What is it? Appears to be a hevy MG round

[TurnipEaterDown]

A what is it?
Cartridge looks like a 50 Browning, with a couple differences:

(Measured casing, may not be exact, would expect to be under min chamber.)

Rim 0.800"
BELT 0.850"
Base ahead of belt 0.790"
Base to shoulder 3.070"
Shoulder 0.700"
Neck OD 0.600"
PROJECTILE OD at case mouth 0.562" (14.27 mm)
Case length base to mouth 3.900"
HS (very faint) K 38 W 7

[HumptyDumpty]

Sounds like 55 Boys, an old British anti-tank round. How many do you have?

[M-Tecs]

Sounds like 55 Boys, an old British anti-tank round.

I believe we have a winner

https://cartridgecollector.net/cartr...ti-tank-rifle/

[TurnipEaterDown]

Thank you both - I did forget about this one.
I knew that people used to convert the guns to 50 BMG, but I didn't look that up.

I have one, could have gotten a couple dozen today. $4 ea.
They were loaded on 5 round strippers.

It was an oddity to me, so it came home w/ me.

[Rockindaddy]

The 55 caliber British Boys Anti-tank guns were imported into this country many years ago. Because they were over 50 caliber they were classified as NFA firearms and required registration under the 1968 Gun Control Act during President Johnson's term. Most were rendered deactivated by plugging the chambers and welding them. Over the years when I held a Manufacturers license, I made up a large barrel vise and an 8' wrench to remove the 55cal barrels and rebarreled many of them to 50BMG. They were great shooters. The 55cal Boys ammo is not in high demand but collectors usually have a round in their collection of various cartridges.

[JSnover]

The 55 caliber British Boys Anti-tank guns were imported into this country many years ago. Because they were over 50 caliber they were classified as NFA firearms and required registration under the 1968 Gun Control Act during President Johnson's term. Most were rendered deactivated by plugging the chambers and welding them. Over the years when I held a Manufacturers license, I made up a large barrel vise and an 8' wrench to remove the 55cal barrels and rebarreled many of them to 50BMG. They were great shooters. The 55cal Boys ammo is not in high demand but collectors usually have a round in their collection of various cartridges.

A good friend of mine has one in .50BMG and you're right; it is a great shooter!