02, 2020 5:53 pm The 12.5" is probably pistol gas length, most are.

Gas tube length can be confusing as there is is about 2" past the barrel into the receiver. A port 7" down the barrel, needs a 9" tube.

Most common failures for carbine gas are, gas leaks or poorly aligned block, too much weight, bad loads, small port. Then you get into things like tight moving parts.

With a good flash light you can generally look through the handgaurd to try and see carbon tracks from under the gas block or where the tube goes in.

You should be able to point the rifle down and push the carrier back about an inch or so, enough to see the bolt face and let it go. Is should drop freely enough to fully seat in the chamber.

Carbine buffer is a bit under 3 OZ and a full auto carrier are what the specs call for.

It's probably worth removing the gas block and checking two thing. Port size and look at the carbon ring. The port should be centered in a ring of carbon. Not uncommon for the block the be placed wrong and restricting the gas flow.

Last, for factory ammo, maybe try the Remington UMC subs. they are not the best, but they are extremely reliable. if they don't cycle unsuppressed probably nothing will.
I am using the Remington UMC subs. And it won't cycle them properly with or without the Suppressor.The Remington UMC supersonic 120gr and 115gr work great.
Well at least you can cross that off the list, and more or less confirms a rifle problem
300 Blackout, not just for sub-sonics.
2BearArms
Silent Operator
Posts: 55
Joined: Thu Aug 23, 2012 11:53 pm
Location: SW CT

Re: 16” barreled 300blk upper problem.

Post by 2BearArms »

If I were to try lowering the weight of the buffer is there a weight I shouldn't go below?
User avatar
dellet
Silent But Deadly
Posts: 6967
Joined: Fri Nov 30, 2012 1:25 pm

Re: 16” barreled 300blk upper problem.

Post by dellet »

I wouldn’t go any lower than the standard carbine which is basically 3 OZ. You end up fixing the symptom instead of the problem. There is absolutely no reason for a properly set up rifle to cycle factory subs without a suppressor. I would find and correct the problem, then decide it I wanted to customize the set up for a given purpose, but just to make it work, shouldn’t take any special effort.
300 Blackout, not just for sub-sonics.
User avatar
plant.one
Silent But Deadly
Posts: 6823
Joined: Tue Jun 23, 2009 8:31 pm
Location: Oakland County, MI

Re: 16” barreled 300blk upper problem.

Post by plant.one »

^^ this.

if its not cycling factory subs with the suppressor - something is absolutely wrong with the firearm itself.

as noted - pull the gas block, check the carbon ring around the port to verify solid alignment, that'd be the first place i would look.


if you want a quick check - by peeking through any holes in your handguard, the gas block should have a small space between the shoulder on the gas block and the block itself. if its sitting directly against the shoulder with no gap - alignment is likely your issue.

good luck and keep us informed what you find.
Reloading info shared is based on experiences w/ my guns. Be safe and work up your loads from published data. Web data may not be accurate/safe.
This disclaimer will self destruct in 10 seconds.
Post Reply

Who is online

Users browsing this forum: Bing [Bot], Google [Bot] and 86 guests