Even though the 20gr Schrifle™ Raptor Alv 270gr and the NeoTec™ Copper 365gr slug will stabilize in the 1-35 twist rate of the H&R; the bore size is still well over the size required to shoot a SAAMI diameter slug with good accuracy. Actually testing of your barrel is the only way to know for sure. After 50+ years, if I have learned one thing, when it comes to rifled barrels- “there are NO certainties”.
I have conformation, from the H&R gun designer, that H&R 960 20 gauge slug guns were indeed a (1-35tr) and were designed for the old Foster/rifled lead slugs. The barrel bore sizes are very large by today’s SAAMI standards. The bore is a full .007ths bigger diameter (on top of the rifling) than it should be. Accuracy Note: With that big a bore only the Lightfield Hybred Exp sabot is designed to adjust to that big a bore diameter, while maintaining proper RPM’S for stabilization, because of its patent key-on sabot design.