300 blackout problem

[Photoguy67]

I have an ar15 with a 556 upper and a new 300 blackout upper. The 556 upper runs 100% and the 300 blackout runs 100% with 125 grain sierra over 17 grains of h110 but when I tried 147 grain bullets I can't get the bolt to lock back or reliably strip the next round off the mag. I started with 16.1 grains of h110 and it seems to catch on the bolt carrier but not the bolt. The chart I have lists that powder as 16.2 grains max load so I backed it down to 15.6 grain with the same problems. I "feels" like the bolt is hitting the end of the buffer tube but still not locking back. I have backed the powder down to 15.1 but haven't been able to get to the range to test that. Any other suggestions? I am getting frustrated at this thing.

[Dolomite_supafly]

Have you tried a different magazine? PMags are notorious for causing problems with 300 Blackout because of the rib in the front of the magazine. It could be the rounds are binding just enough that the next round it not getting up in time or that the following is doing the same.

 

I would watch the primers and start loading. Do not reduce the OAL while you are working up as that can increase pressures. Or you can start by using the load you have been using and seating the bullet deeper to increase pressures but keeping the same length and upper the powder charge is a safer, and probably better, method.

 

There are plenty of people out there using 17 gains of H110 with the 147's but I have no clue what their OAL length is so be cautious in working up to that with your current OAL.

[Photoguy67]

I forgot to mention that, I do not have any p-mags but I have tried 5 different GI metal type mags all with the same results. This is a picture of where it locks everytime.

[Dolomite_supafly]

It is a gas issue, you need to increase the charge weight. I would recommend going up by .3 grains until you get to 17.3 but make sure to watch for signs of pressure. I bet 1/2 way between where you are now and 17 grains it will start acting normal.

 

If you are running a heavy buffer there is not need to run one. They only cause problems and increase the felt recoil.

[Photoguy67]

I honestly don't know what buffer I have, its whatever came with the rifle when I bought it. Stock S&W M&P 15. I will try more powder, Thanks. 

[Lumber_Jack]

I was just gonna say, it's definitely a gas problem, but G beat me to it. The brass marks on the deflector show the brass is being thrown in a pattern indicative of short stroking. Mine did the same thing when the gas block was tuned for supers and I'd shoot subs. Mine was not quite as severe because it would still feed but the bolt wouldn't lock back on subs. Try Dolomite's remedy and see what happens. Is this a factory built upper? Edited July 31, 2014 by Lumber_Jack

[Photoguy67]

No I built it. It's a mossburg barrel, aero precision upper and I can't remember what brand bcg.

[Photoguy67]

I was actually thinking it was over gassed since I have a .107 gas port and primers are flattening, but I have researched that and its apparently normal for some primer flattening with converted brass and wsr primers. 

[Lumber_Jack]

[quote name="Photoguy67" post="1174398" timestamp="1406850265"]I was actually thinking it was over gassed since I have a .107 gas port and primers are flattening, but I have researched that and its apparently normal for some primer flattening with converted brass and wsr primers. [/quote] Have you double checked the gas block alignment? Make sure that the holes are lined up or some gas gets blocked. Is this carbine length gas system?

[Photoguy67]

I have double and triple checked that and yes it's a carbine length

[Lumber_Jack]

[quote name="Photoguy67" post="1174415" timestamp="1406853849"]I have double and triple checked that and yes it's a carbine length[/quote] With carbine length your fighting an uphill battle. The 147gr is a heavy bullet for the amount of powder in .300BLK. I'd definitely take Dolomites advice and push the envelope and she of you can get that extra gas. Or swap the barrel for one with a pistol gas :)

[Dolomite_supafly]

Is the gas block against the shoulder of the barrel? If so you need to make sure it is .040" away. Barrels are made to use the hand guard cap and if you do not have a milspec hand guard the ports can be out of alignment front to rear.

