I started when I was 12 and used a Rem 1100, 12 gauge. I started with a hand thrower. Then trap, then skeet, then birds and bunnies. Kids pick up things pretty darn fast. A nice soft kicking gun, and lots of shells. Light target loads, preferably. If you want to start with an over under, you can get a set of .410 tubes pretty reasonable and shoot charcoal briquettes. I still throw them for my buddies kids sometimes. They go poof. Kids love it.
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"I didn't get the sophisticated gene in this family. I started the sophisticated gene in this family." Willie Robertson