I guess it depends on the rifle and the application and I’m sure many folks will provide a different answer, but I’ll share what I do and why in this post.
Let’s start with the obvious. Any time a scope is mounted to a rifle it should be sighted in and zero should be confirmed. For a new scope, start with bore sighting. In my experience, a local gun shop should be able to do this with little to no cost. One can bore sight using their eyeballs, but I’m either doing it wrong or my eyes just aren’t what they used to be. So I prefer to let my local gun shop handle the bore sighting as I don’t have the tools for it. I have been thinking about picking up a laser bore sighting tool like the SiteLite Laser Boresighter, but I haven’t really had a need for it since my scopes rarely move after the initial mounting.
Beyond the initial bore sighting, I like to maintain a 100 yard zero on my rifles, I do this by taking the rifle out to a local shooting range with a box or two of ammo and shoot 4-5 shot groups at a target designed for sighting in a rifle.
The sighting process is fairly simple:
- Pick one of the four outer targets (not the center one).
- While aiming dead center, take 4-5 shots.
- Find the center of the shot group and determine how far off center you are.
- Make adjustments to the scope.
- Repeat steps 1-4 with another outer target until satisfied or all outer targets are used.
- Finish with another 4-5 shot group on the center target.
Now to answer the original question this post is attempting to answer: how often should a scope be sighted in? In my case, I “sight in” my scope at the following times:
- After bore sighting a new scope (or a scope that came off another rifle)
- When changing ammunition used in the rifle
- Before a significant shooting event like an upcoming hunt or competition – especially if the ambient temperature will be significantly different than the last time it was sighted in
I’ve had a few hunting buddies purchase different hunting ammunition than what was used to sight in their rifle because the typical ammunition was out of stock or another brand had a killer sale. They didn’t consider a different brand, projectile, or projectile weight might change the point of impact. Then when the opportunity to take a shot on choice game at a common distance they completely missed. When I found out about the change in ammunition used to sight in the rifle, I let them know that it can significantly change the point of impact. This has a lot to do with different powders burning at different rates and different projectiles and weights stabilizing differently as it travels down the barrel. One hunting buddy couldn’t believe it would make such an impact. A trip to the range with a box of typical ammunition and the different ammunition showed as a shift on the point of impact by several inches. So yeah, a change in ammo is definitely a reason to sight in the rifle again.
A significant shooting event is another reason to check the zero on the rifle. I’ve heard some powders burn at different rates at different ambient temperatures. If this is true, it makes sense to confirm the zero when planning to use it at a significant event during different seasons. I would consider an upcoming hunting trip a significant shooting event (or maybe an upcoming competitive shooting event but I’ve yet to participate in such an event with a rifle) and frankly I don’t want to miss a shot at choice game. So I just make it a habit to confirm my zero a week or two before such an event. This has translated to confirming zero on my rifle a two or three times a year. Adjustments have been minimal (less than an inch vertically or horizontally), but I’ve had to make them. I suppose if I was hunting or competing with a rifle a couple of times in a month, I might limit confirming zero to once during each of the four seasons (as we go from hot to moderate to cold weather and back). But I would probably confirm again prior to whatever event seems significant to common ones.
I guess I could summarize this advice as one should sight in their rifle anytime they need to be certain they can make a precise shot with a given scope, rifle, and ammunition combination.