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Posted: 5/1/2026 6:07:43 PM EDT
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It has been 15+ years since I have been real active in shooting and it's time I return to what once aas a passion... So... for my home and truck gun I have settled on an AR in 300 Blackout. The 8" has obvious advantages but I'm absolutely open to a 10" or 11" if there is an actual ballistic advantage. Bullet will be either 200 or 220gn. This will be suppressed and it will be dedicated to training and self defense. Any input from experienced users with the barrel length and bullet selection would be greatly appreciated! Lastly... I knownthey have dedicated magazines. I plan to carry a 20rd with several 30s as reloads. Are the dedicated mags actually more reliable or a gimmick? Thanks in advance! |
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It was developed around a 9" barrel but that's a bit long IMO. I have an 8.3" and that's the longest I'd go. 7 or 7.5" is even better if you reload. They are not a gimmick for subs. For supers it doesn't matter but the subs need the profile of the specific mags to help them stay stacked and fed properly. You didn't ask for the opinion on subs but I'm giving it to you anyway. It's your life, not mine, but I'd use 110gr black tips for SD, not subs. Way, way better ballistic performance and still reasonably quiet with a can for limited SD situations. The last thing you'll notice is the sound difference in a situation like that and the first thing you'll notice is the ballistic performance difference. |
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With mostly or exclusively supersonic cartridges, longer is better and 16" offers good ballistics in a relatively handy size (vs like a 20" 5.56 AR-15 to be clear), but with exclusively subsonic cartridges, I would go with an 8" SBR as @mak0 said if I were you, OP. Maybe pick a 12.5" SBR as an intermediate length for mixed use. Per this Ammoland article, you can see how just over 250 fps is gained by going from an 8" barrel to 16" barrel with 125 gr supersonic ammunition, whereas with 220 gr subsonic ammunition, you're only looking at around 80 fps for 8 more inches in length. In terms of bullet selection, I've gravitated toward 110 gr bullets, but I shoot almost exclusively supersonic ammunition. And as -Obsessed- said, the 300 BLK specific magazines seem to be more reliable with subs than standard AR mags, but I have not seen a big difference with supers. |
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Originally Posted By -Obsessed-: It was developed around a 9" barrel but that's a bit long IMO. I have an 8.3" and that's the longest I'd go. 7 or 7.5" is even better if you reload. They are not a gimmick for subs. For supers it doesn't matter but the subs need the profile of the specific mags to help them stay stacked and fed properly. Do you know what the twist rate is on those 7" barrels? IIRC the 5.5" and 6.5" barrels have a much higher twist rate than on the 9-16" barrels. |
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Originally Posted By NN300BLK: Do you know what the twist rate is on those 7" barrels? IIRC the 5.5" and 6.5" barrels have a much higher twist rate on the 9-16" barrels. Usually 7" barrels can come in a 5 or 7 twist. Not important for supers but for subs I'd opt for a 5 if exclusively shooting subs otherwise I'd probably go 7. My 8.3" is a 7 twist. It stabilizes 240gr just fine and I'm confident a 7" would stabilize a 240gr just fine but there are some heavier bullets that may get kinda close to the limit of safely stable when using a can. I general bullets are stable at higher twist rates than most realize. Huge safety factors built in, as can be seen by the commonly available stability calculators out there. Again, a bit of a moot point if you reload, as stability increases with velocity, and you can usually make up the vast majority of velocity losses and load right up to almost breaking supersonic speed with subs anyway. I once downloaded subs to see how low zi could get velocities and still cycle. They were 217gr projectiles, and I got them down to 690 FPS while cycling still and they were perfectly stable out of that same barrel. YMMV. |
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I really like the lancer 300 blk mags, never had an issue with supers or subs. I use lancers for 300 blk and metal mags for 556 so there’s never any confusion what is what by sight or touch. Also, if you want to shoot 100m+, there’s a significant ballistic advantage for 10.5” with subs or supers vs <10”. There are diminishing returns >10.5 with supers and little to anything to be gained >10.5 with subs. Depending on your specific round, most subs are going to max out around 11.5-12.5” |
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Originally Posted By FloridaBob: It has been 15+ years since I have been real active in shooting and it's time I return to what once aas a passion... So... for my home and truck gun I have settled on an AR in 300 Blackout. The 8" has obvious advantages but I'm absolutely open to a 10" or 11" if there is an actual ballistic advantage. Bullet will be either 200 or 220gn. This will be suppressed and it will be dedicated to training and self defense. Any input from experienced users with the barrel length and bullet selection would be greatly appreciated! Lastly... I knownthey have dedicated magazines. I plan to carry a 20rd with several 30s as reloads. Are the dedicated mags actually more reliable or a gimmick? Thanks in advance! I think anywhere from 8-10.5ish inches for a 300blk barrel is ideal for handguard length and maneuverability. With 200 to 220 grain factory ammo subsonic is going to be subsonic but if you're going to load your own