Anyone heard of these; A friend found a case that looked just like a 50 BMG, Except it had a belt. The base dia seemed the same as 50 BMG, and was rimless, but the neck was expanded to maybe about .577ish. It also seemed to have a Mil type base stampings.

British 55 Boys (Boyes) rifle AKA the 13.9x99B (vs the 50 BMG AKA the 12.7x99).

Thanks Tailgunner, I'll pass that on.
John L.

J.d. Jones, of SSK industries make a 50 bmg, necked out to a 585 bullet. He calls it a 14.5mm. It uses an 1173 grain boatail bullet. The case has ho taper and a much sharper shoulder than the 50 bmg. You can get 2850 ft/sec with the 1173 grain bullet. It is a blast to shoot.

Bigdoggy
The big clue is that he said it's a BELTED case, all but two of the military headstamped cases (that I know of) in this size range are rimless. The 2 exceptions are the WW-1 German 13x92R (rimmed) and the 55 Boys/139x99B (belted)

If I was a betting man, I'd wager that in his headstamp is a "K" for Kynoch followed by the year of manufacture at the 12 o'clock position. Any letters/numbers at the 6 o'clock position indicate the original loading of that case.

My "man" is the exectutor of an estate, bought said case, and a live 505 Gibbs around to show me, and now has misplaced both. If ever he finds them I'll record the headstamp and let you know. Thanks again.

Yes, Tailgunner is on target again. (or still)

The case has "K" and "41 or 47" at the top, and
"W II" at 6 o'clock.

Thanks again. John L.