UPDATE SEPT 2021
After hearing from a hundred or so hunters, in the last two years, that have tested the TarHunt 365gr Copper slug in their 20ga H&R 960 Single Shot have some good news. They report that the slightly over size bore of the H&R 960 is able to group the TarHunt bore size Copper slug inside 125yards. It appears that the lighter slug, made of Copper material, is able to get enough grip on the rifling, when fired in a slower 1-35tr, to stabilize the solid/longer bore size projectile. (A 1-35tr is just a little over 1/2 the RPM’S of the normal 20ga 1-24tr.) I am sure that results will vary from barrel to barrel, so testing you gun is the only way to find out for sure.
Bad News for H&R 960 Owners
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 established in 2000. The bore can be as much as 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 and Commander IDS sabot design can adjust itself to that big a bore diameter, thus maintaining the proper RPM’S for stabilization; because of its patented key-on sabot design.