The front sight of a 1377 must have the barrel through hole opened up to .435" in order to fit a longer than stock barrel or the Barrel support from a 760 or 2289 must be fitted.
760 Barrel support is 760C008. Also need 788-004 & 788-005 (bolt and nut)
788-011 is an alternate for the 760C008
788B011 is the third option using the 781A012 self tapping screw.
I think the 760C008 is the best option.
There are also aftermarket metal barrel supports (Blue Fork Designs) which are better than the plastic parts above. No factory metal barrel supports are available from Crosman.
"NO GUNS WOULD BE A RIOT"
Later
Tim
Mac1 Airgun