In addition to those mentioned above, I've seen used Black RTV.
I have also seen advertised these options but have no experience with them, they each have a lip that extends somewhat forward to cover/cushion the gap:
TangoDown pistol grip
Duck Bill Tactical Grip
I have also seen advertised these options but have no experience with them, they each have a lip that extends somewhat forward to cover/cushion the gap:
TangoDown pistol grip
Duck Bill Tactical Grip

Comment