Thanks Dzscubie, but the front sight post definitely needs to be taller. This is exactly the same problem that occurs on flat top ARs that don't have the F-marked FSB when you put on a rear sight or carry handle that's at milspec height (0.04" taller than commercial or A2 height) - the front sight post has to be screwed up so high that it slops around and is ready to fall out.
Bushmaster, ArmaLite, DPMS, Olympia and others use the old A2 FSB and have a commercial height carry handle to match. That works fine but many end up with a different rear sight. Bushmaster has had so many complaints on this that they sell a 0.04" taller post for people who add on a "Colt" or milspec height carry handle or rear BUIS.
I first ran in to this problem with my AR build last year which has an ArmaLite upper and a Colt carry handle (actually made by Cardinal Forge). The Bushmaster taller front post solved the problem and was a cheap fix since it's only $5.50. This even allowed me to go to the Santose IBZ zero which needs even more front post height since you reindex the carry handle to go 4 or more clicks under 6/3.
Umarex is aware they have this problem and in the past has sent out the taller front post for free to people who complain. I'll be complaining!
Edit: Very nice OPS you have there rodent.22! I like the suppressor and stocks! I was temped to go that route but also like the plain M4 look too. And the M4 was in stock but no OPS. No doubt I'll make a few changes, in fact I have a Giles 3-point sling on the way from Phoenix.