Why would he need 15 rounders for range work?
He doesn't need a 20 round mag for range work. It is, IMO, a good option as it is little helpful if you do a lot of shooting. More time shooting per magful. 2-15s and 1-20 = 1 full box of ammo, no loose rounds.
I don't have a Shutgun News or a Gun List at this time but if you can get ahold of one of those, they will be advertised. They are a bit steep in price (generally $100+/ea nowadays) but it is an accessory that is works pretty well for me. (Got mine before the ban @$35/ea. Oh the good old days.) Plus, it is a little bit easier to handle for quick mag changes. And, it's pretty cool looking, IMO.