Part of the problem is that we say it backwards from the US military manner.
WE say "M2 Ball ammunition" and their Quartermaster, same as ours, says "cartridge, small arms, caliber .30, Ball, M2".
The "M" stands for "Model" but, if you really want to, you could think of it as "Mark" and it might confuse you less. THEY are certainly confused by our Marks and Stars, yet they are exactly the same as their "M"s and "A"s and "E"s and so forth and our "P"s correspond to their "T"s and we all become thoroughly confused. But both systems make sense when you get familiar with them.
But it remains that the US refers to things much more by their "M"s than we do, as we (and those Brits, of course) tend to like names for things.
Ammunition is only the very least of the confusion. You can have a pair of Tanks and they are both M3, except that one is 11 tons and the other is 22 tons and the half-track sitting beside them is also an M3 and the Carbine that the truckdriver carries is an M3 and the SMG in the tank is also M3. Small wonder that Honey, Grant/Lee, White, Carbine, Grease Gun and so forth were used so often. Pity the poor chappie who got all mixed up and ordered 100 M2s and ended up with a truckload of Brownings or a couple of acres of tanks when all he really wanted was enough shells to go to the range!
But the BOX says "BALL" and THEN it says "M2" so there you have it: Product and Model, in order.
But I'm back in the previous war and I am building the rifle that never was built (it was rebuilt from whatever they had lying about), the Rifle, Short, Magazine Lee-Enfield, Converted, Mark IV which you will also find listed as the Rifle, Short, Magazine Lee-Enfield, Mark IV, (Converted) and if this topic goes on much longer I am GOING to find that jug with the red label marked "SR-D" and quietly put me out of my misery!
But the ammo is safe, even if I might not be! And as Buffdog has pointed out, it is still too valuable to bang off.... but it is an awfully-nice item for a collection.