Interesting.
From here
https://en.m.wikipedia.org/wiki/.600_Overkill
the muzzle velocity is up to 2450 Fps and the muzzle enrgy is up to 11995 Ft. Lbs. Or 16172 Joules
comparing to other other firearms from
https://en.m.wikipedia.org/wiki/Muzzle_energy
Firearm -
Caliber -
Muzzle energy ft-lbs joules
air gun spring .177 15 20
air gun mag spring .22 22 30
air gun https://www.physicsforums.com/wiki/Pre-charged_pneumatic_(PCP) . 22 30+ 40+
pistol https://www.physicsforums.com/wiki/.22_Long_Rifle 117 159
pistol https://www.physicsforums.com/wiki/9%C3%9719mm_Parabellum 383 519
pistol https://www.physicsforums.com/wiki/.45_ACP 416 564
pistol https://www.physicsforums.com/wiki/.357_Magnum 640 873
pistol https://www.physicsforums.com/wiki/10mm 775 1,057
rifle https://www.physicsforums.com/wiki/5.56%C3%9745mm_NATO 1,325 1,796
rifle https://www.physicsforums.com/wiki/7.62%C3%9739mm 1,527 2,070
rifle https://www.physicsforums.com/wiki/7.62%C3%9751mm_NATO 2,802 3,799
heavy https://www.physicsforums.com/wiki/.50_BMG 11,091 15,037
heavy https://www.physicsforums.com/wiki/14.5%C3%97114mm 23,744 32,000
The https://www.physicsforums.com/wiki/7.62%C3%9751mm_NATO was a Nato round M14,( Vietnam) - apparently not enough to stop a T-rex. ( M16 similar )
The https://www.physicsforums.com/wiki/7.62%C3%9739mm was soviet ie the famous AK47
The https://www.physicsforums.com/wiki/.50_BMG Browning machine gun, sniper ammo - better choice.
Seems like the regular ammo issue for gun enthusiasts and regular army infantry would be lacking in penetration power - maybe only 10 inches through tough skin and flesh.
T-Rex might be annoyed. Lucky shot to vital organs - possible but unlikely.