Rifle -
When I was eighteen, a family friend let me shoot his Model 70 in 375 H&H. He had lightened it up for ease of carrying. All up, it probably weighed 7½ pounds. After a dozen or so practice rounds that had a 220gr. cast at about 1600fps, I tried some 270 silvertips. OUCH. Being eighteen and male, I kept firing. After the first four or so the shoulder started to turn numb. By the time I finished a half box, I couldn’t lift my elbow above my shoulder. The next day I had a bruise the size of my hand with the fingers spread.
Shotgun –
When I was thirteen, I fired BOTH barrels of a SxS 20 gauge at once without the butt firmly in the pocket of my shoulder. Knocked me on my backside, but I kept the kept the muzzle downrange and the gun didn’t get a scratch.
Handgun – three way tie –
Ithaca stakeout in 12 gauge with a 2 3/4†magnum slug, fired one-handed.
Four inch Model 29 with Magna stocks firing LBT 320s at about 1400fps.
Titanium Chiefs Special with full bore .357 mag.