I am looking to build an AR pistol but have some Q's before I start ordering stuff.  My first concern is how the 7.5" barrel attaches to the receiver.  Does the front sight tower have to be removed in order to screw on the barrel nut and put the free float tube on, then the front sight tower re-attached?  I don't know much about free float tubes.  I have built plenty of full size AR's in the past and just from what I know about the standard barrel nuts/ handguards, I am confused about the 7.5" pistol free float/ barrel attachment.  If so would this be the case with a railed gas block in lieu of the front sight tower?  I ask this as I am not sure if I want to go with a flat-top or standard iron sight set up.  I was thinking I would assemble on an A1 upper receiver (haven't seen one so far) instead of an A2 or A3.  The adjustable sights on the A2 or A3 (with detach carry handle) doesn't seem to make sense to me with range increments out to 800M and 600M respectively considering the 7.5" barrel, which is why I was thinking an A1 upper.  With a flat-top upper and gas block I realize some sort of optic would be required, however I would have to go cheap as funds are lacking and wouldn't want to cheap ass out on the optic when I have so much invested in the pistol itself.  Do they make low profile sights for this type of set up?  Input would be appreciated, thanks.

IIRC, most free-float tubes take the place of the barrel nut...

Or maybe that's just the under-handguard type....

yes you will need to remove front sight tower