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5.56 or 300BO SBR (Page 1 of 2)
Posted: 8/15/2026 11:54:42 AM EDT
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I am going to put together an SBR next and can't decide between something like a 11.5 5.56 or a 300BO. Uses for the gym would be HD and shooting 200yds or less. Only draw for me towards 300 is that with subs and a suppressor it's really quite but then I've heard some say they can be finicky to get to run reliable. I'm not going to be out shooting 100s of rounds a week so the added cost of the ammo isn't going to be a factor. |
| My sons 12.5" 5.56 is a pretty handy rifle. However even suppressed it's loud. My only experience with .300 is shooting subs out of my buddy's suppressed sbr. It put a big smile on my face. I'd imagine you'll want to suppress it regardless of caliber if you plan on shooting supers. |
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Discrete Ballistics I settled on these for my hd 300blk sbr. I've never been concerned with an intruder wearing armor or anything like that. You're most likely to encounter multiple intruders rather than one dressed and ready for Call of Duty |
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300 with subs is movie quiet with my suppressor. With supers, it’s OK. 10.5” It works perfectly with subs or supers and with or without the suppressor. No adjustments needed. I have zero interest in a short 5.56. I do need to choose a proper stock and put the brace in the closet. 
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10/22/14 I stand with Canada
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Originally Posted By ZoToL: After playing with a 8" 300blk and suppressor, even an 11.5" with can in 5.56 seems long. And with the NFA thing going down Its hard to want anything longer. It is funny how that works. You really get accustomed to the short barrels. Hardest part is (was) rail space on super shorties, but depending on state laws (and if you are a covered party), 90 degree vertical grips are back on the menu! |
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Originally Posted By alacop: For HD the 300 suppressed is great. I keep 110 Barnes in mine, not subsonic, as subs won't defeat sotf armour. Still easier on the ears than 5.56. 8.5 barrel is the sweet spot. My primary HD rifle is a BCM 9" 300 BLK, suppressed, loaded with 110gr Barnes Tax-TX. I figure it should do the trick. It runs supers and subs, suppressed and unsuppressed, 100% with an H2 buffer and springco blue. I can't think of anything worth replacing it with if I wanted to. |
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Originally Posted By GimmeLibertee: My primary HD rifle is a BCM 9" 300 BLK, suppressed, loaded with 110gr Barnes Tax-TX. I figure it should do the trick. It runs supers and subs, suppressed and unsuppressed, 100% with an H2 buffer and springco blue. I can't think of anything worth replacing it with if I wanted to. exact same setup i have for HD. no regrets. |
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Originally Posted By alacop: For HD the 300 suppressed is great. I keep 110 Barnes in mine, not subsonic, as subs won't defeat sotf armour. Still easier on the ears than 5.56. 8.5 barrel is the sweet spot. A fellow man of culture. Barnes 110g Tac-tx are brutal and I'm okay with them being a bit louder since they're still quieter than suppressed 556. |
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Originally Posted By panthermark: It is funny how that works. You really get accustomed to the short barrels. Hardest part is (was) rail space on super shorties, but depending on state laws (and if you are a covered party), 90 degree vertical grips are back on the menu! I used an oversized handguard so I could tuck the can. 8" barrel and the handguard is 10.7" if I remember correctly. Allows me to have the same hand position for my home defense 300 blk and my 13.9" general purpose 556. |
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Originally Posted By DVCNick: Subs are falling off a cliff by 100yd. Subs are the same performance envelope as a pistol round, should be a little better ballistics than a hot .45acp, but that's about where it's at. Yep, there's a reason 300 blk subs are sometimes referred to as Spicy 45. The main difference is that you can get much nastier wound channels with the flower petal subsonic rounds available with those long 300 blackout bullets. |
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I put together a 10.5” .300 BO just for shits and giggles. Hadn’t ever shot or used the round so I was curious. Honestly I love it. I do regret not dropping down to 8 inches or so. Subs are fun, sending 220 grain bullets to 200 yards makes everyone on the range giggle and ask questions. With electronic ears the bullet hitting the steel seems louder than the gun. Though holding 4 feet high isn’t really practical. My favorite though is the 110 grain class of bullets. Even without a supressor they are much more pleasant to shoot and be around than my 10.5” 5.56 ever was. I didn’t enjoy that upper and everyone around always whined how loud it was. I do really enjoy a good supressed 12.5” 5.56 though. I vote get 2 of each. 
