I haven't picked up a AR15 since Vietnam So I was in the gun shop and they were selling complete DPMS lowers.

Then I found a S&W upper with a Colt carrying handle and a Yankee Hill flip up front sight.

When I put them together I couldn't hit the target at 50yards. I have no idea what it was sighted in for.

I was thinking of either, taking off the Colt handle and putting on a Yankee Hill rear sight or putting on my Super Sniper 10X40 scope with Warne rings.

Or should I just buy a rear flip up sight.

I'm never going to shoot this past 100yards.

Any suggestions?

First, check that the detachable sights are correctly mounted and secured.

Second, Google "zero an M16" and you will find instructions for establishing a 25 meter zero and a 25 meter zeroing target.

Third, get a large (4'x4') target panel upon which to mount your 25 meter zero target so that you can capture the strike of your bullets and place it at 25 meters. Fire at it with three rounds. Observe the group size and location. If round strike is dispursed more than an inch or inch and one half recheck your sight mount or your aiming technique. If the group is satisfactory, follow the directions you got from your Google search and adjust the sights.

If the group is highly dispersed and your sights have no movement at the attachment points, the barrel is probably shot and you need the services of a gun smith.

Good luck to you.
Dan

there are differences in the front sights for the ar. one is a flip up for a rail mount and the other is for a gas block mount. need to find out which you have on the front.

Hard to say for certain, but I'm betting you were shooting WAY high. That flip-up front is likely way too short for the carry-handle rear.