Range report: 5.56 and .300 BLK SBRs

[nfa1934]

Tried out two setups today. First was a 7.5" 5.56. Fired a total of two rounds. Failure to extract both times. One of the guys who works there said someone else was having the same problem recently with another 7.5". New extractor solved the other person's problem. I'll try the brand new BCM bolt/bolt carrier that I have in my 10.5" upper next time.

Second was my 8" .300 BLK. This setup worked perfectly last time using subsonics through my 7.62 YHM Phantom suppressor. This time I tried it through a Silencerco Osprey .45 suppressor. I was happy with the level of sound suppression, but it would not cycle reliably. Not enough back pressure. It would cycle maybe 3 or 4 rounds out of a 30 round mag. The Phantom will be going back on this one since it has proven to be a reliable combination in the past.

[Helen Keller]

what subsonic load are you using?

[nfa1934]

what subsonic load are you using?

Factory Remington 220 gr.

When the Form 4 clears on my 9mm Tirant, I'll try it on the .300 BLK too. Maybe the smaller bore diameter of the 9mm can will generate enough pressure to cycle.

Not a big issue though, it is 100% reliable with the Phantom (it's just that the Phantom is heavy).

[Helen Keller]

seems I'm gonna have to get ahold of a 220gr mould.






How is function WITHOUT a can on with any of your loads?

[L1A1Rocker]

Tried out two setups today. First was a 7.5" 5.56. Fired a total of two rounds. Failure to extract both times. One of the guys who works there said someone else was having the same problem recently with another 7.5". New extractor solved the other person's problem. I'll try the brand new BCM bolt/bolt carrier that I have in my 10.5" upper next time.

Second was my 8" .300 BLK. This setup worked perfectly last time using subsonics through my 7.62 YHM Phantom suppressor. This time I tried it through a Silencerco Osprey .45 suppressor. I was happy with the level of sound suppression, but it would not cycle reliably. Not enough back pressure. It would cycle maybe 3 or 4 rounds out of a 30 round mag. The Phantom will be going back on this one since it has proven to be a reliable combination in the past.

That's odd. It should function sub-sonic w/o a can. Do you know if your bolt carrier is full or semi-auto? I've read of some problems with the full auto bolt carrier. Also, I'm assuming the gas port is in the pistol location, correct? Who made the upper/barrel? Have you checked to make sure the gas key is properly installed/staked? (a known problem with CMMG) Also, the gas block may not be correctly alligned - another problem that has surfaced in the various 300 BLK builds.



seems I'm gonna have to get ahold of a 220gr mould.



I'm partial to the 247 gr cast bullets myself.
http://www.gunsnet.net/showthread.php?14531-Hollow-Point-Cast-Bullets-for-Sale
But you need a user servicable can so you can clean out the lube build up from time to time.

[nfa1934]

That's odd. It should function sub-sonic w/o a can. Do you know if your bolt carrier is full or semi-auto? I've read of some problems with the full auto bolt carrier. Also, I'm assuming the gas port is in the pistol location, correct? Who made the upper/barrel? Have you checked to make sure the gas key is properly installed/staked? (a known problem with CMMG) Also, the gas block may not be correctly alligned - another problem that has surfaced in the various 300 BLK builds.




I'm partial to the 247 gr cast bullets myself.
http://www.gunsnet.net/showthread.php?14531-Hollow-Point-Cast-Bullets-for-Sale
But you need a user servicable can so you can clean out the lube build up from time to time.

Bolt and bolt carrier are LAR side-cocker. Heavier than a normal bolt and carrier. I assembled the upper myself. Noveske barrel, pistol length gas system. Gas block was installed already, didn't make any adjustments.

[L1A1Rocker]

Bolt and bolt carrier are LAR side-cocker. Heavier than a normal bolt and carrier. I assembled the upper myself. Noveske barrel, pistol length gas system. Gas block was installed already, didn't make any adjustments.

Noveske is quality stuff. I have not heard of any problems with their barrels or gas blocks. You may have a problem with weight in the BCG. I wonder if there is a way to trim off some weight. If not you may consider opening up the gas port one drill size at a time until it functions correctly. I wonder if Noveske would be willing to share any suggestions. . .

[nfa1934]

Actually, one more difference with the .300 BLK that I didn't think to mention. Before when it was running 100% with the Phantom, it had an A2 stock with a rifle buffer and tube. Switched to a carbine buffer tube and a standard weight carbine buffer for today.

