and maybe gap or GAP?

Sorry if it is blatantly obvious.

45GAP ammo is very rare and probably expensive (I've never checked prices). 45ACP ammo is very common though fairly expensive (probably much less so than 45GAP due to being common).

Basically, new cartridges/calibers come out periodically and are usually started by one gun manufacturer with hopes it'll become a standard. .40 Smith & Wesson came out in the early 90's and has become a very common standard with many manufacturers producing guns in the caliber. 45GAP came out whenever it did but no other manufacturers cared and it never really caught on.

It's similar to standards used in other types of products/manufacturing. TRX tires from Michelin (metric sized wheels) are similar in the way they never caught on.

<ul><li><a href="http://en.wikipedia.org/wiki/.45_GAP">Wikipedia - 45GAP</a></li></ul>