Lessons learned from Bushy.
According to Bushy the Colt A3 flattop sight plane is 0.040" higher than the Colt A2 sight plane.
So Colt uses a different front-sight/gas-block, Bushy in all their wisdom decided to use a A2 front sight and a lower carry handle on their flattops.
So when using a Knight, GG&G, ARSM, etc… BUIS on your Bushy you need to get a new front sight post from Bushy that is +0.040" higher. This puts your front sight post approx level with the protective ears, instead of below them like it should be.
Hope that helps, I bought the front sight post when I ordered my upper, but I'm going to replace my front sight post anyway with the GG&G tactical flip up front sight and I didn't get the Bushy carry handle anyway so long term the problem solved for me, and in the future if I decide to get a carry handle, I'll just get a Colt spec handle and no need to re-adjust zero to switch between my ARMS #40 & the carry handle. (Although I see no need to get a carry handle other than prosperities sake...)
Hope that helps