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Its a Geisele .300 8" " Phazed Array" barrel. Its pretty light. This is my favorite setup, over years of building and fads I think I'm happy till integrally suppressed barrels start showing up. Right at 6lbs unloaded. the optic is a place holder till the AEMS Macro comes out, and on the fence to see what happens but also a OC Hydrogen L Suppressor.... Carbon handguard because its nice in northeastern winters. I'm not dumping money into thermal and etc. Just need a solid rifle. Attached File |
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I vote 300 BLK. While it can be finicky, its not at that barrel length. A decent barrel in the 9-10.5+ range will run reliably with both subs and supers. When you start getting down into the 7-8 inch range it can be more involved to get to run with both. But that's also where 300blk shines. Don't sleep on 5.56 though, 11.5 is near optimal for suppressed use. Do you plan to shoot a lot of subs? Maybe even hunt deer? Shorter than 10.5?--- 300 BLK Supers only? 11.5+? --- 5.56 |
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Originally Posted By Vorgath: Because the 11.5 has roughly 40 percent more dwell time, compared to a 10.5. That bcm marketing schtick needs to be put into conext. How many 10.3" .07" gas port sized mk18s were/are failing regularly? Sure, 11.5" has 40% more dwell time, but was it necessary to begin with? Why not 12.5" for 60%(number pulled from 4th point of contact) more dwell time, because more is better and all that? I'm not trying to shit on bcm, i own and like bcm, but context needs to be applied to marketing. |
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Never had a single issue with my 10.3s/10.5s. I’ve been shooting them without any issues for a very LONG time in many iterations. I love that length. I also love my 11.3s/11.5s. Honestly Iove my 300BLK SBRs, but I would own a 556 SBR before a 300BLK, even though 8.3/8.5 300BLK rifles are my HD go-tos. GET MULTIPLES OF BOTH. |
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Originally Posted By HobbitHunter: That bcm marketing schtick needs to be put into conext. How many 10.3" .07" gas port sized mk18s were/are failing regularly? Sure, 11.5" has 40% more dwell time, but was it necessary to begin with? Why not 12.5" for 60%(number pulled from 4th point of contact) more dwell time, because more is better and all that? I'm not trying to shit on bcm, i own and like bcm, but context needs to be applied to marketing. In my opinion, yes... the "marketing schtick" is very true in this case... the 10.5 is already out on the edge of true useability, so that 40% increase and little bit of extra barrel makes a big difference in pretty much every aspect of how the rifle runs. This is why I dumped 10.5 for 11.5 in my shortest 556s (not BCM) Whereas 300blk is right in its "good" zone in 8-10" DIs. No ridiculous carrier velocities, mega gas clouds suppressed, etc. I did have a 6.5" DI 300blk I sold because it was too high strung, similar to a 10.5 556. Now I have a Rattler... piston runs better a those super short lengths. |
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Originally Posted By DVCNick: In my opinion, yes... the "marketing schtick" is very true in this case... the 10.5 is already out on the edge of true useability, so that 40% increase and little bit of extra barrel makes a big difference in pretty much every aspect of how the rifle runs. This is why I dumped 10.5 for 11.5 in my shortest 556s (not BCM) Whereas 300blk is right in its "good" zone in 8-10" DIs. No ridiculous carrier velocities, mega gas clouds suppressed, etc. I did have a 6.5" DI 300blk I sold because it was too high strung, similar to a 10.5 556. Now I have a Rattler... piston runs better a those super short lengths. Originally Posted By DVCNick: Originally Posted By HobbitHunter: That bcm marketing schtick needs to be put into conext. How many 10.3" .07" gas port sized mk18s were/are failing regularly? Sure, 11.5" has 40% more dwell time, but was it necessary to begin with? Why not 12.5" for 60%(number pulled