Originally Posted By: Blackhawk43I have wondered the same thing, I have bore sighted a lot of rifles with them and have never seen any damage. I think barrel material is a lot harder than we give it credit for.
I've looked in the barrel with a borescope after using one of the boresighters with a nickel or chrome plated spud with a spring steel "keeper". What that spring steel clip and plated spud can do to barrel steel is ugly. I used the boresighter in several barrels, all of them have the damage to the lands. The rifles I didn't use it in, my lever actions that don't have a scope, do not have the damage.
What you will see is gouges and smearing of the lands in the area from the muzzle back to where that spring steel clip sticks in the barrel. The gouges are on the part of the barrel where the spring steel keeper is located.
The type of boresighter that has the expanding spud may not cause damage but since I've not used one and then looked in the bore with a scope, I don't have evidence one way or the other.
The laser type with the aluminum spud with rubber or plastic expanding plug don't cause damage that I've seen, neither does the Leupold version that attached magnetically to the barrel crown.
I boresight all my bolt actions by peering through the bore at a target 50 yards away with the rifle on a rest. I center the target bullseye in the bore, then adjust the mounts for windage with the scope centered in it's travel for windate, then set the scope cross hairs to be on the bullseye while holding the rifle in place. That always gets me within a couple of inches at 50 yards and centered in windage travel, after which the transition to 100 yards is a piece of cake.
Fitch