I have the LaserLyte Boresight, it has "toggle" style inserts for a variety of calibers. It inserts into the barrel, rotates to snug and works better in a large indoor warehouse or garage, with a rest.
When I installed my P4 NcStar Scope on my Sig556, I was inside our large covered shop. With a bench and rest, it turns on, and by looking through the optics, at a given range, say 50yds, you can "walk" the scope to the bore.
I was high and right, but very close. Crosshairs to red dot. When I got to the range for live fire, it was windy and some windage adj. was necessary, but I was on the black right away, and after a few adj. in the 10ring. YMMV, but for a "Black Rifle" even some pistols, they work fine.
Of course you can always "zero" at twilight or a cloudy day, as full sun makes the "target" hard to spot at anything past 25-50 yds.
It got my rifle very close, in short order. If your planning it for a long range rifle, you'll need the light right, preferably dim.
Hope this helps. It is just my .02 cents, but it did help.