adult .22
Well yeah, the Mosin, STG are indeed gimmicky, but I suspect still fun if not all that practical. While I'm at it, I will declare that all serious shooters need a good .22, really, two good .22's, rifle and handgun. I call those guns "adult .22's". The .22lr is still the best deal shot for shot in ammo, allowing for quiet, affordable practice.
What's an adult .22? Good trigger, good sights, and will last a couple of lifetimes. On the rifle, full size stock and weight to approach the feel of a centerfire sporter. Mine is (are) a Ruger 77/22 and a Ruger MkII standard 4" model. The 77/22 is w/o question the firearm that I have shot the most in my assortment, mechanically reliable and with a tweak....good trigger. Add a Leupold rimfire scope, and all the boxes are checked for a rifle.
The MKII is a bit of a flyweight and ideally a HB model would likely better suit. But my standard model came with such a wonderful trigger right from the start that I've kept it all these years. Feeds and functions with nearly any ammo, and more accurate than I am.
Regrettably the 10/22 trigger is deplorable with out a lot of work these days. In it's common carbine version, a bit of a flyweight also. That all can be fixed, but at additional cost. I'd mentioned the T-bolt in a previous post and have always wanted one. The Browning SA22 is a candidate but again a bit of a lightweight. I've got a Savage MkII HB that might qualify, but the magazines are junk and it is not reliable. The old Remington Fieldmaster pump is adult sized, but again, the trigger leaves alot to be desired. The Henry levers are a hoot, acceptable triggers, can be scoped, but I wish they were a pound heavier. Not to say any of the above are not fun rifles, they are, just not quite fully mature.