I've had a Frontier Armory lower for a few years. Never got around to making/getting an upper for it. I've tried it with .223 upper and it works great.
It came with a second hammer intended for .22LR uppers.

An upper receiver (smooth, no forward assist) I ordered came yesterday. I planned on ordering other parts as I find them in a money-saving offer. Problem is, it looks like individual parts when added up may equal or exceed purchasing a complete upper...in which case I've just increased the costs by buying this receiver.

One of the things that drives up assembling may be shipping on each item. Oh well.

Anyway, I have a spare charging handle, delta nut/spring, carbine hand guards and cap and now a receiver. I have Black Dog .22 magazines which I am hoping will work. I think all I need now is a BCG and barrel, which I am uncertain about. I see collars for .22 conversions. If I am reading correctly, barrels come with a proper extension and the equivalent of a collar is already on the barrel?

And, if I am going to use the carbine hand guards, I'm limited to a 16" setup, if 20 inch, I'll need rifle-length guards. In either case, I'll need a front sight or gas block for the hand guard cap to rest against?

I think I'll be buying CMMG, but looking for suggestions on other makes.

Thanks for any advice.