from 4th point of contact) more dwell time, because more is better and all that? I'm not trying to shit on bcm, i own and like bcm, but context needs to be applied to marketing. In my opinion, yes... the "marketing schtick" is very true in this case... the 10.5 is already out on the edge of true useability, so that 40% increase and little bit of extra barrel makes a big difference in pretty much every aspect of how the rifle runs. This is why I dumped 10.5 for 11.5 in my shortest 556s (not BCM) Whereas 300blk is right in its "good" zone in 8-10" DIs. No ridiculous carrier velocities, mega gas clouds suppressed, etc. I did have a 6.5" DI 300blk I sold because it was too high strung, similar to a 10.5 556. Now I have a Rattler... piston runs better a those super short lengths. More than anything, the extra inch probably makes it more forgiving for a wider selection of ammunition. Been eyeballing a 7-3/4" rattler in 5.56, either that, or the 9" MCX for a bag gun with a bit more punch than a 9mm. I like 300Blk in concept, but the ammo is pretty fucking steep compared to 5.56, with rather limited options for good subsonic loads that are all stupid expensive. On top of this, there are plenty of soft points that still have a 100+ yard effective range out of a 7" barrel, plenty for the intended use case of such a tiny PDW. Far as sound signature, supers are supers, either one is bad juju. But for subs, 9mm JHP are more consistent than all but the most expensive of subsonic 300 options. I figure if I want the most discrete option possible, a suppressed PCC is going to be a better bet; small enough to fit in a bag with the can attached, and several have good FRT options, which I think narrows the difference in firepower. I'm unaware of anyone getting subsonic 300 working with a FRT... Even FMJ would be sufficient in that case, assuming one works out the logistics of tuning and gear setup. |
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11.5 5.56 with a modern low-back pressure suppressor, like the Rhodie6, CAT WB, Echocore, Fireteam 5.56... etc. And a FRT/FRS/SS. I know mine is "hearing safe"... well, enough so that I can shoot without hearing protection and do not experience any ringing or pain/discomfort. I'm very, very impressed with the performance of modern, 3-d printed, flow-through/low-backpressure suppressors on 5.56, They actually work now. |
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Originally Posted By Brahmzy: Never had a single issue with my 10.3s/10.5s. I’ve been shooting them without any issues for a very LONG time in many iterations. I love that length. I also love my 11.3s/11.5s. Honestly Iove my 300BLK SBRs, but I would own a 556 SBR before a 300BLK, even though 8.3/8.5 300BLK rifles are my HD go-tos. GET MULTIPLES OF BOTH. This is the way. Get both. |
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I went down the 300BLK in an AR rabbit hole. Could never get the gun the run reliable. 300BLK just does not have the gas pressure/energy to power through problems. It will get dirty very fast and then start having problems. 5.56 SBR is the way to go. I built a 10.3" SBR. Has a 10.3" Geissele barrel which is super accurate. OCL Polonium for suppression. Superlative Arms piston conversion because I got tired of getting gassed out by even unsuppressed AR's. Maybe consider something a bit different. Ruger American Gen2 in 308Winchesher. Several versions of this bolt action to choose from. I have the Scout version that has excellent AR style sights. And there is a lot of 308Win subsonic such as the excellent 200gr S&B. But you can also shoot full power 308 as well. Threaded barrel for adding a suppressor. |
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Originally Posted By Inverted_Polarity: Maybe consider something a bit different. Ruger American Gen2 in 308Winchesher. Several versions of this bolt action to choose from. I have the Scout version that has excellent AR style sights. And there is a lot of 308Win subsonic such as the excellent 200gr S&B. But you can also shoot full power 308 as well. Threaded barrel for adding a suppressor. 
