Hey guys im pretty new to the AR world and have a question. I have a Black Rain Ordnance 5.56 and ive noticed that the mags and lowers are the same with 5.56 and .204Ruger so i was wondering if i could just do a barrel swap to switch between calibers or if i needed to have a complete upper assembly?
Thanks,
Parker

I'm not familiar with Black Rain Ordnance specs, but if  they use a standard barrel extension, gas tube and bolt carrier, then it should work fine.

There are a lot of rifle calibers such as .300 blackout, and 7.62x39mm where the bolt, barrel asembly and mag are all you need to change, as long as the rifle uses a standard upper and lower receiver, gas tube and bolt carrier.

I would ensure that those things are standard first, however.