I was at the range today with a friend enjoying vintage Garands and AR's. My friend decided to try out his "new" M16 slickside upper on his Bushmaster lower. Had a very difficult time even getting it on paper. After dialing the sight to it's max left windage we got on paper. We had to aim about 8-10 inches from center just to keep rounds on target. Not being familar with A1 sights, I suspected a canted FSB for the windage to be off that much. Any suggestions?

More detail needed, because if the barrel on the piece is indeed a milsurp Colt, it's highly unlikely that the sight base is itself canted on the barrel.  It may, however, be a case of the barrel itself being canted in its' position on the upper receiver.

Was the barrel/upper assembly purchased as a unit, or were the two pieces bought separately and assembled in the civie world? Are they both milsurp, or is one of the pieces commercial? Look at the feed ramps at the rear and see if they are centered re the mounting lug just underneath them.

When we get more info, we can provide more accurate fixing help.

I don't have the upper with me, but I can tell you my friend bought it at a gunshow and the upper supposedly came from Sarco. It appears to be an original Airforce m16 upper, but has some unusual markings. There is no C or MP on the barrel, just a 12  on the bottom of the barrel near the flash hider.  There are no forge marks on the upper that I could notice. The bolt carrier is marked with a D, not the usual C, and is a full-auto carrier. I didn't inspect the bolt, but it also was not marked according to my friend. The finish on the upper also was not the usual Colt grey, but some sort of dark grey/black anodizing with sort of a "rough" texture. The barrel and the upper also has the original style delta ring, not the current tapered delta ring. One more thing, the upper was not the "large hole" upper that Colt switched to for awhile. Sorry, I couldn't provide more details. I told my buddy I'd ask around and see if anyone might be able to help.

i would check the bbl. and if the front sight is canted just hit it with a mallet.