So much to say about this. First off, no fault of their own that their release date is during the great ammo drought. It is easier to find guns and mags than ammo right now.
Second, the mags for M1 Carbines are as available as anything else, but they are going to have to use a different mag, as 30 Carbine mags are much too long for 9mm.
Don't know about the others Jesse listed, but KelTecs are as hard to find (for a reasonable price) as an honest politician. You would be better off with a 9mm upper for your AR.
I'd like a threaded barrel for my M1 Carbine so I could mount a suppressor. Hmm, you would have to regulate the gas or hand load....