One other answer (and the cheapest and best) is to order/buy a 20 gauge modified barrel from H&R for his .410. a youth model in 22" or a 26" standard model 20 gauge barrel. It will have a bead sight and will shoot inexpensive foster style slugs great and accurate out to 40-50 yards with the bead sight or have the chamber area drilled and tapped for a H&R scope rail. No problem with a 20, plenty of steel to drill into. Barrel $39.00, scope rail w/hammer extension $18.00, return shipping $5.00, a better deal could not be found anywhere. If you get the Tracker II barrel you will probably want to shoot sabot slugs which are much more expensive than the foster style, nice but the barrel is more money and so are the slugs. The way to go is to save money, have fun, and shoot deer...<><.... 
(Of course you could look around for a used 20 Gauge H&R Topper, Model 158 or Pardner, cut the choke off (only needed if it is a full choke, mod shoots slugs fine) and set it up for foster slugs, you might be able to find a used one for about $50-$65...)
(Of course you could look around for a used 20 Gauge H&R Topper, Model 158 or Pardner, cut the choke off (only needed if it is a full choke, mod shoots slugs fine) and set it up for foster slugs, you might be able to find a used one for about $50-$65...)
