ELR, Extreme Long Range shooting competitions. That’s a handmade wildcat cartridge called 14.9mm SOP, with a lathe-turned solid brass projectile. The casing is made from 20mm autocannon brass. Shooters compete in three-man teams to rapidly calculate bullet drop and wind drift, then fire to put three shots on a 1x1m steel plate more than a mile away.
This particular round is notable for throwing a very low drag projectile that weighs 1,690 grains at 3350fps, generating roughly 42,000 ft-lbs of muzzle energy. This is almost twice the kinetic energy of 14.5x114 Russian, and it remains supersonic out to ~5km (5400 yd).
The cartridge, rifle, ammo, and ballistic data were all created by a few guys on the long-range enthusiast forum Snipers Hide. The entire project was privately funded by civilian shooters.