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Another trouble shooting thread, 300 Blk Out

SmartDonkey

Sergeant of the Hide
Full Member
Minuteman
Oct 18, 2018
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314
Same old story- I got a 300 from a guy and am having trouble making it run... It seems to all be AR-S parts, but not sure if it was factory or a build. I don't see the exact option available so I am assuming build rather than a complete upper. It is a 16" 1/8 twist with a pistol length gas tube. With that gas system and barrel I was expecting it to be over gassed but not sure if it is. Non-adjustable gas block, but it is properly lined up with the port.

It will run subs fine if I have a can on it- but if I pull the can it will have problems with about every 4th round. The round will be nose high with the bullet running into the top of the chamber and the bolt stopped on the body of the case. Not sure how it is getting enough purchase on the round to strip it 3/4 out of the mag and then skip over it? It is hitting hard enough that it will ripple the case mouth and some of the cartridges have a slight bend. That would make me think it was a mag or feed ramp issue if it wasn't for the fact that it runs perfect with a can.

So that makes me think it is a gas issue? I only had 1 box of super-sonic ammo and it ran them fine, but I can't remember if I had the can on or off with them. All I want to do is get it to run subs with no can-- just for a stay in the house defense gun that wouldn't over penetrate and probably never get used. It has a H1 buffer, I didn't weigh the whole thing- I pulled the weights out and the three of them were 1200 grains, so approx 2.75 ounces. Any advice?
 
You’re going to want that can if you need to shoot the gun inside.
 
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What magazines and ammo are you running and how full are your mags stuffed?
 
I'm sure the can would be nice if I ever had to shoot in the house- but I don't want to tie up my only can on a gun that will hopefully never be used inside. Also, with the 16" barrel it gets a little difficult to maneuver inside with the can on it. And i'd like to know that it is still functional without the can.

I've tried it with magpul and 2 amend (or amend 2, however you say it). When loaded up I could see the mag flexing a little on the front so I tried it with less rounds- about 6- and I also tried metal mags.... but they are just standard 223/556 mags.

The ammo is Rem UMC 220 grain, S&B 200 fmj, and Geco 220 grain HP. The Geco ammo was by far the best- but still had a couple jam up.
 
But all the mags worked fine- as long as the can was on. And they all had issues without it.
 
I normally shoot with USGI 20 rounders with about 10 in them with no issues. Went to a few matches with Pmag 30s and had all sorts of issues with jamming. Recently bought 300 specific mags and they worked like a champ. Mainly shooting the S&B 200.

Supers are short enough they don't interfere with the rib inside the mag. Subs are different story.

RE can: Is there a discernible difference on back pressure? Subs can be finicky with out can. Did you try loading one round and confirming lock back (with AND without)? It may be the can is getting just a bit longer stroke out of the BCG and allowing the mags a few more millisecs to pop the rounds up.

Also, I don't run a longer barrel, but I have always used an H2 for 300 BO.
 
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Can equals longer gas impulse. My 8.5” 300 BO will not run 220 subs w/o the can. Runs supers just fine can or no can. Buffer is Spikes T2 (4oz), mags are full 20 round USGI.

OFG
 
Weird, my 8.5” pistol runs supers and subs, with and without a can. I’d think that a 16” would have more than enough gas...
 
But all the mags worked fine- as long as the can was on. And they all had issues without it.
Port is smidgen too small to run subs reliably unless can is attached.
16”, pistol port sized to run subsonics w/o a can is going to be really overgassed w/super sonic ammo.
Ammo, different burn rate powder, can fix the subsonic cycling issue most probably.
Might also try lighter buffer (possibly empty buffer) and if not enough, a lighter K buffer spring. Just make sure you put heavier buffer and higher K buffer spring back in before using supersonic ammo.
 
Weird, my 8.5” pistol runs supers and subs, with and without a can. I’d think that a 16” would have more than enough gas...
16” w/pistol port is harder to balance between super and sub ammo because of the longer dwell time.
8.5” is actually quite a bit easier. Counter intuitive but has been my experience.
 
16” w/pistol port is harder to balance between super and sub ammo because of the longer dwell time.
8.5” is actually quite a bit easier. Counter intuitive but has been my experience.
Must be. Id like to say I’m some sort of ar15 yoga, but I just put the thing together and started shooting. It’s a shame really as it has a SLR gas block on it, but it isn’t being used- could do with a low pro non adjustable. But, I have a cherry bomb brake rock setted to the barrel that I don’t want to deal with removing, just to pull the gas block. Then again, that prevents me from looking at an expensive gas block that doesn’t have a home and saying, “I should really build this into a rifle...”
 
Just got back from trying some of your suggestions..... and although I haven't figured out the specifics- I think (although being wrong in not a rare occurrence for me) that it is a mag issue.

I loaded one round up -in 3 different mags- three times and all 9 times the bolt locked back without the suppressor. I then loaded the mags with 10 rounds and I would get 1 to 3 jams per mag. On some of the jams I could see that the bolt face was behind the back of the case so it must be as it slams forward that it is pushing it down and passing it. I would pull the mag out and it looks like the rib on the mag is pushing the front of the top bullet a little high and to the center.

I did get my first failure with the can- but I had to load 20 in the mag to do it, and it jammed on the 3rd shot. Not sure how the can changes things if it is a mag issue- unless it changes the recoil enough that the cartridges in the mag don't move out of position as much?

