I have a new Bushmaster 20" A3 rifle.  I am replacing the upper recevier with a new Bushmaster A-1 upper recevier.  Will I have any problem with the front-sight tower hieght not being compatitable ?

And for future reference,  are the Bushmaster A3, A2, and A1 upper recievers interchangable in regards to Bushmaster barrel assemblies ?  In other words, if I order a Bushmaster barrel assemble, can I mount it to either a Bushmaster A1, A2, or A3 upper reciever and not worry about sight problems (other than the dreaded index pin alignment problem - but let's save that for another thread) ?

Should work.  

The only sight height problems I am aware of are with the Colt M4.  They require a Colt removable carry handle to get correct sight alignment.  I don't know if all Colt M4's are this way.

Answer is YES.  I had this discussion with a BM tech the other day. The Colt front is 40/1000" inch higher causing the problem.  BM is uniform.