Are 9mm and .223 receivers interchangeable?

[CarlosDJackal]

I am wondering if anyone has tried to see if a .223/5.56 upper work with a 9mm lower (after the mag block is removed and the correct buffer and buffer spring is installed, of course) and visa versa?

Thanks in advanced!!

[LMTRocks]

This could get complicated....
If a 9mm lower had the mag block removed and buffer and spring changed, then it probably started it's life as a .223/5.56 lower. It probably had the special 9mm hammer added to it, and the 9mm upper needed the special 9mm bolt. If you want to find out if the previously 9mm lower can go to .223/5.56, call the manufacturer and see. buffer, spring, and bolts and triggers usually make the weapon what it is intended to be. if you have a 9mm lower and want to see if
.223/5.56 can go through it, just get a stripped lower and build one.

[TangoChaser]

9mm and 5.56 lowers are identical once you remove the 9mm specific parts. The only exception is the Oly Arms Glock mag dedicated 9mm lower.

[kalikraven]

To further confirm this, I run a 300 whisper upper on a Colt 9mm marked lower and she runs fine.

[CarlosDJackal]

To further confirm this, I run a 300 whisper upper on a Colt 9mm marked lower and she runs fine.

Did leave in the 9mm hammer? Or did you swap this out with a regular AR (.223/5.56) hammer?

Sorry guys, I should have explained my intent. I expect to have a Transferable Colt SP1 convert in a few months and I was wondering that if I were to install a 9mm hammer into it if it would still work with a .223/5.56 upper (once I remove the mag block, 9mm buffer, and buffer spring)? That way I can shoot the less expensive 9mm ammo with it should I want to (I also have a .22LR Conversion Kit that should work - but I won't know for sure until the ATF approves my Form 4).

I have been trying to test this but due to the early sunset, I have not had the chance plus I did not want to go through all that hassle to remove the mag block and replace the buffer and spring if it turns out the 9mm hammer will not work with a .223/5.56 upper and BCG.

Thanks!!

[Jo Mama]

A 9mm hammer will work with a 5.56 upper. It won't work on full auto.

[kalikraven]

Well I also have a 9mm marked lower with a 9mm upper and I'm using regular 5.56 FCG. The trick is to have your bolt ramped. Then all you have to do to convert it is swap out the buffer, upper, and remove the mag block.

[CarlosDJackal]

Thanks guys!!

[CleverNickname]

9mm and 5.56 lowers are identical once you remove the 9mm specific parts. The only exception is the Oly Arms Glock mag dedicated 9mm lower.

RRA also has lowers that are 9mm-only. I don't really see the attraction though.