I'm always surprised by how many hunters I meet that consider bore-sighting their rifle to be sufficient sighting in for hunting season. Their logic is apparently that since the bore sighting device is measuring point of impact based on the barrel it must be quite accurate. Right? Absolutely not.
Bore sighting devices were never intended to be a replacement for sighting in a rifle with live ammo at the gun range. Because there are numerous different variables affecting a bullet's flight and point of impact that is impossible to replicate in a bore sight device it is physically impossible to engineer a bore sighting device that is consistently more accurate than 10-15 MOA.
A rifle barrel has spiralling horizontal grooves cut into it to stabilize the bullet in flight by causing it to rotate. When a rifle bullet first begins its acceleration following the firing pin striking the primer, because of the incredible amount of g-force involved in a bullet accelerating from 0 to 3000 fps the bullet tends to bulge slightly. This forces the copper jacket into the rifling and creates a gas-proof seal. A bore sight device sits on top of the rifled grooves in the barrel, rather than enveloping them like a bullet would. This minor discrepancy between actual point of impact and where a bore sight device measures point of aim contributes to their innaccuracy.
Another aspect of accuracy that bore sighting can't calculate or replicate are the barrel vibrations caused by a bullet travelling down the barrel. When a bullet is fired the barrel of the rifle vibrates ever so slightly just like a tuning fork. This oscillation as the bullet goes down the barrel can have significant impact on the point of impact.
And last of all, some rifles just will not shoot a particular bullet type or weight accurately. For example, my Remington 700 7mm Rem. Magnum absolutely will not shoot any better than a 4 MOA group with 150 grain Nosler Partitions. However, when I switch to 175 grain Nosler Partitions, I can shoot 3/4 MOA groups all day long. In another case I had a Winchester Model 70 in 300 Win. Magnum that wouldn't shoot 180 grain ballistic tips better than 3 MOA but shot 165 Barnes Triple Shock X Bullets almost as accurately as a bench rest rifle.
Bore sighting devices are a great way to get a gun on the target so you can further refine your sights from there but it should never be used as a replacement for time at the gun range. Is it really worth taking the risk on missing a shot at the buck or bull of a lifetime because you weren't willing to burn a couple of bucks in ammo to sight a rifle in properly?
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