12.7x108 caliber is technically legal to own in the US, as a rifle. Problem is chicken vs the egg. Ammo is not readily available since no one is building guns and no one is building guns since ammo is not readily available. No one wants to make the first move.
14.5x114 caliber requires a complex application process to own, 6+ months to wait, plus a $200 tax to even apply to purchase. This applies to rifles or MG. Once all cleared, you then have to pay for the firearm.
Here are some representative examples of MV and barrel lengths. Note that manufacturer of the ammo, even of the same type, varies in velocities observed. Ammo of different types also have different velocities. lengths in inches, velocity in ft/sec.
Caliber Length M/V Notes
.50 BMG 29" 2865 Mk211 Lake City
.50 BMG 29" 2750 Mk211 Winchester
.50 BMG 29" 2830 Mk211 Fabrique' Nationale'
.50 BMG 36" 2815 Hornady AMAX Match
.50 BMG 45" 2910 M8 API M2HB
.50 BMG 45" 2910 Mk211 M2HB
12.7x108 42" 2822 API Dshk
12.7x108 43" 2756 Gepard Rifle
14.5x114 53" 3297 API KPV MG
14.5x114 58" 3280 Gepard Rifle