I have a Colt MT6400C with an IOR M2 4x scope. With the flattop and ARMS #22 high rings, the scope did not clear a #40 BUIS (#40L wasn't out yet), so I opted to get a ARMS #38 Swan in the S-Ex size. The built-in BUIS fits under the scope perfectly and I can forward mount a red dot sight in the future.

To get the #38 to fit over the M4 handguards I had to sand the top, near shooter side just a bit to slide the #38 over. I didn't think much about this until I started considering free floating the barrel with a different handguard. (See picture below).



The installation is snug and puts downward pressure on the handguard. Is this a problem that I should be concerned about? If so, would a particular free float tube fit or do I need to get a #38SPR?

My experience with the ARMS#38s-ex is that the brush breaker alone can affect accuracy. Groups opened about .5 MOA on a 20" rifle. Removed it after 20 shots or so.

A little more sanding, regular carbine handguard (top half), or a standard length ARMS #38 would also work for you.