Need some help with .300 Blackout

[Rob62]

Hello Everyone,

I recently purchased a 7.5" barreled .300 Blackout upper for an SBR.

But doing research for various Blackout loads show 16" barrels as the standard used with published reloading data. And what those published loads show varies greatly.

So I am posting here in the hopes that someone who is running an SBR chambered in Blackout will share what they have learned.

I plan on shooting both Sonic loads (not suppressed) and Sub Sonic, either suppressed (ideally) or not suppressed if I can get the thing to function with SS loads.

Powder / bullet weight suggestions are greatly appreciated. Specially interested in Sub Sonic Load data with bullets in the 147-180 grain range.

(While it may not be relevant my gas bleed off hole in the barrel is .105")

Regards,

Rob

 

[TheAreopagite]

I just started reloading 300 BLK, but I can throw my .02 in. Published load data is always the place to start, working up from the minimum charge. What that means in your case, is it may eventually take a bit more powder (or a different powder) to get reliable function. It was helpful for me to google around and see what people were using in similar lengths and projectile weights, while knowing that a lot of factors could differ (barrel could be .308 to .310 or so, gas port size which you've already mentioned). If you look at the thread I posted last week you can see what I was using, and that I had good results with one projectile needing additional work.

As for subsonic projectiles in the 150-180gr range, you could probably do it with a bolt gun but I don't see any way you'll get a semi to cycle with those. It's just basic physics: bigger bullet requires more energy/powder, so you get enough gas to cycle the action. Maybe if the carrier was lightened considerably, with a lighter spring, but even then function will probably suffer. Stick to the big boys 203-245 .

For supersonic loads, 115-125 seems to be the preferred weight for good velocity on par with 7.62x39. A lot of people use 150gr pulled bullets as inexpensive plinking rounds. You could load 180s supersonic, but I doubt they would perform as well as lighter projectiles.

Good luck!

 

[jpm2953]

You are gonna need heavier bullets than 180 to run subsonic in an AR. I like the 208gr A-max. They are cheap. I use 10.3gr Accurate 1680 for my 8.5" barrel. For my 16" I had to drop the charge to 10gr to keep it subsonic.

 

[Rob62]

Today was the first day I took this rifle out. Initial function tests with 150 gr FMJ commercial loads was less than stellar. I had multiple function problems and only got off a few rounds.

Failure to feed, Failure to eject, and double feeding were the main culprits. (The magazine was carefully inspected and it's good, so I have ruled that out as being the problem - but will try others also next time)

When I get back out to the range (who knows when that will be) I will be starting slow - loading and trying to fire one round per magazine at a time to narrow down what is going on.

FWIW - the few rounds that did get fired were incredibly mild. And recoil was practically a non issue.

 

[jpm2953]

What length gas system and what buffer?

 

[Rob62]

The buffer and buffer spring are standard for what would be in a regular "carbine" - not sure how to ID them more accurately.

The gas port (bleed off hole) is about 4.5" from the chamber mouth. (Barrel length is 7.5")

 

[TheAreopagite]

You are gonna need heavier bullets than 180 to run subsonic in an AR. I like the 208gr A-max. They are cheap. I use 10.3gr Accurate 1680 for my 8.5" barrel. For my 16" I had to drop the charge to 10gr to keep it subsonic.

You are just the person I have a question for . I loaded AMAX 208 at 2.12" over 10.7gr A1680 and had lots of FTFs in my 8.5" barrel, pistol length gas upper. I was thinking it was undergassed and short-stroking, but now I'm thinking maybe it was too much powder since you're running 10.3grains. Any suggestions? Other possibilities I was thinking for the failure was magazine (Gen2 Pmag) or overall length was too short or too long. Any help would be appreciated, thanks!

edit: After looking around a little more, it looks like some people are loading at 2.2 OAL so maybe I was too short?

 

[TheAreopagite]

So I know I said it probably wouldn't work, but if you google around there are a handful of people who are loading 178 grain projectiles for subsonic 300 BLK, some are even claiming to run them in short barrels unsupressed. So I guess it is possible, but I'm going to stick to the heavy projectiles while I get a little experience under the belt.

 

[SMLEshooter]

A couple of links to some previous threads I started:

The first one had my results of loads through a 16" barrel and the same load through an 8" barrel.

http://www.theoutdoorstrader.com/threads/300-blackout-loads-16-8-0-barrels.782179/

I have similar data somewhere on 220 grain subsonic. Never posted it cause nobody seemed too interested in the first thread...

Second link is just some interesting reading.

http://www.theoutdoorstrader.com/threads/300-blackout-150-grain-load.446642/

Good luck.

 

[jpm2953]

You are just the person I have a question for . I loaded AMAX 208 at 2.12" over 10.7gr A1680 and had lots of FTFs in my 8.5" barrel, pistol length gas upper. I was thinking it was undergassed and short-stroking, but now I'm thinking maybe it was too much powder since you're running 10.3grains. Any suggestions? Other possibilities I was thinking for the failure was magazine (Gen2 Pmag) or overall length was too short or too long. Any help would be appreciated, thanks!

edit: After looking around a little more, it looks like some people are loading at 2.2 OAL so maybe I was too short?

That seems way short. I load mine to 2.240" drop that powder charge a bit too if running suppressed.