I am sure this has been answered a hundred times on here but I am asking again and hope I don’t get crushed. I have a Radical Firearms 10.5" barrel pistol chambered in 5.56. I would like to also have it chambered in 300 blackout. I know the barrel has to be changed but what else has to be or should be changed. I want it to function properly and at some point I will have a suppressor on it but I will not to start off with. Any advice would be appreciated. Once again I apologize if this has been covered already but I am new to the forum. Thanks in advance.
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Honestly just the barrel. But a few pointers…
- Safety issues: 300blk WILL chamber in a 5.56 with a false headspace. Always make sure you have the right ammo for the right upper.
- Because of 1, I strongly recommend using a color or some sort of marking system for your 300blk upper and mags. Use something different than your 5.56. You might want want consider mags as well, you can use the same for both rounds, but decide now how you want to mark your mags. I originally used pmags for 556 and metal for 300blk.
- although it is only a barrel change, buy or build a 300blk upper. 300blk usually uses a different buffer (heavier) than a standard carbine, and as cheap as complete blem lowers are, just buy one and have 2 guns.
- I recommend a new gun because 300blk can be finicky. Subs probably won’t run without a can, and the addition of a can means you may have to change something again. An adjustable gas block will be helpful, but not required.
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I agree with all that was said.
For #2 - I purchased some PMAGS made for 300AAC - same price as other PMAGS - I suspect the follower may be a little different. Anyways, they state 300 AAC on them and I use the rubber magazine bands to identify them. I also fitted one of those on the lower (around the mag well, have to remove the trigger guard to add it). That way I just match colors - as long as the mags are loaded correctly.
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Just my 2cents worth. Just build another pistol. That way you don’t have to worry about changing barrels, mags, buffers, whatever. I was going to have a couple uppers and just move my lower back and forth… but then it’s like why. Just get another lower. Done.
In any scenario I can think of, 2 pistols are better than one. 
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I guess my thinking was to save some money by just getting a 300 barrel. Wouldn’t it be a much higher cost in building a whole new AR pistol? I think I would probably keep as a 300 blackout most of the time because that cartridge I think will do better ballistically from such a short barrel compared to the 5.56. I have a Galil that is chambered in 5.56 so I wouldn’t be missing out much. What would be a round about cost of just building a complete AR chambered in 300 versus just a new barrel and even a new buffer?
$500-600 minus lower for whole pistol
i have a .300 upper sitting in the safe for when want to shoot BO
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Yes it is easier to just swap a barrel. But I would also get a new BCG for it so you have a dedicated bcg for each barrel and each round. Funky things can happen when you start swapping a bcg between two barrels. You also will need a different buffer set up also, so although it is easy to swap a barrel, not necessarily cheaper.
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Are you planning to swap the barrel and resight your rifle every time you plan to change calibers?
That’s gonna be a lot of time and ammo wasted.
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But how often can you actually switch barrels before the threads on the receiver start to feel the pain of being torqued and removed several times. And be sure to have gauges to check headspace each time. I don’t know, maybe it’s just me but I kind of like getting a gun together and setting it up just right for it’s intended purpose. I then identify that gun for it’s intended purpose. Immediate defense, CQB, mid-range battle or long range tack driver.
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So how about if I get a 300 barrel, change my buffer and get another BCG to go with the 300 cartridge, barrel and buffer and just wait until I have a little more cash and then build another 5.56 pistol later.
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That would work…but by the time you buy a barrel ($100+), bcg ($75-100), and buffer ($30), you can almost buy an upper. PSA has a 16" for $289 (would need bcg) and they go up from there…
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those fancy NiB PSA BCGs are on sale for under $125 today…
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your choice. And if your the type of guy that likes to tinker with your guns, it’s a good choice. Make sure you get good tool kits and gauges. Torque wrench and armorer’s tool or crow foot wrench set, headspace gauges, vise block. That sort of stuff. Or you can just have a gunsmith swap out the barrel and get you a new bcg and check the headspace for you. Oh, and check that your feed ramp is sized for a .30 cal and not the .223.
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Build or buy another upper. Then you can swap back and forth by just popping the pins.
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You’ll also likely want a pistol length gas tube, as its the most reliable gas system for shooting subs and supers.
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Will this kit have everything I need in it? It’s PSA model # 5165449132B. It is a blemish item but as long as it is mechanically functional that’s all I am worried about.
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It’s a complete pistol upper with a charging handle and a BCG. Put it in a complete pistol lower and you’re ready to rock! Though yo might need mags, I’m not sure if 5.56 mags will feed 300 blk reliably.
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My 5.56 pmags have been gtg with 300blk. Lancers and Troy’s also.
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I use 5.56 mags exclusively with 300 Blackout. Zero issues so far…
Hexmags and PMags
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