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Originally Posted By rb889: More than anything, the extra inch probably makes it more forgiving for a wider selection of ammunition. Been eyeballing a 7-3/4" rattler in 5.56, either that, or the 9" MCX for a bag gun with a bit more punch than a 9mm. I like 300Blk in concept, but the ammo is pretty fucking steep compared to 5.56, with rather limited options for good subsonic loads that are all stupid expensive. On top of this, there are plenty of soft points that still have a 100+ yard effective range out of a 7" barrel, plenty for the intended use case of such a tiny PDW. Far as sound signature, supers are supers, either one is bad juju. But for subs, 9mm JHP are more consistent than all but the most expensive of subsonic 300 options. I figure if I want the most discrete option possible, a suppressed PCC is going to be a better bet; small enough to fit in a bag with the can attached, and several have good FRT options, which I think narrows the difference in firepower. I'm unaware of anyone getting subsonic 300 working with a FRT... Even FMJ would be sufficient in that case, assuming one works out the logistics of tuning and gear setup. Originally Posted By rb889: More than anything, the extra inch probably makes it more forgiving for a wider selection of ammunition. Been eyeballing a 7-3/4" rattler in 5.56, either that, or the 9" MCX for a bag gun with a bit more punch than a 9mm. I like 300Blk in concept, but the ammo is pretty fucking steep compared to 5.56, with rather limited options for good subsonic loads that are all stupid expensive. On top of this, there are plenty of soft points that still have a 100+ yard effective range out of a 7" barrel, plenty for the intended use case of such a tiny PDW. Far as sound signature, supers are supers, either one is bad juju. But for subs, 9mm JHP are more consistent than all but the most expensive of subsonic 300 options. I figure if I want the most discrete option possible, a suppressed PCC is going to be a better bet; small enough to fit in a bag with the can attached, and several have good FRT options, which I think narrows the difference in firepower. I'm unaware of anyone getting subsonic 300 working with a FRT... Even FMJ would be sufficient in that case, assuming one works out the logistics of tuning and gear setup. This is why we have choices, but I disagree with basically every part of this. Anything under 10.5, 300blk is way better running, way better suppressed (supers or subs), better terminal performance (super or sub), works great with DI down to about 8", and less than that with a piston. 5.56... just not meant for these barrel lengths. Can a 5.56 rattler run at these lengths, I'm sure it can, but I don't consider it close to optimal when 300blk exists in the exact same systems. As far as ammo cost... 300blk is a reloader's dream. All the components are readily available/shared by other cartridges and blasting ammo is cheap to load, second only to 5.56. If there was ever a reason to start loading, it is 300blk. Top shelf defensive ammo... you just need a few mags of it, so I don't really consider cost as a significant factor. vs 9mm: Talk about functionally finicky, 9mm subguns are up there; at least most of them. Even with subs, you're giving up a lot of mass to 300blk. Supers (what I keep in mine) no comparison at all. FRT- these are range toys with no business being in a "serious use"/defensive gun, in my opinion. I went down the 300BLK in an AR rabbit hole. Could never get the gun the run reliable. 300BLK just does not have the gas pressure/energy to power through problems. It will get dirty very fast and then start having problems. I don't know what you had, but get a BCM 9" with a Sprinco white and H3, and it will run any good ammo you can throw at it. Like any DI AR SBR, it will get dirty especially suppressed and will need to lube it occasionally to keep it running its best. |
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12.5 5.56 SBR Attached File 300 blk subs are basically a pistol round with pistol ballistics. |
Isaiah 1:18 - "Come now, let us reason together," says the LORD: "though your sins are like scarlet, they shall be as white as snow"
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Originally Posted By Fooboy: 12.5 5.56 SBR https://www.ar15.com/media/mediaFiles/113559/IMG_9426_jpeg-3816981.JPG 300 blk subs are basically a pistol round with pistol ballistics. "Expanding" subs? Somewhat.... most (188gr, 190gr, 194gr and 200gr) fall between 9mm and 45acp expanding subs, but with more accuracy and range (and cost). If you have a 1:5 twist, 242gr subs (Maker Rex) are available. Some non-expanding subs (especially the 190gr TUI's) can get through soft IIIA armor. What about supers? |
