you've got metal bushings already, dont bother buying more.
I just did a custom build m16 on sunday out of tokyo marui m4 and other random parts. The metal bushings i got seemed to not fit in. I double checked and saw the plastic ones just slipped in no problem.
Well good news yuhaoyang! They do fit!
you just need to be a tad aggressive. What i did was put a hard surface behind the mechbox casing, (in this case the floor @.@) and tapped it with a hammer on the flat surface of the bushing. They will fit very snug, but it doesn't matter!
Try it out