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Posted: 6/30/2026 9:21:34 AM EDT
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I am thinking about a pistol build specifically as a bag gun. While 5.56 would be kinda sorta OK, given the muzzle blast and flamethrower effect I am hoping for a more-bullet/less-powder option. Here are my wants: 8-10 inch barrel supersonics only 0.378" rim diameter runs in standard, unmodified GI mags a cartridge that cannot be forced into a 5.56 chamber (i.e. not .300BO) .277 Wolverine seems to check the boxes, I think. Brass may be a PITA. I like the .25 Copperhead on paper, but I think it'll chamber in a 5.56. If you know different, holler. Not sure about the 6mm-223/6x45 wildcats. Not sure about the 7.62WT. Any recommendations? thx |
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Originally Posted By CreakyKneesM193: I am thinking about a pistol build specifically as a bag gun. While 5.56 would be kinda sorta OK, given the muzzle blast and flamethrower effect I am hoping for a more-bullet/less-powder option. Here are my wants: 8-10 inch barrel supersonics only 0.378" rim diameter runs in standard, unmodified GI mags a cartridge that cannot be forced into a 5.56 chamber (i.e. not .300BO) .277 Wolverine seems to check the boxes, I think. Brass may be a PITA. I like the .25 Copperhead on paper, but I think it'll chamber in a 5.56. If you know different, holler. Not sure about the 6mm-223/6x45 wildcats. Not sure about the 7.62WT. Any recommendations? thx 300 Ham'r meets all your requirements except the mag. It requires 300 Blackout magazines to get 30 rounds with no feeding issues. |
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You want a short barrel biased cartridge (like 300blk) that: Uses a 5.56 bolt (300blk, 300Ham'r, 350 Legend, 6MAX) Uses a 5.56 mag (300blk) Isn't 300blk I don't think a modern cartridge exists that meets all of that criteria, especially if you are trying to incorporate the "less powder/more bullet" criteria. |
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.300 AAC is the round you're looking for, although I understand the reservations. I have become pretty anal about keeping the ammo and magazines separate. I even ditched all of my 5.56 Pmags so that my .300 Pmags are the only ones I have, and they have mag bands for further ID. When I take the .300 out I usually leave the 5.56 guns at home, especially if shooting with other people. But the short .300 is still one of may favorite AR rifles. It's what the cartridge was designed for and it shines within it's performance envelope (with supers). |
| I have a 12” 6.5 Grendel. I shoot 123 grain ELD-M or 90 grain TNT. Granted the 6.5 Grendel requires a 6.5 Grendel bolt, barrel and magazines but the performance is there with mild recoil and less blast than a short 5.56. The 6.5 Grendel is limited to 52,000 psi pressure but even at 2200 FPS the 123 grain ELD-M or 123 grain SST the down range effectiveness is surprisingly good. |
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Originally Posted By JKinAZ: .300 AAC is the round you're looking for, although I understand the reservations. I have become pretty anal about keeping the ammo and magazines separate. I even ditched all of my 5.56 Pmags so that my .300 Pmags are the only ones I have, and they have mag bands for further ID. When I take the .300 out I usually leave the 5.56 guns at home, especially if shooting with other people. But the short .300 is still one of may favorite AR rifles. It's what the cartridge was designed for and it shines within it's performance envelope (with supers). One additional thing to think about for prevention of jamming a .300 round into a 5.56 chamber, might be to put Ranger baseplates on all your .300 mags. Very strong visual and tactile difference. Something that occurs to me that I never see mentioned; use of a loading block. You know, one of those wood rectangles with a grid of holes. For one's "boutique" cartridge, if you make a habit of going from box to block to mag in a very controlled environment, there would be a very clear visual "one of these things is not like the others" indicator. Since I already have a whole bunch of Pmags and GI mags, both 20- and 30-rounders, I do plan on tasking the GI mags for an alternative caliber. |
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6x45 would be my choice if you don't go 300bo. You can load straight from 5.56 brass without any extra steps, just well lubed cases. 300bo is my preference though. I use lancer mags for my 300's, and gi/pmags for all the 5.56 guns. |
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Op wants everything 300 blackout does but doesn't want a 300 blackout. Here's what I did when I built my 300 blackout pistol. I took ten mags and dedicated them to 300 blackout with these. Do they look funky? Sure. Will it stop me from making a mistake? Absolutely. mag bands  
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Originally Posted By Crash1433: Op wants everything 300 blackout does but doesn't want a 300 blackout. Here's what I did when I built my 300 blackout pistol. I took ten mags and dedicated them to 300 blackout with these. Do they look funky? Sure. Will it stop me from making a mistake? Absolutely. mag bandswww.amazon.com/dp/B0BJBKW4CX Mag bands will help, but I don't think the error is from loading mags full of .300 Blackout into a .223 gun. It's from loading a magazine with .223 on top with a single .300 Blackout round in the mag. That's why I advocate for clear mags in 5.56, so you can visually check each mag. And even that's not going to save you at the range when your buddy asks to shoot your gun, you say sure, step off to take a piss, and he decides he doesn't want to burn your ammo, so he'll shoot his ammo out of your gun. If you ditch your H3 buffer and your forward assist, you're much less likely to grenade a .300 Eldest Son round in your 5.56 rifle. OP, you're likely going to have the same issue w/ any 5.56 derived cartridge, ie, 6x45mm, 6.5x45mm, 7x45mm, etc, as you would with Blackout. If you'll step up to a larger cartridge base, it becomes impossible to muscle the wrong case into the chamber - 6.5 Grendel, 6.8 SPC, even 7.62x39mm all do well out of fairly short barrels. An alternative would be to put a Fold AR upper on your rifle, that folds to 17" or less. |