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Originally Posted By Fooboy: 12.5 5.56 SBR https://www.ar15.com/media/mediaFiles/113559/IMG_9426_jpeg-3816981.JPG 300 blk subs are basically a pistol round with pistol ballistics. This guy, I like what this guy recommends. Use a rifle rounds to get rifle terminal ballistics. Plus 5.56 is better for any over penetration concerns. |
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Originally Posted By DVCNick: This is why we have choices, but I disagree with basically every part of this. Anything under 10.5, 300blk is way better running, way better suppressed (supers or subs), better terminal performance (super or sub), works great with DI down to about 8", and less than that with a piston. 5.56... just not meant for these barrel lengths. Can a 5.56 rattler run at these lengths, I'm sure it can, but I don't consider it close to optimal when 300blk exists in the exact same systems. As far as ammo cost... 300blk is a reloader's dream. All the components are readily available/shared by other cartridges and blasting ammo is cheap to load, second only to 5.56. If there was ever a reason to start loading, it is 300blk. Top shelf defensive ammo... you just need a few mags of it, so I don't really consider cost as a significant factor. vs 9mm: Talk about functionally finicky, 9mm subguns are up there; at least most of them. Even with subs, you're giving up a lot of mass to 300blk. Supers (what I keep in mine) no comparison at all. FRT- these are range toys with no business being in a "serious use"/defensive gun, in my opinion. I don't know what you had, but get a BCM 9" with a Sprinco white and H3, and it will run any good ammo you can throw at it. Like any DI AR SBR, it will get dirty especially suppressed and will need to lube it occasionally to keep it running its best. Originally Posted By DVCNick: Originally Posted By rb889: More than anything, the extra inch probably makes it more forgiving for a wider selection of ammunition. Been eyeballing a 7-3/4" rattler in 5.56, either that, or the 9" MCX for a bag gun with a bit more punch than a 9mm. I like 300Blk in concept, but the ammo is pretty fucking steep compared to 5.56, with rather limited options for good subsonic loads that are all stupid expensive. On top of this, there are plenty of soft points that still have a 100+ yard effective range out of a 7" barrel, plenty for the intended use case of such a tiny PDW. Far as sound signature, supers are supers, either one is bad juju. But for subs, 9mm JHP are more consistent than all but the most expensive of subsonic 300 options. I figure if I want the most discrete option possible, a suppressed PCC is going to be a better bet; small enough to fit in a bag with the can attached, and several have good FRT options, which I think narrows the difference in firepower. I'm unaware of anyone getting subsonic 300 working with a FRT... Even FMJ would be sufficient in that case, assuming one works out the logistics of tuning and gear setup. This is why we have choices, but I disagree with basically every part of this. Anything under 10.5, 300blk is way better running, way better suppressed (supers or subs), better terminal performance (super or sub), works great with DI down to about 8", and less than that with a piston. 5.56... just not meant for these barrel lengths. Can a 5.56 rattler run at these lengths, I'm sure it can, but I don't consider it close to optimal when 300blk exists in the exact same systems. As far as ammo cost... 300blk is a reloader's dream. All the components are readily available/shared by other cartridges and blasting ammo is cheap to load, second only to 5.56. If there was ever a reason to start loading, it is 300blk. Top shelf defensive ammo... you just need a few mags of it, so I don't really consider cost as a significant factor. vs 9mm: Talk about functionally finicky, 9mm subguns are up there; at least most of them. Even with subs, you're giving up a lot of mass to 300blk. Supers (what I keep in mine) no comparison at all. FRT- these are range toys with no business being in a "serious use"/defensive gun, in my opinion. I went down the 300BLK in an AR rabbit hole. Could never get the gun the run reliable. 300BLK just does not have the gas pressure/energy to power through problems. It will get dirty very fast and then start having problems. I don't know what you had, but get a BCM 9" with a Sprinco white and H3, and it will run any good ammo you can throw at it. Like any DI AR SBR, it will get dirty especially suppressed and will need to lube it occasionally to keep it running its best. No idea what was screwed up on his, but my 10.5” shoots supers and subs, with and without the suppressor. No adjustments or changes needed. It runs everything, all the time. |
