Personally, I would include the unit with each number you write.
12mm x 12mm x 4mm
I like that a lot better, because some day, all the numbers in one expression
may not all be the same unit.
12 ga. has a far more diversified variety of shells (loads) than 20 ga. If you don't want to reload, there are many shell types to choose from. Bigger, hotter, faster 12 ga. loads are best for...