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Originally Posted By Crash1433: Op wants everything 300 blackout does but doesn't want a 300 blackout. Here's what I did when I built my 300 blackout pistol. I took ten mags and dedicated them to 300 blackout with these. Do they look funky? Sure. Will it stop me from making a mistake? Absolutely. mag bandswww.amazon.com/dp/B0BJBKW4CX I put mag bands on EVERYTHING...uppers and all. All 22LR uppers have 22LR mag bands. All 5.56 uppers have 5.56 mag bands All 300blk uppers have 300blk mag bands All 22LR mags have 22LR mag bands. 22LR mags are obvious All 5.56 mags have 5.56 mag bands. I don't shoot 5.56 much anymore, but the plan was to use 5.56 pmags for these. All 300blk mags have 300blk mag bands. Translucent Lancer 5.56 for 300blk (I do have one 20 round 300blk specific pmag that I wanted for any problem rounds) In theory it makes more sense to have 5.56 in see-through mags, but I don't shoot it much, and Lancer's don't have the ribs that pmags do. The ribs can interfere with certain 300blk rounds. Got an email from Phantom Defense today, they had their black NAS3 300blk rounds in stock again (OOS-they sell out in hours). I almost bought some. I have some black ADI 300blk rounds from a long time ago. Hard to mix up rounds that have black cases. 200gr Maker and 120gr Tac-tx  I still have some of this:  |
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I did some quick research on the .300 mag situation. People generally like Lancer's .300 mags, though I read one claim that they don't feed supers as well as subs. Generally I couldn't care less about translucent mag bodies, but in a mixed 5.56/.300 world, it makes sense to use the translucent Lancers for a boutique round. I'm not worried about the "friend slapping a .300 mag into a 5.56 build" scenario, they can bring their own damned guns. I only let someone else shoot one of my guns about once a decade; I stop what I'm doing and watch them like a hawk. If I do go with .300 for 8 or 9 inch pistol builds, I'd need to get rid of my two pistol length 5.56 barrels so that when I'm in pistol context, I'm in .300 context only. Why is it every time I hop on ARFCOM my credit score goes down? You bastards! |
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Originally Posted By jjz3: 30 carbine would be neat. Hydra Arms had one on display at shot A few months back I looked at 5.7x28 for this sort of PDW use case, I found it underwhelming. However, its uncle, the .22 Spitfire, is pretty cool; https://en.wikipedia.org/wiki/.22_Spitfire .30 Carbine case necked down to .224, nice shoulder, plenty of neck length. |
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Originally Posted By Crash1433: Op wants everything 300 blackout does but doesn't want a 300 blackout. Here's what I did when I built my 300 blackout pistol. I took ten mags and dedicated them to 300 blackout with these. Do they look funky? Sure. Will it stop me from making a mistake? Absolutely. mag bandswww.amazon.com/dp/B0BJBKW4CX This is what I did. All my .300 mags have bands on them, but I also took a band and dropped my trigger guard and put a band on the the magwell of my lowers. I have one .300 carbine and one in Pistol. Both have bands around the magwell. I also bought "subsonic" bands so mags with subsonic .300 have 2 bands on them. |
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Well, shoot. Looks like .300 it is. .300 HAMR and .277 WLV are probably fine, but probably better for rifles than pistols. Seems like when someone develops an interesting wildcat, .300BLK sneaks in and strangles it in the crib. Looking at brass, bullets, and factory ammo, .300 cannot be beat at this time. I've made the command decision that AR pistol(s) will only be .300, rifles will only be 5.56 (and .350L). I'm thinking black 20-rd Pmags with Magpul Ranger plates for .300, FDE 30-rd GIs and Pmags for 5.56 (which means I need to get some FDE paint). |
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Originally Posted By jjz3: 30 carbine would be neat. Hydra Arms had one on display at shot There are several SSK Whisper cartridges based on the .221 Fireball (like the .300 Blackout) from 6mm through the 7mm. Will they feed reliably? Don't know, but the OP isn't missing much with his .300 barrel. . |
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Originally Posted By CreakyKneesM193: Yeah, not doing that. Has to be a 0.378 bolt face. I forgot to mention, I'm mainly interested in the 5-25 yard fight, unlikely as that is (I hope). I have no interest in 200 yard ballistics or any of that. Originally Posted By CreakyKneesM193: Yeah, not doing that. Has to be a 0.378 bolt face. I forgot to mention, I'm mainly interested in the 5-25 yard fight, unlikely as that is (I hope). I have no interest in 200 yard ballistics or any of that. 300 Blackout is the obvious and ideal option for what you are describing, other than magazine compatibility. Pretty much any alternative cartridge is going to work best with a suitable magazine. Originally Posted By Crash1433: Op wants everything 300 blackout does but doesn't want a 300 blackout. Here's what I did when I built my 300 blackout pistol. I took ten mags and dedicated them to 300 blackout with these. Do they look funky? Sure. Will it stop me from making a mistake? Absolutely. mag bandswww.amazon.com/dp/B0BJBKW4CX I used color tape bands on both the firearms and magazines, kind of like the Israeli IFF bands idea. |
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