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Posted: 11/19/2025 10:50:16 PM EDT
| I have 2 complete lowers and 2 pistol braces. What length barrel should I be looking at? Choices are 7.5, 8.5, and 10.5. I'll be be looking for a suppressor once it is built and sorted out. This will be a home defense weapon. |
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7.5 or 8.5 is plenty. Maybe deciding how much hand guard length you need would be a good starting place. I would go 7.5" barrel for home defense if you are shooting SUBs. What is important is the twist rate. The faster the better. My 300BO pistol is a Sig Rattler LT (6.75"). It has a 1:5 rate which makes stabilization of a heavy slow round in a very short barrel possible. I would not go below 1:7 twist. I don't know, but you might want an adjustable gas block or something like the adjustable Bootleg BCG to control gas. It's tricky with subsonic, especially if you want to switch between subs and supers or with and without suppressor. On setup, if you are going to use subs for home defense you might need a suppressor to make it run right. Some builds need a certain amount of gas pressure to get subs to cycle properly in a short barrel build. If you set it up to run subs or supers w/o suppressor, you'll have to start all over again when you get a can. It's far different than setting up a 5.56 gun. I'm sure someone on the forum has successfully built a 300blk from scratch, but there are headaches. The EASY button here is buying a BCM or Geissele (or whatever) complete upper and using a configuration they recommend. Or you could spend a lot of time (and possibly money) trying to figure it out yourself. I think Geissele and BCM are 10", but I'm not up on 300blk upper offerings. Hope this gives you some ideas Good luck |
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I have barrels in: 7.5" (Faxon) 8.0" (Wilson Combat SS) 8.2" (Rosco SS) 8.3" (BA - Hanson) 8.5" (PSA) 9.0" (BA) 10.5" (Faxon SS) 12.5" (Green Mountain - CL) 16.0" (RTB) All three of the sizes you have will work for HD, and I agree with the responses in the thread. A few questions: What are the twist rates of your current barrels? Are you planning on shooting subs or supers, or both? Are you using a suppressor? Will you have a light on the gun? The biggest benefit of the 10.5" is more space for hand rails along with the boost you get with NAS3 ammo, it takes that weapon "beyond" HD. I LOVE the weight/profile of the Faxon's, but both of mine are 1-8 twists. That works for me for now because I shoot mostly supers. They make the save versions in 1-5, but I would prefer 1-7 for my usage. Everything else I have is 1-7. The 7.5" Faxon is super light, but rail space is limited on that short of a barrel. That is the biggest knock on that barrel length. After playing with all types of sizes, I think the best HD size is in the 8.2"-9" range. Mostly because there seems to be quite a difference in real estate between a 7" rail and an 8" rail. |
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I prefer anything I'm holding/shouldering like a rifle/SBR to have a 10.5-12" barrel as I feel like I have more room for the guard and more room to reach out for my front hand purchase. With a Sub 8", I feel bunched up, like I don't have room to reach out. I just got a 8.9" AP5, MP5 Clone and its going to take a bit of used too. I went with the 8.9" as I felt like the AP5-P and M, would look really cool, but be to bunched up for my liking. YMWV... |
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Originally Posted By Boom_Stick:
""AKs are for villagers you have to tell not to shit in their water supply.""
NRA Endowment Life Member
Originally Posted By Boom_Stick:
""AKs are for villagers you have to tell not to shit in their water supply.""
| I've used everything from 7.5 to 10.3. Personally 8" to 9" is the sweet spot IMO for reliability, performance and size with a can. My two favorites are my Rosco 8.2 Bloodline build and my 9" BKings (Ballistic Advantage) build. Both have been excellent performers, very accurate, and well gassed for supers and subs with just a standard gas block, standard buffer spring and carbine or H buffer. |
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Originally Posted By bootleg15: I have 2 complete lowers and 2 pistol braces. What length barrel should I be looking at? Choices are 7.5, 8.5, and 10.5. I'll be be looking for a suppressor once it is built and sorted out. This will be a home defense weapon. If it's strictly a home defense gun, then might as well make it as short as possible, 7.5" suppressed gets my vote. If you plan to use it elsewhere, then i'd go longer for more versatility in multiple ways (longer rail without shrouding suppressor, extra velocity for supers, etc.). |
| 8.5 and 10.5. Both using subs and suppressor. Killed deer with both. Not much difference. I did go with a silent spring in both. Can’t remember the brand. |
17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.
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Originally Posted By Network_Daddy: I have a 8.5" but its been SBR'd. I like it a lot. Gets good results from 110gr AAC but the barrel isn't ideal for that round I'm sure. I shot it with a FRT and that was all kinds of fun. If you are using the AAC 110gr V-max, it is perfect. The 110gr V-max is probably the most popular super for 300blk not named "Tac-tx". 110gr Hornady V-max, 110gr Nosler Varmageddon, and 115gr Sierra Varminter/JHP are all perfect in 300blk. The lower velocities take them from .308 explosive varmint rounds, to 300blk controlled expansion/fragmentation rounds. The 110gr V-max is my HD load. AAC 110gr Vmax  300 Blackout (7.62x35mm), 110gr V-MAX, AAC (America's Ammunition Company) AAC 115gr Varminter/JHP  115gr Sierra JHP for 300 Blackout? AAC 110gr V-max vs 110gr Varminter in an 8.5"  .300 Blackout V-Max vs Varmageddon Ballistic Gel Test & Chrono With An 8.5" AR! |
| I have 2 10.5 barreled 300BO’s. One has my sandman s shooting Hornady 190 subs with my Bering Vibe 35mm thermal on it. Whisper quiet. No need for hearing protection. My other has a eotech 512 on it shooting Hornady 110 v max and 110 Tac Tx. Both are sub MOA. Figure out what pill you want to shoot, then decide on the twist. |
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Originally Posted By ScottsBad: ... I'm sure someone on the forum has successfully built a 300blk from scratch, but there are headaches. ... Good luck with zero experience I assembled a 5.56 and 300blk in 2015, no major issues. diy 300blk was fairly new so carbine gas 16" barrels were not yet "do not recommend" and I went with carbine gas. had to put a reduced spring rate buffer spring in to get it to load the next round - would eject but not cycle back far enough to strip the next round out of the magazine. works perfectly with subs and supers. if I did it today I would use pistol gas on a 16". actually, a 16" 300blk is ridiculous unless you're avoiding pistol/sbr rules for some reason (live in a commie state, for isntance). since then I've assembled a 10.5" 300blk pistol and a 11.3" 6.8spc pistol, no issues. if you're shooting supers, I would go ~10-11" or you'll get a fireball. if you're shooting subs only the extra barrel length just adds some muzzle velocity, and you want to keep it subsonic anyway. I haven't built up a 300blk shorter than 10.5" but I would expect issues from a 5-7" barrel - the pistol gas port is going to only leave a couple of inches of barrel past the port. with that short time for the gas system to get pressure, it's going to be less tolerant of mistakes like poorly sealed gas blocks, gas tubes, incorrect size gas port, etc. if I was building another 300blk pistol for subsonic only and a suppressor I would go 8" barrel. |
The deuce you say.
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 How Much Does Barrel Length Affect .300 Blackout Bullet Velocity? |
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A 10.5" here...but I use it for both subs and supers. The 1:8" works just fine, 220 MKs group 2" at 50 yards which is more than enough for HD. Heck, I have a 10" Contender with a 1:10" barrel and it groups the 220s at 1080 fps into 3" .but the holes are very slightly not round. . |
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