The 3.5 lb gun is going 34ft/sec as soon as the gas exits behind the back of the bullet(conservation of momentum). That's 60 ft-lbs of E involved in the recoil that the shooter has to dissipate . Catching the 15 lb weight moving at 11.3ft/sec is a good comparison. I'm sure the kid, or the guy who gave him the thing to shoot had no idea. Holding the gun tighter with stiff arms may just take out a kid's thumb instead of ripping other sinew.
See post 73.
So it's about on par with catch a bowling ball dropped from about 20 feet up.