If they are Lead bullets, you may get a lot of gas blow-by, which may cause Leading to beat the band. If the bullets 'slug up' on firing, which is more likely to happen if the bullets are really soft and the powder is really fast, you should get a decent gas seal and should not see Leading. When the bullet metal is a low melting point metal like Lead, blow by gasses act just like a cutting torch, then the cut Lead is deposited in the bore.
If jacketed, you'll still get blow by, but not a significant problem.