Which pistol do you currently have?
As far as I'm aware, most RAM pistols are 0.43" or 0.50"
0.43" was more prolific for pistols aimed as training aids as its closer to real life calibres and easier to make a comparitive size in pistol form
This made 0.43" popular in the milsim community
0.50" has become more available as magfed has become bigger in the U.K. and as 0.50" as a paintball has come back and made a success in the rental / low impact market
0.43" paintballs aren't commonly found or available at general events, it's similar for 0.50" but it's more available in recent years.
Magfed events are more likely to have or allow these calibres
There does remain the issue that organizers & staff at non specific events won't know how the legislations foot pounds allowance translates into more of a fps velocity (or how it fits with the court case that stated 300fps maximum)
You will therefore find yourself mostly limited to 280fps (or perhaps 250 fps indoors or at night) with a harder paintball that gives you more shots per 12 gram but limits your effective range
Going to a 0.68" paintball pistol will mean a bigger, bulkier pistol with a magazine of 7 or 8 balls, and somewhere in the region of 25 effective shots per 13 gram. But usable at any paintball event
There are many 0.68" pistols about, the current leads are the tpx & Tiberius designs:
On a remote line using an air cylinder instead of 12 grams I prefer the tpx
As a single pistol as your main gun, using 12 grams, I prefer the Tiberius (but carrying a minimum of 3 magazines, preferably 4 or more, using a lot of 12 grams, and 'wasting' as many 12 grams as the number of magazines carried
My overall preference is to use two tpx pistols on 12 grams
http://p8ntballer-forums.com/threads/milsig-pistol.178410/#post-1610846
