More about sighting-in: How to determine the two intersection points
By B.B. Pelletier
The following question was a comment to the June 1 post, At what range should you zero your scope?
How do you figure out what the two intersections are? Is there a formula?
Just shoot & see
No doubt, a formula could be constructed, but I do it the easy way. Simply aim at a specific point at a known distance, shoot and see where the pellet impacts relative to the aim point. That's where I came up with the distances I gave in my June 1 post.
Each pellet has a different trajectory
My distance figures in that posting were not exact. They change slightly because each type of pellet has a unique ballistic flight. If it has higher drag, it slows down faster and the trajectory is more pronounced. The figures I gave were for a domed pellet, like the Crosman Premier, which is pretty standard for field use.
If you shoot wadcutters or hollowpoints that have a sharp shoulder, such as the RWS Super-H-Point, your trajectory will be more pronounced because the pellet is less aerodynamic. The difference will be small at close range but will increase rapidly as the distance passes 30 yards. That's why wadcutters and hollowpoints are not as good for long-range shooting (unless you take the time to learn their performance characteristics well).
Twenty yards is a common zero distance
The initial point of intersection I gave in the posting (20 yards) is based on seven years of competitive experience shooting field target. I had to learn the performance of a great many pellets (all .177, but that makes little difference to what we are doing). Although there are a LOT of variables, I soon noticed that nearly every shooter had a first zero point of around 20 yards. When I tried it myself, I discovered why.
What a precision target shooter wants is the most forgiving zero possible. One where, if the yardage is not exactly what the shooter estimated, the difference in pellet impact point is very small. If you zero your gun so the first impact point is 20 yards, you'll get another 10 to 20 yards of flat trajectory and any apparent rise or fall of the pellet is less than one pellet diameter. The first intersection is at 20 yards and the second is at 30 yards (a 10-yard flat spot) with a gun shooting around 725-750 f.p.s. When you get it up to 950 f.p.s., the second point will be all the way out to 40 yards. If you go even faster, you'll have a flatter trajectory but also get blown groups and leading. It's not worth it.
You CAN stretch out the first point of intersection to 30 yards, if you like. All it takes is a scope adjustment. BUT, your flat spot will be MUCH shorter than when you were zeroed at 20 yards. It may only be 5 yards long. Zero at 40 yards for the first point, and the flat spot MAY be yards long. Get it?
What's really happening is that the pellet is dropping the moment it leaves the muzzle. You have about 30 to 40 yards to play with before the trajectory is a real downward slide. By zeroing at 20 yards, you get a nice long flat spot and can still adjust your scope or hold-over for distances outside that range.
Will zeroing at 5 feet give me a really long flat trajectory?
At this point you may be wondering why I'm saying 20 yards is a good zero point. Why not zero at 5 FEET and enjoy as much of that flat trajectory as possible? If your scope looked straight through the center of your barrel, you could do that. Because you have to mount your scope high above the barrel, you can't make it work that way. The separation of the scope axis and bore axis introduces parallax that has to be accounted for when you sight-in.
This is already a long post, so I'm going to end it here and continue this discussion on another day. I hope I've answered some of your questions. Please feel free to post comments or additional questions!