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Posted: 2/19/2026 4:49:36 PM EDT
| What tips, tricks, and parts do you have to suggest? All of my builds have been either 223 or 556 so this is new territory for me. Right now, I have old, never-fired 16" bbl and an adjustable gas block....any 300 bo specific parts needed? Buffer weight? |
"Don't wrestle with pigs. Everyone gets dirty and the pigs enjoy it too much."
"It is easier to get forgiveness than permission"
RIP EDWARD AVILA
"It is easier to get forgiveness than permission"
RIP EDWARD AVILA
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If you're going to run subs make sure you get the Lancer 300 blk specific mags. They prevent feeding issues with the heavier sub type bullets. 300 blk can be kind of picky if you're trying to run suppressed/unsupressed subs and supers. But really if you keep suppressed 100% an H1 buffer should be okay, just gotta try it out and see what works for yours. |
| I have had good luck running 8.2" Rosco and 9" BKF (Ballistic Advantage) barreled 300 blackout builds with standard springs, carbine or H1 buffers, and fixed gas blocks. Both run supers and subs, with and without a suppressor. Although I did swap the "enhanced" BCG I originally used in the Rosco build to a standard BCG to get it to reliably run subs unsuppressed. The weight difference and notably tighter gas seal of the replacement BCG did the trick and she runs like a top. Maybe I'm lucky. |
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Originally Posted By DancingInAshes: 300 blackout really doesn't like flow-through suppressors with subs, so if you have one, find a way to increase the amount of gas so it will reliably drive that bolt back far enough to eject AND grab the next round. This does not reflect my experiences at all. I'm using Griffin reduced backflow cans with non adjustable gas blocks with no issues. |
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Full powder burn for .300 blk is, what 8" barrel? If you are looking for extra velocity out of your rounds, stay a little above that. Some people find those short 6" and 7" barrels don't allow for much rail space, including me. (If you have a rail designed to have a suppressor under it, it's less of an issue.) I've come to prefer the 9.5"-10.5", myself. When you get your barrel, check your feed ramps. Clean up the any sharp corners or edges and polish the feed ramps before installation. It's easy, and it's notably harder to do after installation. .300 BLK has wider projectiles than 5.56, and some big subs with fat bodies can have feeding issues that can be avoided by giving them beautiful, slick feed ramps with no sharp edges or corners. (I've had several .300 blk barrels with feed ramps that dug into either the projectile and/or the brass causing feeding issues.) If corners/edges are sharp, use a small metal file and high grit emory paper. If possible, polish with dremel, too. Clean well to remove any debris. This is usually less of an issue with supers. This may not be an issue with every barrel, but it will not hurt any barrel, either, and it can save you work later. I saw an earlier suggestion regarding feeding, that some magazines don't play well and the suggestion for lancer .300 blk specific mags. I've had big subs that don't like some 5.56 magazines, too. |
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If it is an old barrel, I have two questions. 1. What is the twist rate? 2. More importantly, it is carbine gas or pistol gas length? Some old 16's used carbine length, but you often have better results with pistol length. 16" is a bit long for an AR build (non-bolt action) unless you have a specific use-case. I have a never fired 16" barrel that has been sitting in a box for a long time. On the flip side, I have FIVE different barrel lengths alone going from just 8 inches to 9 inches. Honestly, there isn't much difference from building a 5.56 other than a shorter gas tube and a different muzzle device thread on the (different) barrel. Same bolt, can use the same mags for a lot of supers and some subs. More finnicky to tune unless you tune for all supers or all subs. Note: Mark your mags Mark your uppers Keep your 5.56 weapons and ammo far away from your 300blk stuff. You want to avoid the dreaded danger noodle. 
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