None traditional sight in options

[psalms144.1]

There are tons of "offset" targets online that are printable and allow you to shoot at 25 yards but zero the rifle to 50 or 100 yards. Get some, go to your indoor range, and zero.

[Darth_Uno]

If you can remove the bolt and look through the bore, continue reading.

Stabilize the gun while looking through the bore at a distant target. For instance I will place the upper on a towel looking out my front window to a fence post 100 yards away. I can adjust the towel to slightly increase or decrease height. Once the tip of a fence post (or other small POA) is centered in the bore, simply adjust the optic to that same POI.

Yep, the 'poor man's boresight'. It's surprisingly effective. Fortunately I'm able to clamp the upper in a bench vise in my garage and aim it into the woods. You still need to dial it in with live ammo, but it's definitely good for COM.