 

And a lot of people open up their gas port to .120" on carbine length gas systems. And although I generally do not recommend going to .120" right off the bat you might consider going up to ~.112"

 

Your issue is 100% a gas issue. That is there is the bolt is unlocking too late because when it unlocks there is not enough pressure left in the barrel when it unlocks to push the bolt carrier back far enough. The bolt needs to unlock earlier and there is two ways of doing that, open the gas port or use more powder.

[Photoguy67]

The bolt is against the hand guard cap. I have removed the gas block several times and the carbon circle is completely surrounding the gas port of the barrel so I am sure its correctly positioned. I will load a few more rounds up closer to the max and try that. Thanks for all the advice everyone. :)

[Dolomite_supafly]

Something else to do when you have the gas block off is chamfer the edge of the gas port. Do not use a drill, use your fingers, and take a larger drill bit an break the edge of the gas port. I have had guns run better doing that. I think it helps smooth out the gas flow.

[Photoguy67]

I am trying a different bcg and more powder first then I may look at breaking out the tools. It just confuses me by running so well on the 125 grain bullets with a relatively light charge. But at this point I may try just about anything. LOL

[Dolomite_supafly]

It is also about the bearing length of the bullet. It is possible to have a lighter bullet develop more pressure.

[Photoguy67]

hmmm I wonder if its possible the the 147 grain bullets are slightly undersize from being "pulls".... Just thinking out loud. Now where is that mic...?

[Photoguy67]

Just an update. I have tried upping the powder charge all the way to 17.3 grains and finally blew out a primer so I can't go any higher on the powder charge. Still no function on 147, 155, 168 or 220 subs. Maybe a different powder? I have only tried h110, thinking maybe lil gun or a1680. other wise, new barrel or new gas system first. Maybe the gas is restricted slightly and just don't realize it. heck I don't know.

[Lumber_Jack]

I'm not sure about the powder but I'd pull it apart and make sure the gas port and block aren't obstructed. And then I'd look at opening the gas port as Dolomite mentioned above.

[Photoguy67]

Last time I had the gas port off I blew through it and didn't feel anything, but thats not to say it isn't undersized or something and i did open the gas port to .125

[Dolomite_supafly]

So you opened the gas port to .125" and you are still having problems?

 

If you opened it up to .125" I would suggest picking up an adjustable gas block to tune it. I know it is easier just to drill to .125" but in most cases that is too large. It won't hurt the gun to leave it unrestricted but it will run dirtier, it will also increase the felt recoil.

 

When I build my guns I slowly drill the port out until it cycles, this ensures the gun is properly gassed.

 

H110 should work fine.

[Photoguy67]

I finally found the problem. I took the gas block off again and was inspecting the carbon ring when I noticed it was perfectly centered but a bit forward, so I removed the end guard cap and ran gas block all the way against the barrel shoulder and worked perfect. Locked back with no problem on only 16 grains. So now I guess I have to order a free float hand guard. Thanks for all the help everyone. I really do honestly appreciate it!!!

[Dolomite_supafly]

It was not the misalignment issue that fixed it unless part of the carbon ring showed the port was partially blocked. it was the .125" port that likely fixed it. Had you had problems with the.125" port?

 

Try putting it back together with the handguards and see what happens.

[Lumber_Jack]

[quote name="Dolomite_supafly" post="1177557" timestamp="1407598721"]It was not the misalignment issue that fixed it unless part of the carbon ring showed the port was partially blocked. it was the .125" port that likely fixed it. Had you had problems with the.125" port? Try putting it back together with the handguards and see what happens.[/quote] I think that's what he was saying. The end cap pushed the block forward partially blocking the gas port. Would that not cause enough gas blockage, even with the bigger port?

[Photoguy67]

I still had the problem even with the enlarged port. I removed the handguard cap and that's when the problem stopped. I am guessing the cap was pushing the gas block forward and slightly blocking the port. I kept looking for it to be centered (left right) but never really paid attention to if it was centered front back which it was not.