AR barrel axis problem

[Vurtle]

Just put together an AR and having problems with barrel axis vs scope axis. Once it is sighted in at 100 yards it is 4" low at 25 yards. Zero it at 25 and it is over 12" off at 100. Picatinny rail and scope centerline are parallel.

The upper is a DPMS 16" with stainless oracle barrel. Scope mount is a Burris P.E.P.R. and scope is a Bushnell Banner. Distance between scope center and barrel center is 2.5". I believe that if zero'd at 100 yd it should only be off center at 25 yd by less than 2".

Is this a common problem for ARs or do I have a receiver or barrel that is no good? Is there an easy fix for it. I am obsessive with torque wrenches and details so everything was installed properly.

 

[Vurtle]

I found a lapping tool from brownell's that can square up the receiver to the barrel if it is off. I think I will give it a try. I am open to other suggestions though.

 

[MistWolf]

Study these posts. It will explain why what you are experiencing is normal
http://m4carbine.net/showthread.php?t=65679
http://m4carbine.net/showthread.php?t=107572

 

[ac6916170]

Do you happen to have the scope mount half on the receiver and half on the forearm?

 

[Jackal]

Bushnell Banner sounds like it could be the culprit. Cheap scopes are not to be trusted. Try a different optic in its place to rule this out.

 

[holdencm9]

Something isn't making sense. It has gotta be the scope. Either that or your scope/bore axis distance is way more than 2.5" or your "zero" is not. You are right that if you are zero'd at 100 it shouldn't be off that much. There is no way.

Sometimes, it helps to stop worrying about trajectory and bullet drop, just pretend (for the sake of understanding) the bullet travels perfectly straight forever. At less than 100 yards, it is more or less true with .223 anyway.

If the scope-bore axis distance is truly 2.5", then in order to zero at 25 yards, you will be adjusting it so that the bore is pointed up, toward the scope axis, quite a bit. Even if the bullet has no drop, it will be following this trajectory

0 yards -2.5"
25 yards 0.0" (this represents a +2.5" rise per 25 yards)
50 yards +2.5"
75 yards +5.0"
100 yards +7.5"

As you can see, if you are truly zero'd at 25, there is no possible way for the bullet to be 12" high at 100. Unless you have a magical bullet that has little wings and can adjust its trajectory mid-flight.

Likewise, if you are zero'd at 100, there is no way it should be 4" low at 25 yards. Due to your 2.5" barrel axis to scope axis, you would actually have to have the bore pointed AWAY from the scope axis in order for that to happen. The bullet won't start on a downward trajectory and then turn upward. If it is 4" low at 25, it will be at least 8.5" low at 100 yards, as such:

0 yards -2.5"
25 yards -4.0" (this represents a -1.5" per 25 yard drop)
50 yards -5.5"
75 yards -7.0"
100 yards -8.5"

 

[BADUNAME37]

It could be the scope.
I will not say for sure, however I will say THAT IS WHY I DON'T TRUST INEXPENSIVE SCOPES.

I have better things to do than run back and forth to the range, trying to figure out why my gun never seems to get sighted-in or doesn't maintain the sighted-in settings once I (wrongly) think the scope IS sighted-in.

Since I have been purchasing Leupold, that problem is HISTORY.

You can pay more by wasting your time, or pay more outright. I prefer the latter.

 

[Vurtle]

I ordered the lapping tool for the receiver. I will give it a try first. The reviews for the banner looked good, I may have to just pony up and buy a good scope if the lapping tool doesnt work.

Thanks for the replies. I will try to post results in a few days.