just start high and work down to the velocity you're wanting. I used to have a 12.5" 300blk and all the factory ammo was still subsonic through that barrel so anything shorter should stay subsonic. If you're planning on reloading, the Maker bullets or Lehigh Defense for subs would be an ideal choice. And Accurate 1680 is dirty but dependable. Also IMR 4227 was good and noticeably cleaner. I've heard the Hornady 190gr Sub-x ammo sometimes wouldn't expand properly. The Lancer 300blk mags are meant for fixing the issues a regular 5.56 mag presented when loaded with subs. The ogive on the subsonic bullets are different than the 300blk supers and the Lancer mags address that issue. The nubs inside the magazine lips are in a different location to accommodate the subsonic profile bullets. I've never personally had a failure when using those mags with subsonic rounds. |
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I have what feels like every 300blk barrel length on the planet (6", 7.5", 8", 8.2", 8.3", 8.5", 9", 10.5", 12.5" and 16") while trying to figure out what I liked. I agree with the earlier statement on handguard length and 8"-10.5" being a fantastic range depending on use. I've been trying to consolidate and just recently decided to an 8" for HD and a 10.5" for SHTF. (The 8" recently replaced a 7.5" backpack gun and a 9" HD). If you are running JUST subs, I would go towards the shorter end. There is no benefit to extra barrel for just subsonics shooting. But I would question getting into 300blk for just subs unless you reload or came across a crate of subsonic 300blk that fell off the back of a truck. 147gr 9mm (FMJ and high quality HST's) are a lot cheaper. Some of the fatter/heavier subs don't always play well with 5.56 mags due to the "Oh-Jive" and the ribs in 5.56 PMAGS. So dedicated 300blk mags might be useful depending on which subs you use. Go with at least a 1-7 for just sub shooting, although you might want to consider 1-5. I have 5.56/300blk/.22LR mag bands on all magazines AND my uppers. Whatever you do, keep your 300blk stuff far away from your 5.56 stuff less you enjoy danger noodles. I'll add this. Up until a year or so ago, I thought my 10.5" barrel was near useless. Limited velocity increase over the 8"-9" barrels. BUT, Phantom Defense made a round specifically designed for supersonic 300blk in the 8" to 12" range. Their NAS3 cased 120gr Tac-tx was an absolute game changer, and is how my 10.5" went from useless to my SHTF/do (almost) anything set-up. https://phantomdefense.co/blog/making-300-blackout-great-again/ Their FPS claims are a little ummm.....optimistic to say the least. (I don't think anyone is getting an advertised 2397 fps out of a 12.5" barrel. I took three shots in a brand new 12.5" and was more than 100 fps slower). But I did get a solid 2250 fps average out of rarely shot, unsuppressed 10.5". That was close to a video I had watched of someone getting something like 2267 fps out of a suppressed 10.5". Given the BC of the 120gr Tac-tx (G1=.357) and the low, low, low expansion floor of Tac-tx (1350-1400 fps, but real expansion is probably closer to the 1550-1600 range), with the right ammo, that 10.5" is a good dual purpose length. They also offer the more popular 110gr Tac-tx in an NAS3 case. I have some but never chrono'd it. Based on reviews, it runs a good 100 fps faster than the Barnes brass versions. Does not have the same BC as the 120's, but if you are running short barrel it makes good sense at closer ranges. I'm sticking with cheaper brass options in my 8" for HD (110gr V-max, 115gr Varminter/JHP) since I don't need the range. |
USAF (Ret) 2003-2024
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I have an 11.3" wilson combat primarily for use with supers (110 tac-tx) for hunting. My subs-only SBR is a 6" barrel. It was a little finicky at first but 208's with CFE BLK shoot very nicely (1.5-2" at 100, which is more than adequate as it's mostly for 25-50 max, only occasionally going further). Then I have a Solo 300 but that's primarily shits and grins only.  As for mags, I've only ever used GI 20s and magpul straight 20s (occasionally some 30-round pmags) and never had an issue. All pmags were "intended' for 5.56 |
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you didn't mention if you have any other AR - if you have a 5.56 you need to be careful to not mix magazines. supersonic 300blk will chamber in 5.56 or wylde chanbers and it's a bad day. if this is your only AR, not an issue. all my 300blk is in pmag30, all my 5.56 is in pmag40. supers work fine in 5.56 magazines. the fatter bullets in subsonic rounds are pushed sideways by the internal ribs at the front of the magazine. my bag gun has all 300blk specific magazines. range mags are a mix of various pmag30 I bought before I standardized on the 300blk specific mags. |
The deuce you say.
USAF (Ret) 2003-2024
Joined:
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Posts:
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CO, USA
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Originally Posted By racer765: you didn't mention if you have any other AR - if you have a 5.56 you need to be careful to not mix magazines. supersonic 300blk will chamber in 5.56 or wylde chanbers and it's a bad day. if this is your only AR, not an issue. all my 300blk is in pmag30, all my 5.56 is in pmag40. supers work fine in 5.56 magazines. the fatter bullets in subsonic rounds are pushed sideways by the internal ribs at the front of the magazine. my bag gun has all 300blk specific magazines. range mags are a mix of various pmag30 I bought before I standardized on the 300blk specific mags. Paying attention to what one is doing is also a really good way to avoid any issues. 
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