When I first built the upper, I used one of those extruded slick side dpms uppers.
When I got it built and everything together, I marked where the 12 o' clock position was and had the freefloat tube engraved with the form1 info for the integral suppressor and got the picatinny rail threaded and installed, and the handguard that I was going to use to keep my fingers from getting hot.
Fast forward to when I was going to zero everything and found out the slick side upper pic rail was slightly canted. Everything you mounted to it (iron sights, scope rails, or red dots) were all canted slightly off. It was the upper that was fubar'd. Nothing I did, so I had to swap to a different receiver (one with a standard forge mark so everything was straight and in spec)
Well, the difference in the barrel nut thread caused the timing to be off on my handguard/silencer tube, so I had to take it apart and lap the face of the receiver, and then I had to chuck the barrel itself up in my lathe and remove enough of the surface so everything would time back up in the proper place when the barrel nut was torqued down in place.
This isn't a standard build. It's truly an integral suppressed upper. Not some large internal diameter handguard where someone fits a silencer underneath and calls it an integral upper. This one truly is all built into the upper
All of this could've been avoided if dpms didn't sell crappy stuff with crooked rails. Only reason I wanted a slickside in the first place was because 9mm uppers don't use a forward assist and I didn't want to junk up the build with one that was pointless though that's what I ended up having to do so anything mounted on the receiver would be straight.
Anyway, a picture is sometimes worth a thousand words. Maybe that would help you see why it was important to time the handguard back right? At the very least, if it didnt' time back up at the 12 o' clock position, the front sights would've been closer to the 5 o clock position because I had built everything according to the POS dpms upper.