With a Bushmaster flat top reciever, do you have
to re-sight the weapon each time you switch from
the scope to a night site or the old carry handle sights ?, don't know if I want to buy a flat top reciever AR15 or not, I know I'm going to buy a Bushmaster not sure which model

pointtarget,

Interesting question.  There are scope mounts that promise to hold zero when removed.  I believe some of them actually do.  For example, the US Optics posi slide and lock mount is one I use and it appears to lock perfectly.  Based on quality I believe it would too.  However, I admit I have never removed it.  I have no reason.  Warning, these do NOT fit all Weaver rails.  They follow Picatinney standards.  And these are not cheap.  Go to:

www.usoptics.com/usPosi.htm

As to night sight, I assume you are talking about something like night vision coupled with an Aimpoint reddot or the like.  If so, personally (since you have bucks to burn to buy  nightvision), I would simply buy a second upper.  

I see no reason to ever use  the carry handle sights.  You can mount flip up rear sights on a flattop that can be used in the event of damage to the primary sight.  If done right they can be used while the primary sight is still mounted.

My advice would be get the flattop.  Put on a primary sighting system (ACOG, Aimpoint, Xx scope, etc) and worry about mighta coulda wouldas later.  I wouldn't even buy the carry handle.  You won't ever use it.

$.02 worth

James

Quoted: www.armalite.com's one-piece mount is extremely good, exactly the right height, and will repeat zero when removed and reinstalled. -Troy

I second that!  I use them, never any problems unless you remount back of forawrd "one slot."  (meaning, just remount it the same exact spot every time)