I'm looking at building a recce-type upper, and I'm considering the YHM flip-up front sight gasblock with bayonet lug. I have a Matech 600M BUIS I will be using on this upper, and I'm wondering:

1. Is the YHM front sight the correct height for this BUIS?
2. Is there a crosspin to secure the gasblock on the barrel so it won't slip when impacted?

Thanks-
BB

1)yes, as far as I know
2)No...it's more or less a "clamp", 4 screws hold the upper and lower halves together.  If you loctite the screws and crank down on it, I can't see it moving under normal wear and tear.  

I've got this gasblock/sight.  It's nowhere near as cosmetic as their older model, but it works.  It's also pretty heavy as well.  

I really preferred the older design..I don't know why they changed it.

Here's the one you're talking about mounted...


here's the older style, for comparison