Can these be used for small game, if so are they better than steel because they break apart?
I would not recommend them for small game.
Are these smaller in diameter? And do they leave fragments in the barrel?
They measure about the same diameter around the band (the widest part) as your standard steel BBs. And no, they do not break apart in the barrel.
What other Umarex Legends guns will these work in other than not in the MP40?
They work in the M712, UZI, P08, Makarov....I think that's everything else?