First you have to search for:
C:\Users\XXXX\AppData\Roaming\SpaceEngineers\Mods
XXXX is your username

inside you find files like this:
566142426.sbm

these you can open via winrar, what i use
then you see a folder structure

browse thru it
DATA/AmmoMagazine_MK3.sbc

click on show icon with glasses and

inside the editor you have someting like:
<AmmoDefinitionId Subtype="MK3CannonAmmoHE" />

which is in my eyes the correct value for the ammo

LAST NOTE: the folder C:\Users\XXXX\AppData\Roaming\SpaceEngineers\Mods contains every mod you ever uses with space engineers so maybe it takes some time to find out the right one.
idealy the modcreator publishes the subtypes he creates to make it way more easier for players to track down the things

Expanding on what HadesDuren said, you can find the mod numbers for each mod by going that mods steam workshop page on your Web browser and that mod number will be in the url. :)