I purchased a new DPMS Oracle AR-15 flat top. Ordered the iron sights and now am real frustrated trying to sight it in.
Vertical is fine. However with the rear sight dialed all the way to the left I am still shooting 3 inches to the right at 50 yds. I suspect that it is a problem with the alignment of the rail on the gas block but it is permanently mounted to the gas block.
Is there any adjustment in the gas block?
Thank you!
EDIT Read this first -
Your gas block is probably not pinned. I found this picture of an Oracle:
There are two or three set screws on the bottom of your gas block. They may have have some loctite on them. If so, you may need to put some heat on them to loose them. Remove your stock, sights and handguards. Loosen the set screws and set your rifle upside down on a known flat surface. Rotate your gas block carefully until it is perfectly level and on plane with your upper receiver. Add a dab of rockset or loctite to the set screws, and torque them back down. Don't monkey fist them. Use an allen wrench, not a screwdriver. Then you should be able to zero your rifle with irons.
Do not be concerned about this small amount of rotation in your gas block from a function standpoint. Gas blocks are drilled with oversized holes that are larger than the gas port which is drilled in the barrel. This allows for some wiggle room when installing gas blocks to ensure that full gas is still available to cycle the rifle.
Rotate your gas block but try not to move it fore or aft. It should still be snug against the collar that retains your handguards.
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I typed all of this below, when I thought you had a pinned gas block. I am not deleting it because it may be helpful to someone who has a pinned gas block.
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There is no adjustment in the gas block once it is pinned. However, there is a way you can true the gas block closer to the true center of your upper receiver. When your barrel is installed in your upper, there is a notch where the pin that is in the barrel extension sits. This pin almost always has some play from side to side. It can be shimmed, and the notch in the upper can be filed(if necessary), until the front sight post is trued to be square with the top of your upper receiver.
If you do not have the tools to do this yourself, contact John Thomas at Retro Arms Works in Illinois. He does fantastic work, he is fast, and his prices are excellent. I have the tools to do it and would offer to do it for you but I am not licensed to do gunsmithing work. It is an easy job, but it is made much easier by years of experience.
Since this work would require just the upper and that is not legally a firearm, you can ship it directly to him in a flat rate box without hassling about FFL issues.