if you tweak you marker right you can fire a normal .68 cal pb at about 400fps but anything more then that you need different pb's.
You can freeze normal pb's but like said they have a hard time staying that size.
Now shooting marbles is dangerous...they don't break period lol.(believe me i have done it lol,,,and they Ricochet)
Edit: i played in the winter once trotter...and you are right those pb's hurt like ****, they leave a nice mark...i even have a few scars where they broke the skin because they freeze up here in norther Ontario and don't break. It isn't very fun but at the same time it is lol.