The heavy recoiling stuff...the really NASTY stuff like 3.5" Mag Blends and Nitros really belt you weather it's in a semi or not and I just found the rear edge of the mount to just a wee bit too close to my on hand thumb. I moved it out to the rail and never have to wory about it again, plus, I now have the gaurd around it which you just can't have with the Speed Bead.