Actually he can load a slug and not permanently disfigure his son's shoulder. 20 ga. slugs don't kick like mules, especially the 2 3/4" loads. The sabot loads are unbelievably effective at 100 yards, quite accurate too. Hell, Federal loads a Barne's sabot to nearly rifle velocities (1,900 ft/sec) in a 3" shell. Enough to bag quite a few different types of critters.
For a 12 ga, Federal and Remington both make reduced recoil loads that cut the kick down a
lot. Federal's Truball low recoil load is effective out to 75 yards or so (non-rifled slug) and kicks about as much as a light target load. Same goes for the Remington Copper Solid, except it works in a rifled bore. Either load can take deer or larger game with little trouble... at the right distance. The velocity difference between the low-punch and full-house is pretty large. 320 ft/sec for the Federal, around 250 ft/sec for the Remington; stock versions move at 1,600 and 1,450 respectively. So some "thwack factor" is lost, especially at longer distances. Still, it's enough of a difference for my neighbor to let his 13 yr son to fire an adult 870 with slugs and the kid doesn't get kicked silly.
Me? I'll stick to smoke poles and soot shooters; they smell better

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Flakbait [Delta6]