@Flummoxed Phoenix  
Hah, you’d have to be at least 91 (17 in 1944 so born 1927). 91-year old brony. I’d be impressed that’s so far outside the normal demographic.

@TexasUberAlles  
Not in WWII, not that long ago.

@Flummoxed Phoenix  
How long ago did you serve? Looks like maybe a 75x640R to me, which is, uh, not in current use. Definitely WWII Wehrmacht at any rate, and the driving bands give it away as a cannon round.

That is no artillery round that I know of, and I was a gun bunny for a year.

@HippoBirdy  
This is super late, but it looks like an AZ23/28 fuze on that bad boy, which means the shell is a HEAT round for an 88 or 105mm tank gun. Either that or it’s an AZ23/26 and none of the War Department/War Office manuals I have know what that means.

@HippoBirdy  
Dang. Impressive work, man. I think I’ll just ask the artist what he referenced for the round, lol.

@Cirrus Light  
Okay, here we go.  
After intense googling, the markings are similar to those found on WWII era ammunition. Unfortunately, I couldn’t find an exact match for the markings, so everything past this point is increasingly unsound speculation (but here I go anyways). The design (large brass case, shell fixed into the case) imply that it’s a high velocity, quick firing gun (anti-tank or anti-aircraft), not a howitzer or mortar. The general proportions most closely match the KwK 42 gun used in the Panther tank (disclaimer: proportions are subject to artist’s interpretation). Most of the markings, however, are more similar to the ones on the KwK 40 gun used on Panzer IV tanks, but there’s no exact match. In particular, all that ammo uses “14” where this one has “13”. The “III” appears to show an explosive round, not a penetrator round (again, not 100% on this).
 
TLDR - It’s German WWII ammo. Test your Google-fu to find out more.

@adatron  
That’s what I was wondering.
 
Oh wow, I just realized that 120mm = ~5 inches. So the gun on the Abrams (and some European tanks) is almost the same caliber as a standard WWII destroyer armament O.o
 
The M109 Howitzer uses a 155mm, so that’s even larger. Scootaloo is a small child-pony, so even smaller than regular ponies, so perhaps that could be a 155mm. I guess it depends on how tall you think ponies are. I put adult ponies around 1 meter, myself, scalp to hoof-tip standing upright.
 
Might be able to find out what kind of round it is from the stuff on the bullet.  
“AZ23/2…
 
13GW35  
III
 
Sm.34.42.6…”

This thing a tank/howitzer/cannon shell?

“…I’m not brave enough.”