[Schuetzenman]

I would say that the buffer and spring combo is often more fussy on these weapons than most of us think. When I built my middy carbine I had the long SOCOM stock and tube on it, so I went rifle buffer and spring. Only would function with 69 gr. Sierras. Put in a carbine spring and buffer and it would run everything. Since you're already at a carbine spring and buffer, not sure where to tell you to go fromt here. Is it not kicking the case out all the way or is it ejecting but not picking up the next round? If ejecting but not picking up the next round it needs more gas or less resistance to move the bolt group back.

[nfa1934]

I would say that the buffer and spring combo is often more fussy on these weapons than most of us think. When I built my middy carbine I had the long SOCOM stock and tube on it, so I went rifle buffer and spring. Only would function with 69 gr. Sierras. Put in a carbine spring and buffer and it would run everything. Since you're already at a carbine spring and buffer, not sure where to tell you to go fromt here. Is it not kicking the case out all the way or is it ejecting but not picking up the next round? If ejecting but not picking up the next round it needs more gas or less resistance to move the bolt group back.

It's short stroking, ejecting the fired case but not picking up the next round. I'm betting it has something to do with the weight of the LAR bolt carrier. I probably had enough pressure from the .30 can to overcome the weight, but not quite enough from the .45 can.

[Schuetzenman]

It's short stroking, ejecting the fired case but not picking up the next round. I'm betting it has something to do with the weight of the LAR bolt carrier. I probably had enough pressure from the .30 can to overcome the weight, but not quite enough from the .45 can.

Sounds plauseable to me as well. Lighter bolt carrier or lighter buffer should do it.

[nfa1934]

Sounds plauseable to me as well. Lighter bolt carrier or lighter buffer should do it.

Not possible to go lighter with the bolt carrier in this upper (side cocker). Does anyone make a buffer lighter than a standard carbine buffer? I know you can go heavier, but I've never seen lighter.

[Schuetzenman]

Not possible to go lighter with the bolt carrier in this upper (side cocker). Does anyone make a buffer lighter than a standard carbine buffer? I know you can go heavier, but I've never seen lighter.

Does anyone make alighter buffer, I'm thinking no. You might have to modify one yourself. I think some of them have several weights in them, and some have just one big weight in them. Looks like a cross pin / rivet / roll pin holds the head buffer on that smacks the rear of the tube. If you could get that off and remove some of the weight, that might do it. I would also say that if that were done you might want to put some sort of low mass filler in, maybe a spring to hold the weight in position so it doesn't slap aournd front to back. If it was one large weight, drill out some metal to lower the weight would be my plan. Or the other means to attack this would be to have the gas port opened up and use an adjustable JP gas block.

[L1A1Rocker]

Not possible to go lighter with the bolt carrier in this upper (side cocker). Does anyone make a buffer lighter than a standard carbine buffer? I know you can go heavier, but I've never seen lighter.

This thread where I made a buffer might give you some ideas on the buffer. (You'll need to scroll down to the part where I was making a heavy buffer but you should get the idea) It wouldn't be that big of a deal to replace the led slugs in one with aluminum slugs to lighten it. http://www.gunsnet.net/showthread.php?3995-300-Whisper-Fireball-Project-Part-5-it-Shoots!!

Regarding the bolt carrier. Would it be possible to mill off some metal in a few places to lighten it? I've lightened a few Ruger MkII bolts in order to reliably run sub-sonic ammo. It should be possible but I've never seen your bolt carrier.

[Schuetzenman]

So any new info to report on function?

[shadow65]

I had to open the gas port on my 9" CMMG barrel to reliably cycle subs. While it might be slightly over gassed, it will run any ammo now., suppressed or unsuppressed.

[nfa1934]

So any new info to report on function?

Haven't been back to the range.

Going to polish the chamber on the 5.56. If the extractor and extractor spring are bad, I don't think it would be ripping a chunk of the rim off the case.

I'm going to try the .300 again with the Phantom to see if it eliminates the switch to a carbine stock as the problem. If it runs with the Phantom (like before), then the problem isn't the carbine length buffer setup. Then I have to decide if it is worth it to open up the gas system, since I don't plan to run it unsuppressed. Worst case scenario, I'm limited to rifle cans.

Edit: checked out Noveske's website. The 8.2" barrel is only intended to lock back when running suppressed (with subsonic ammo). That would explain it short stroking with that huge hole through the bore of a .45 pistol suppressor. It runs like a top with a rifle can, as is evidently intended. It also isn't recommended for use with suppressors with an asymmetrical baffle stack (the Osprey is about as asymmetrical as it gets). The huge bore diameter is probably more forgiving of a poorly stabilized bullet than a .30 can though.

So, if it's running as intended, I'm not going to mess with it. When my Form 4 clears on my 9mm AAC Tirant, I will probably see if it will cycle with a 9mm pistol can. I think it might.