First real 300 blackout build

[DDadams]

I had one the year before last but honestly didn't even use it after realizing I did not want to stock the ammo at the prices it was at. So this is going to be my first one and I've heard/read that they're pretty finicky.

It's going to replace a short 556 as a backpack gun so I'd like to go much shorter on this one.

The shortest 2 options I'm looking at from Faxon are 6" or 7.5". I'm going with Faxon as they're notoriously on the higher side for gas port sizes to give me the best chance at avoid cyclic issues. Although I primarily plan on using subs for it I'll likely take it out to practice and probably use supers to save on cost - therefore I'll be using an adjustable gas block rather using a BRT eztune gas tube like I have been on my 5.56 uppers.

Is there any big potential difference in reliability issues with 6 vs 7.5" - if anyone here can speak on either length and their experiences, good or bad?


I do have several suppressor options and neither are very low back pressure so I think cycling a faxon barrel should be fine but I'm just not sure of the cartridges limits with barrel length.

I also don't know what kind of buffer / spring setup I should use. On the vast majority of the 556 uppers I have I use a lower with an A5 vltor extension + H0-H3 and sprinco green. I will be using a law folding adapter on this as well.


Any help/advice is greatly appreciated.

 

[BR8818]

I had a 6" Right to Bear barrel and SLR agb, I had to open up the gas port to .105" to get all subs to cycle. I used a lw bcg, sprinco white, and a carbine buffer, with this setup it cycled every subsonic round I tried suppressed, but was over gassed with supers suppressed. If you're planning on using a law folder and full weight bcg you might end up needing a yellow sprinco.

I've had alot of different 300blk's 16", 9.5", 10.3", 6", and currently an 8.3". I've had this 8.3 for about 3 years now it's been my favorite length. The only way I would go shorter is if I planned on only running subs suppressed. This 8.3 is alot quieter than the 6" I've also noticed less "unburnt" powder and crud in the lower. The 6" was also just too short for me to shoot comfortably my hand found the HOT suppressor a couple times. Also if you plan on using any lights/ ir lasers you run out of rail space really quick.

 

[edwea]

I used a 7.5" faxon barrel on my last build and love it. I used an adjustable gas block which is nearly all the way open and captured buffer. I played with buffer weights for fun to see how they behaved. The c and h buffers were perfect, no failures and ejection was at 4 o'clock and so consistent that once I knew where it was landing, I could set up a coffee can to catch them. The h3 buffer was too heavy. Ejection wasn't consistent and the bolt occasionally didn't hold back after the last round. Pay attention to the twist rate. Faxon offers a 1:5" and a 1:8".

 

[DDadams]

I used a 7.5" faxon barrel on my last build and love it. I used an adjustable gas block which is nearly all the way open and captured buffer. I played with buffer weights for fun to see how they behaved. The c and h buffers were perfect, no failures and ejection was at 4 o'clock and so consistent that once I knew where it was landing, I could set up a coffee can to catch them. The h3 buffer was too heavy. Ejection wasn't consistent and the bolt occasionally didn't hold back after the last round. Pay attention to the twist rate. Faxon offers a 1:5" and a 1:8".

What did you use on yours? The 1:5 or 1:8?

I was leaning away from the 6" barrel already and after both of the replies so far here I think I'll probably go with the 7.5 unless I really cheap out and just buy a factory Aero 8". I'm pretty sure they use a 1:7 twist.

 

[edwea]

Mine is a 1:5". I've never shot supers out of it. I love it.

 

[masterdekoy]

I have a 7.5 1:5 from Faxon. It shoots great. It wears an adjustable gas block but it’s almost all the way open, so you could probably get by with a non adjustable if you want.

As far as buffer weights, especially with a law folder you probably want to start with carbine and may have to remove the weights to get it to run reliably with subs, you just have to try it out. I’ve been using the LWRC UCIW stock and it works for me, not as short as a folder but keeps it simple and lightweight.

Only other thing I would caution is if you plan to use the riflespeed gas block, a 6” barrel might not provide enough clearance in front of the gas block for a muzzle device to fit. There’s not much extra room.

 

[Jarvitron]

I had a factory Aero 8" in 1:8 iirc and it was mediocre with subs and better with supers (which were more plentiful at the time), I would absolutely advise a 1:5 if you're gonna feed it subs exclusively.