Here is a shot of my Spike's BAR rail with a pistol position TP555 block. No mods done to the rail and you can get in there easily to adjust the block.
I just thought I would post this in case anyone is thinking about this setup.
I had it cut at 9.5 so it would sit right at the end of the rail. I think it makes a nice clean look. I took a deer with it a few days ago. 100yds with 110g Vmax at 2200fps. Killed her almost in her tracks. The heart looked like someone set C4 off inside it.