For the first time in my life this past weekend I had my favorite FFL boresight an LPV on an M4 for me.  Usually I'd just shoot until I got it right, but with ammo shortages/price increases I decided to do this for this one rifle.

I've always heard that it "gets you on the paper", but really, just how close does it normally get  you, especially for a caliber that shoots pretty flat, a rifle that's known to be accurate, and a good quality Leupold 1-4 variable?

The range I frequent is still closed and it may still be weeks before I can live fire it.  I'm curious that if the need suddenly arose to fire that weapon in a SHTF situation how useful is it going to be if it was just boresighted and I never got a chance to fine tune the sighting in?

Depends what system he used to boresight and at what distance.

Remove the bolt and look down the barrel, aiming at something easy, like a car tire. Compare that to reticle. (keep upper fixed steady, obviously)

He did it with a laser and also told me, "I'm sighting you in at two inches low."  We were in his shop so the distance wasn't that great.

Should be good. It is just to get you in the ballpark. I would go out to the desert or some open land and dial it in

Bore sighting gets you on paper. Sometimes, you get lucky and be just a minute or three off.

FWIW, I bore sight using a PEM-1 Laser Borelight at 10m with the gun secured in Tipton Ultra gun vise.

No need to laser boresight - remove the bolt and put the barrel on something about 50 yards away, sync the optic reticle.  

You'll be on for windage and within a couple of inches in elevation (usually a tad low).  

Unless you suck at shooting, make single round adjustments to get you where you should be at 25 or 50 for the zero distance of your choice (100 for me).

Bump out to your zero range to confirm.

It shouldn't take more than 3-5 rounds to get it any better.

And remember, a boresight isn't a zero.  It's just to get you close and there is no substitution for putting rounds on paper.

Honestly, it'll be minute of man out to 25-50, but beyond that it could be anywhere from spot on to completely worthless.

I boresight mine at 100 yards by looking thru the upper while it is supported and then put the cross hairs on the target and always hit paper - usually in a pie plate size area.

I always found with laser bore sighting is that it gets near to the horizontal plane as your shots would.

I created a topic a few days ago with a similar question. The bore sight is essentially where your barrel is pointing and a question I have is always what sort of measurement can be computed where you can know your bore in relation to some distant known target. I'm not sure if there is an answer to this, but what I do is take a known zero rifle of equal length and twist sight height, and simply use the bore sight across a long hallway. It just so happens this bore laser and sight line lines up to this table leg to where it touches the floor and the base of the drawer. So I would use that as a reference to zero my rifle.

Turns out it is fairly accurate way to do it. Granted I would adjust a little more but doing it for a few rifle builds, I had success.

Another way to zero the rifle is with a mechanical zero. Just dial it down all the way one way, then count how many clicks it takes to the other end. You divide it by half and click backwards to the center point. With this kind of zero, you'll be sure to hit a human sized target within the yard, and home. You won't be winning matches though.

And finally, a lot of top tier optic companies tend to zero their optic to the rifle they have their BDC for. In some cases the optic is mechanically zeroed in the fashion I mentioned above. I do know an A4 acog that I used on my M16a4 clone had zeroed at the factory and when I took it to the range it was spot on. So read the manual before going crazy.

I call BS on that. If he know you were "2 inches low" then why do that? Why not make it completely dead on instead of 2 inches low?

I always clamp the upper to my deck rail and align the dot/reticle to something around 50 yards away.  It always has me on a pie plate between 50-100 yards.  From there I can finish zeroing.  Some rifles need a couple MOA adjustment and some are a click or two off, but it's always close.  

ETA:  I've tried the laser boresighters and found them to be no more accurate than my normal mk1 eyeball boresight.  YMMV.

Because 2" low at 10 yards or under is way closer to a real zero than dead on at that distance...

2" low at 7 yards or so is about right for a 25 yard zero. Which sounds like what he was going for.

+1 on everything said so far. Boresighting does take a lot of the guesswork out of the initial stage of the process. I do recommend it if you have the opportunity. It goes from walking the shots slowly towards the center to “go right 3 clicks and up 2.” But it’s not a substitute for real rounds downrange.

My concern is ammo. Sometimes, you theoretically should be fine, but your gun just throws a particular brand/weight like 3 MOA to the side. So for no good reason, you’ve got groups going high and left (or whatever). Things are funny like that.

An average of the dozens of "gunsmith" boresighted guns I see at the range each year is about 15 MOA radius.

I usually boresight by taking the upper off. Remove carrier. Place upper on sandbag on the back deck. Look through bore at a birdhouse about 75 yds away. Adjust optic to match what I'm looking at down the bore.
Nearly always gets me pretty damn close at 100 yds.  2 shot group, adjust. Another 2 shot group.

Usually get zeroed in 4 or 5- 2 shot groups.