Want to shoot Foster slugs through my '90s Browning 3" A5 smoothbore (28" VR with multiple choke tubes). Of course the only sight is the front bead. A local gun shop told me the receiver can't be drilled and tapped and the only available rear sight is a clamp-on not strong enough to be woth a flip, so I should buy a slug barrel. Can anyone suggest an alternative for a rear sight? Thanks