Sorry to say but people are assholes and will usually take the easy way out (ie. just using .25's cause they have a bag of them sitting at home) and you can't give people preferential treatment (ie. senior members who you can "trust" can bring their own BB's).
The only feasible way is to only allow field BB's for safety's sake. It sucks but people will hopefully understand. Maybe if you lower the costs for field entry and sell the BB's in a way that you would recover the cost of the BB's and the field fee would be about equal that could be a strategy. Or include the BB's in with the field fee.
As well one thing could be to have players "pre purchase" BB's. I have heard that in Japan people will buy whole bottles of alcohol at a bar and the barkeep will keep it there so they can come whenever they want and drink it. A similar strategy could be implemented for players that frequent your venue. ie. They'd buy a whole bag of BB's (2000 BB's) and you store it in a back room or something with their name on it. Obviously you could run out of room quite easily but maybe only offer this service to people who play say at least 3 times over 2 months or something.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE