What I have experienced with .243 bullets in reduced loads is they have a very abrupt velocity threshold. I mean, they don't seem to be as forgiving as the larger caliber bullets. My vote is the 30-30 AI with the 125gr SST or a Nosler B-T. Work the loads up gradually till he can tolerate a muzzle velocity of 2100 to 2300 fps and you essentially have a 300 Blackout fired from an AR. Those bullets have proven very effective at those speeds by countless hunters. Actually, your 500 S&W Encore with the mouse fart loads would work too. Just a thought.