As for the metal ball, if I understand what you are saying, some barrel lugs come with a hole drilled on the distal face of the lug. It doesn't go through the lug but is deep enough to accept a spring and a little ball, which are kept in place with a $12 staking tool. The ball catches on little notches on the corresponding face of the magazine cap. This is what comes on my old style 7 shot short barrel model.
Some barrels that don't have the spring and ball are nevertheless drilled to accept them. Some aren't drilled. I believe, but it may have changed, not long ago it was a $25 additional charge to order a barrel from the factory with the ball and spring inserted and staked.
Nothing keeps the magazine spring from flying out when the cap is removed.
If you want to use a barrel without a ball detent, get a 3 pack of wave washers from Brownells. You need to use one to keep the magazine cap from backing off.
http://www.brownells.com/.aspx/sid=4...000021_d_10510