I have a Colt 16 inch barrel I want to free float.
Looking at getting low profile gas block to fit under rail,
will the set screw type mess up the barrel cuts for fixed FSB?
Probably doesn't matter, it's not likely to go back to stock.
PA has Radical blem gas block for 4.99
Ideas...  thoughts?

Bob the existing FSB, you know this one works and is pinned in place correctly.
Whether or not it is "butchered" is up to you...

It can damage the rear taper pin hole because the rear set screw usually bears down on the same area.
If the rear taper pin is cut with half its diameter or less into the barrel, then no damage to worry about (the trench will act like a dimple for the set screw), otherwise the set screw will hit the edge of the taper pin trench and cause a little damage, but nothing a rat tail file can't fix if you wish to reinstall the pinned block.