I have Redi-Mags on both of my M4gerys and would really like to put them on my AR pistol builds but I have a question. I know that the Redi-Mag has the cut-out for viewing of the serial number but do I need to modify them to allow the "PISTOL" marking be visible also or can the serial number provide them this information?

I doubt it, My lower doesn't even say pistol on it but it is registered as a pistol on the 4473, which is all that counts since Mega doesn't manufacture full firearms. I guess it would just be a personal preference/ CYA.

Just my $.02

good to know. I didn't think about pistol lowers that don't have it marked on them. oh well. If anyone else has any info or experiences feel free to chime in. -JBP

ETA: I have read all over until I feel as if my eyes are about to bleed and cannot find the answer to my question. Is there a limit on how long overall a AR pistol can be from end of buffer tube to tip of barrel? I only ask because I see all these AR pistols with very short buffer tubes and haven't seen many with the carbine length buffer tubes like the kit Spikes Tactical sells. Am I ok having that buffer tube and a 10.5" upper or do I need to change it? Thanks

A pistol cannot have a STOCK on it.  There is no limit to barrel length or mag capacity.
oh yeah, IANAL.