Since everyone has covered safety I figured I'll just tell ya something different, I use to play airsoft a lot (like every weekend for 4 years) I had a daisy shot .177 pellets or bb's. I figured why not try low velocity metal bb's, everyone hated it so I put it away (never played on a regulated field just back woods) so after my TM M14 broke in half I salvaged the barrel took off the plastic insert on the back of the barrel did a little work on it and after a that it fit in the receiver of the BB gun. The issue I had with it was no hope up, also with some of the airsoft bb's if you had to much pressure the BB's would just go hay wire and you couldn't hit the broad side of the barn. I would just stick with the official made guns just for the pure fact that they're designed for what they do, If anything one of your best bets would be to modify a daisy single pump those things have some crazy range for a bb gun, plus with single pump you don't have to worry about over pressure or anything like that.
"come on you sons of b***hs! do you want to live forever?" Sergeant Daniel Daly, USMC, Belleau Wood France June 6th 1918
"You can be as mad as a mad dog at the way things went. You could swear, curse the fates, but when it comes to the end, you have to let go" Benjamin Button