That is my home defense system, bought a PSA 8.5 upper and a multi lower. You can use reg mags for super sonic only, stick to Lancer Mags, Troy mags, and the mil-spec metal mags work best, Pmags are a hit and a miss. If you load subsonic you will have issues. I put an Omega 300 and it is super quiet. Labeling is a must like mentioned before, always double check, triple check.
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Very well said and very good advice. 
This won’t work for everyone but I run all 20 round mags for .300 Blackout and 30 round mags for 5.56.
Even my wife knows this rule so we never have any mix ups.
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I like that idea for my wife.
Thanks for sharing it.
i didn’t know this, thank you i only ever shot super sonic, this could help prevent future confusion
If you want to use 300 Blackout ammo, you’ll need an upper that is suitable for that caliber. The top PSA you are looking at is suitable for this purpose. However, if you have a desire to own a separate 300 Blackout rifle, this could be a convenient option, especially if you plan to use it for target shooting at the range. The decision depends on your personal preferences and purposes of use.
You can use 556 mags. They make 300 specific ones that feed better with subsonics i believe but regular pmags should be good
I myself use dedicated mags for 300. I used to hunt 220 subs. Keyhole at a 100 yds but still on target, massive entry but no exit. Hornaday 110 V max massive exit. Works well for deer too. It was my second all PSA build 16" rifle. Subs only for a 10 1/2" pistol for me now.
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Are you using a suppressor? Subs should not be keyholing at 100yrds…
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Absolutely awesome!!
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I was not
Subs are harder to get to work without a can, but that should not be keyholing at 100yrds.
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I stand corrected sir. Called my son and he said it was a 163 yd shot (we were hunting together), my bad. 100 yd was the 110 V max shot. Duh l hate getting old’r
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I do the exact same thing and I use the wristbands listed above. I also put a wristband on the rifle barrel so the family just needs to match the mag to the gun (same wristband).
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Welcome to the PSA community 
@matt2
Hey New here, but I have and have built up several ARs. Everything 5.56 except the barrel is the same. If you want to get into .300Bk buy an upper ready to go. I recommend labeling the upper somehow so you don’t accidently load the wrong ammo and have a bad day.
I installed a new ejection door dust cover that simply says .300 Blk. Easy and permanent label. I also use Mag Bands to label my .300Blk and Subsonic from my regular 5.56 mags. You could also dedicate a colored mag for fastand easy ID.
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you can’t accidentally load the wrong ammo in your 300blk… so, you actually need to do that to all your 5.56 instead…
Welcome to the forum!
I get his logic. If it’s not specifically marked 300 don’t even do it.