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Just get an M&P15-22
i bought cmmg .22 kit for plinking indoor when it's cold out. it was cheaper than an entire.22 upper and i'm happy with it.
didn't see a need to have dedicated upper. but it's really up to you.
keep in mind that when you use a kit you are using barrel that has ~1:9 twist for jacketed 60gr .223 bullet, which is too aggressive for small 30gr unjacketed slug. more aggressive rifling will cause increased barrel leading
dedicated upper on other hand will have correct twist for .22LR bullet which i believe is 1:16 therefore you get the most out of it accuracy wise
just my $0.02