Hey everyone.  I bought an upper that didn't have any sights.  Most of the time I see these they are optic ready sights.  I went with a SIG Romeo 5 red dot.  The issue I had was I tried to sight it in and couldn't get on paper.  When I looked around how to sight it in the best it seemed everyone said you need to have a front sight and use that to adjust your red dot.  I plan on putting sights on the rifle but I didn't think I would need to have them to get the red dot working.  I don't mind them co witnessing but im not sure how to get a front sight post that isn't a flip up or without removing the gas block with a front gas block front sight.  Any advice how to fix these issues would be great.

What range are you trying to get it on paper at? On my the .22lr I built for my daughter I sighted the red dot in at 25 yards for final sight in. Started at 7 yards to get it on paper then out to 25 to get it to where it is mainly going to be shot at.

Use a laser boresight for a rough approx of where on paper you want to be at 25 yds, then increase the distance to where you want. I use my laser boresighter a lot and don’t need iron sights to sight in a new optic.

Unload and make safe the rifle.

Separate the upper from the lower and remove the bolt carrier.

Place upper on bench.  Looking from the chamber towards the muzzle, adjust the upper's position until the bore is centered on the target.

Once the barrel is centered on the target, your red dot should also be on the target (or at least on the paper).

If red dot is not on target, use adjusters to bring the dot onto the target.

After that, fine tune using live fire point of impact information.

Separate upper, remove BCG, rest upper on bags, boresight on a 6-10 MOA target with plenty more backing. Turn the adjustments opposite of where the dot needs to move (dot needs to move right-adjust left).

The above two posters are correct - it is super easy to bore-sight an AR.  No laser needed, just look down the barrel from the chamber end at a target downrange and adjust the sight so that it sees what you see looking down the barrel.

I've done this twice just this week, and both times I was no more than six inches from where I wanted to be - and that was at 100 yards.  Just had to fine tune from there.  Took no more than three or four shots to get and confirm my zero at 100 yards with this method.