Re: Do you have a NightForce 12 - 42 x 56 on your AR?
I don't care what anyone chooses to put on a rifle, personal preference, what one likes another may or may not.
What matters is performance and whether your equipment will allow you to get the job done!
Per above, with the 12x-42x BR, you have the issue of available internal adjustment, which if you want to go to 1,000 yards requires an angled mount (limited options & expensive). If you add a 20-30 MOA base to get to 1,000, you may not have the internal adjustment for a 100 yard zero. Also factor in your lowest magnifcation is 12x which brings up mirage and field of view issues. Also consider your adjustments are 1/8 MOA, which makes for a lot of clicks/dialing if you are shooting at a lot of different ranges. The issues above are some of the reasons why I said that it would be the last scope that I would put on an AR.
Now if you want to make it a dedicated bench gun, you can address the issues above, and you are good with the limitations, then best of luck to you.
Not sure what your ultimate goal is, but food for thought:
There is no signifcant number of 1,000 yard F-T/R shooters standing on the winners podium at a national level who are running a .223 rifle. If they can't do it with a bolt gun, best of luck doing it with a gas gun!
12x-42x BR Scope was purpose built for use on a dedicated 1,000 yard rifle, not for use on a rifle that is going to shoot 300-1,000 yards. Per above, there are "mid range scopes" that are better suited for that application in terms of available internal adjustment, magnification range, available field of view, and an adjustment value that does not require tons of clicks/dialing.
I run Nightforce NXS 8x-32x and Sightron SIII 8x-32x for 100-1,000 yards, because like you I want the higher magnification for the longer range shots, but they don't have all of the issues that a 12x-42x BR does.
If your decision is functionally sound, then run with it. If you are trying to make something work that is not going to be functionally sound, then you may want to reconsider.
Best of Luck,
M Richardson