10/22/14 I stand with Canada
| I'm going to do the 11.5" 5.56 since I'm flush with ammo for that. Just submitted the Form 1 today so the wait begins. Honestly, I wouldn't care if it takes a couple months because I wasn't wanting to start another expensive project until winter and it'll be hard not to finish it as soon as it's approved. |
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Originally Posted By gunner76: I'm going to do the 11.5" 5.56 since I'm flush with ammo for that. Just submitted the Form 1 today so the wait begins. Honestly, I wouldn't care if it takes a couple months because I wasn't wanting to start another expensive project until winter and it'll be hard not to finish it as soon as it's approved. I submitted a Form 1 yesterday that was approved today. Individual. I didn’t even have time to get a stock lol. |
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Originally Posted By alacop: For HD the 300 suppressed is great. I keep 110 Barnes in mine, not subsonic, as subs won't defeat sotf armour. Still easier on the ears than 5.56. 8.5 barrel is the sweet spot. this I have a super shorty 300blk bedside gun |
I wouldn't stand in front of a piss-filled supersoaker. Does that make it a good pistol? - Caboose314
I thought I was covered for 22 cans, but the NFAids is a bitch when it mutates - themagikbullet
I thought I was covered for 22 cans, but the NFAids is a bitch when it mutates - themagikbullet
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Originally Posted By gunner76: I'm going to do the 11.5" 5.56 since I'm flush with ammo for that. Just submitted the Form 1 today so the wait begins. Honestly, I wouldn't care if it takes a couple months because I wasn't wanting to start another expensive project until winter and it'll be hard not to finish it as soon as it's approved. Nothing wrong with that. Get one of the new generation short and thicc cans like the Tisha or Rhodie and that'll still be a nice short setup I like the Hornady 75gr "SBR" ammo out of my shorties, it's noticeably quieter If you reload, you can get factory 2nds projectiles from Midway and Lovex SBR powder makes a pretty close recipe |
| I had this debate and went between a 10.3-12.5 5.56 and a 8-10.5 300.....settled on a 11.5 5.56. The Short 300s are handy, but you run out of handguard REALLY quickly. I did tucked cans, unconventional mounting of accessories, all kinds of stuff. I like the 10" handguard length, so the 11.5 barrel ended up being perfect. Training is also cheaper w/ 5.56 ammo and honestly the ballistics on a 11.5 is perfect for what I'm wanting from the gun. |
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Originally Posted By panthermark: It is funny how that works. You really get accustomed to the short barrels. Hardest part is (was) rail space on super shorties, but depending on state laws (and if you are a covered party), 90 degree vertical grips are back on the menu! Rail space solution - get a rail that extends over the suppressor |
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Originally Posted By gunner76: I'm going to do the 11.5" 5.56 since I'm flush with ammo for that. Just submitted the Form 1 today so the wait begins. Honestly, I wouldn't care if it takes a couple months because I wasn't wanting to start another expensive project until winter and it'll be hard not to finish it as soon as it's approved. https://bkingsfirearms.com/shop/complete-upper-receivers/bkf-ar15-11-5-5-56-nato-1-7-twist-m4-commando-colt-model-935-complete-upper-receiver/ |
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Qoute : FCSD - Hold up! I could have tip toed around here with just my hand covering my ass, instead of jamming my thumb in my asshole? Y'all motherfuckers need a user manual or something.
Montani Semper Libre
Qoute : FCSD - Hold up! I could have tip toed around here with just my hand covering my ass, instead of jamming my thumb in my asshole? Y'all motherfuckers need a user manual or something.
5.56 or 300BO SBR (Page 1 of 2)
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