I had to file down the parts on the 5ku BBU to get the dytac sight to fit, doesn't take much though, best bet is to take the BBU out of the slide and try to fit the sight directly to it (see if the hole matches up with the part under the slide) then slowly file away the offending sections until it fits.
Took me about 5/10 mins? I should suspect the hi-capa sight has the same issue, I think the problem is the 5ku dimensions (I may however be wrong!)
Had to do this to two different BBU's.
Wings