And maybe something more is still going on.... I think quite a few people get by with regular mags..... but buying some new mags is first on my list.

Thanks.
 
My 5.56 mags did similar and it pissed me off beyond belief the few times I took my rifle to matches and it sucked.

Load the mags most of the way and feel with your finger if they are dragging on the ribs. Upon comparing the 300 BO mags and 5.56 ones, you could also see the nose of the bullet being out of alignment as they went down the magazine.

Edit: Found older pic. Left = 5.56, Right = 300 BO. Compare the areas of red circles. Rounds are S&B 200.
 

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Just got back from trying some of your suggestions..... and although I haven't figured out the specifics- I think (although being wrong in not a rare occurrence for me) that it is a mag issue.

I loaded one round up -in 3 different mags- three times and all 9 times the bolt locked back without the suppressor. I then loaded the mags with 10 rounds and I would get 1 to 3 jams per mag. On some of the jams I could see that the bolt face was behind the back of the case so it must be as it slams forward that it is pushing it down and passing it. I would pull the mag out and it looks like the rib on the mag is pushing the front of the top bullet a little high and to the center.

I did get my first failure with the can- but I had to load 20 in the mag to do it, and it jammed on the 3rd shot. Not sure how the can changes things if it is a mag issue- unless it changes the recoil enough that the cartridges in the mag don't move out of position as much?

And maybe something more is still going on.... I think quite a few people get by with regular mags..... but buying some new mags is first on my list.

Thanks.
Might be a mag issue, MagPul 300BO definitely worth trying.
Could be enough different ogive profile and/or OAL of the supersonic ammo that minimizes mag issue.
 
My 5.56 mags did similar and it pissed me off beyond belief the few times I took my rifle to matches and it sucked.

Load the mags most of the way and feel with your finger if they are dragging on the ribs. Upon comparing the 300 BO mags and 5.56 ones, you could also see the nose of the bullet being out of alignment as they went down the magazine.

Edit: Found older pic. Left = 5.56, Right = 300 BO. Compare the areas of red circles. Rounds are S&B 200.
My 20 round USGI magazines load 300BO centered like the 5.56 mag pictured. Haven’t tried any plastic mags.

OFG
 
I’ve only ever run lancer 5.56 mags and never had an issue. Can’t speak to magpul mags...
 
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I’ve always ran the magpul 300 blk mags with 0 issues with subs or supers with a can or without, I do have an adjustable gas block also.
 
lancers are all I use . I have a 16 inch carbine with carbine gas and a 8.5 inch pistol . I reload most of my ammo and can run subs and supers suppresses or not .I use 1680 powder and I think that helps with the carbine gassed gun .
 
Yeah mags or more importantly COAL. Had this problem if many different builds. If bullets too long and with less gas, it will hang up. instead of dragging the bullet tip along the top of the barrel and chambering the round.. Can also have issues with setback in that situation.

Feedlots not designed for 200+ grain bullets.

Lancer 300 blk mags are flawless ime. A fistant second being 300 blk pmags.

If you reload you could adjust COAL.

Otherwise subs without a can. Meh
 
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Same old story- I got a 300 from a guy and am having trouble making it run... It seems to all be AR-S parts, but not sure if it was factory or a build. I don't see the exact option available so I am assuming build rather than a complete upper. It is a 16" 1/8 twist with a pistol length gas tube. With that gas system and barrel I was expecting it to be over gassed but not sure if it is. Non-adjustable gas block, but it is properly lined up with the port.

It will run subs fine if I have a can on it- but if I pull the can it will have problems with about every 4th round. The round will be nose high with the bullet running into the top of the chamber and the bolt stopped on the body of the case. Not sure how it is getting enough purchase on the round to strip it 3/4 out of the mag and then skip over it? It is hitting hard enough that it will ripple the case mouth and some of the cartridges have a slight bend. That would make me think it was a mag or feed ramp issue if it wasn't for the fact that it runs perfect with a can.

So that makes me think it is a gas issue? I only had 1 box of super-sonic ammo and it ran them fine, but I can't remember if I had the can on or off with them. All I want to do is get it to run subs with no can-- just for a stay in the house defense gun that wouldn't over penetrate and probably never get used. It has a H1 buffer, I didn't weigh the whole thing- I pulled the weights out and the three of them were 1200 grains, so approx 2.75 ounces. Any advice?

This is actually a pretty easy one - your rifle is probably gassed/sprung just about perfect for practical use. It's under gassed for subs without the suppressor, but face it - there's zero point in using subs without a suppressor. You can make it run that way, but if it doesn't, don't worry about it because there's no need. Gassed the way it is, it'll be slightly quieter with subs suppressed than if it ran them without a can, and with supers suppressed you'll get a little less gas in the face. Those are good things; I'd leave it the way it is.

FWIW, pistol gas is correct for a 16" 300 Blk if you want it to be reliable with subs suppressed. Carbine gassed systems tend to have more trouble with subs, although they're fine for a supersonic-only setup. I've set up my personal 300 Blk uppers with pistol gas on all of them for that reason, but have dealt with a few carbine systems. You got the right setup with yours, so you can feel good about that.

As for mags - 5.56 Pmags are great for supers. There is a 300 Blk Pmag that works well, but only in 30rd variety. The 20rd 300 Blk Lancer mags are great for both subs and supers; if you're stocking up on mags those are the ones to get. They also run 5.56 just fine.