Exactly, @Dan_DeColumna !
As for figuring out how powerful a particular gun is, you have to also specify the ammo. Muzzle energy depends on the factors that @Dan_DeColumna mentioned, and how much energy it has at range depends on the range, the aerodynamics and mass of the round, and atmospheric variables (temperature, barometric pressure, wind velocity…).
Generally, bigger rounds do better at range partly because they have more powder behind them and partly because they have more mass for the same cross-sectional area thanks to the “Square-cube law” so they decelerate less quickly. That is part of why a Desert Eagle .50 AE can’t compare to a .50 BMG despite them being the same diameter.
Also, barrel length plays a pretty big role, so even if you know the propellant energy of the ammunition used in your old man’s .45, than still wouldn’t help much unless you also took into account the gun itself.
And yes, the 88mm AAC uses a big round, though it’s more of a shotgun shell.
Now for the confusing part!!!
While small-arms use “caliber” to refer to the diameter of the round, artillery uses the term “bore” for that and uses “caliber” to describe the ration of barrel length to bore. For instance, the main guns on the USS Missouri are also “50 caliber” because their 800” (or 66’8”, or 20.3meter) barrels are 50 times their 16” bore.