I recently changed a Bushmaster A2 Handle upper to a Yankee Hill A3 flattop upper. I have an A2 front sight that came with this Ar. I now cannot get accurate results with an Yankee Hill flip up rear sight along with the A2 front sight, My front sight is adjusted as far as possible for bullet impact but is 2 1/2 in low on impact and my rear sight is adjusted all the way to left side of rear sight and cannot get any more accuracy than that, Please help, Do I have a front sight issue or am I doing all this wrong. How should I resight this AR after the changes I have made ??

Bushmaster and RRA both sell a "Tall Front Sight Post" that fixes this.  The problem is the front sight may well be taller then the ears on the flip up front sight.  So, you other option is to buy a "F-type" front sight assembly.  We can all thank Colt for this insanity!



EDIT - I may be confused here ––- sounds like you have the OPPOSITE issue going on?  Sounds like you need a shorter front sight to get your groups up...  Have you tried screwing it in?  The front sight is used to fix elevation issues.  The left to right issue means your front sight is canted or you bent the barrel pin which I thought only Bushmaster could do (they had issues some years ago with some gorilla bending barrel pins)

It sounds like your front sight base is  canted for one, also by going to a flat top upper you will need a F marked front sight base or buy a .40 taller front sight post, Bushmaster sells them. My opinion I'd just replace the whole front sight base with a F marked one and be done with it.

For a quick fix, file your front post down to bring impact up.  As far as your rear sight being all the way to the left, if you look, you may find your fsb is canted to the left and should be addressed if you want it more centered.

Your front sight canted to one side then you need to reset your barrel.

Remove the barrel and reinstal without tons of torque to get it tight- ensure the front sight stays upright and not canted to left or right. If your upper receiver should have a square hole for the barrel index pin, if it is buggered up, shim that side with thin metal (soda can) to get the area built up again. This will help prevent the barrel from turning in the receiver while torque is applied.

When your barrel front sight is straight, center your rear sight and go shoot again.

If your rifle still shoots low, ensure the front sight post is being turned down (lowered) to raise your impact. If the front sight post is bottomed out (screwed all the way down) then you will need to file the top of the front sight post. Or just try a different rear sight.

Thanks alot for the help guys. My problem now is my flash suppressor is welded on so there goes replacing the front sight with a F marked front sight. I Also pulled the delta ring and the barrel lug is set in the upper just fine with no play. Wish I could find a good gunsmith near Temple,tx to fix this issue.

Don't worry about replacing the front sight base, just buy a taller front sight post.

Its too tall already - he's shooting low.
If you have to move the rear sight left, it means the front sight is twisted left (which throws the shot to the right), which is the typical result of torque rotating the barrel against the indexing notch in the upper. I've seen armorers fix this by whacking the sight base with a big leather mallet which twists the barrel back like it should be. But since you have the tools for installing a barrel, you can loosen it and put a thin shim in the notch to do the same thing.
For elevation, make sure the front sight is screwed all the way down. If it still won't zero at 25 yds, you'll have to file it down until it does. I have never heard of anyone having to do this on an AR, so I'm wondering if your new rear sight is good quality?

My bad for not reading the whole thread. But still if you're shooting low, you really don't want to install an F-marked FSB. As you said, 1saxman, file the front sight post down.

The rear flipup sight I"M using is YHM, Last night I remove delta ring again to check spacing in the upper and barrel lug. It fits perfectly with no allowed play but did put on a scope and the FSB is in the left part of the scope picture and not centered. I figured to fully fix this problem is put on a YHM Flipup front sight and center the gas block to give me that accuracy and sight picture I've been looking for.

"center the gas block to give me that accuracy" Go figure.

You can also use a small file to open up the notch in the receiver where the barrel fits. It doesn't take much. But before I did that, I would use a leather mallet on the fsb to move it in place. Like I said, it doesn't take much.
Also, make sure the YHM rear sight is sitting flat and straight on the receiver. It wouldn't be the first time a rear sight or receiver rail was off for that matter.

Yea , I would think you would center the gas block since bushmaster canted it to the left when the installed it. So what was the "go figure " comment for ?  Sorry not as intelligent as most people believe the are. So educate me partner !!

Shave down FSB, ditch old handguards, add Troy TRX rail, add new front sight to rail.