Failure to adjust scope parallax degrades crossbow group size by 3 to 4 inches
I had two phone calls yesterday asking for assistance with accuracy problems using their new TAC 15/15i crossbows.
Both fellows reported getting 4 inch or greater group sizes at short distances of around 25 yards. As you may know, my groups (and those of others) have consistently been less that an inch at 25 yards.
With the first fellow it sounded like he was a pretty experienced rifle shooter the way he answered my questions that were trying to respectfully figure out whether he might not know how to adjust out normal parallax error. I spent a LOT of time o nthe phone with him before it became clear to me that he knew a little about parallax but almost nothing about to to eliminate it. I told him how and I'll tell you how in a minute.
Later the same day I had a second guy call with the same lament about poor groups at short distance. This time I cut right to the chase and told him politely and explicitly that it sounded like parallax error and told him how to eliminate it.
Guy #2 called me back today to say he tried my parallax correcting method and his groups at 25 yards dropped from 5 shots into 4 to 5 inches down to 5 shots (at different dots) down to way less than 1 inch.
The dial on the left side of the Accushot scope that ships with the TAC crossbows is the parallax dial. It has yardage marks on it out to 500 plus yards. It adjusts focus of the target as you turn it and more importantly it reduces and just about eliminated parallax errors when done properly.
The yardage numbers on the dial are only guidelines. Rarely do I find they coincide with the actual optimum correction factor (for either target focus or parallax) at a given distance -- regardless of how much you paid for the scope. It would be nice if you could count on the image focus adjustment done by the parallax dial to be best at the same dial location as that of the parallax adjustment -- but that also rarely coincides perfectly.
WHAT IS PARALLAX?
It is easier to describe than to define so here goes. Set up your scope and bow (or rifle) on a steady rest with the reticle aimed at an object at say, 25 yards. Now, without touching any part of the weapon and scope, look through the scope at the object and move your eye back and forth, up and down. Kind of like a "bobble head"). As you move your eye, notice carefully whether the reticle seems to move around on the object (target).
The reticle movement you probably see is called parallax. Now let's think about what happens when you try to shoot a group of 5 shots with this condition. You put your eye up near the scope adjust the bow, fire, put the bow down, reload, etc.
When you take your 2nd shot, your eye may be in a slightly different position or angle on the scope so the reticle points a little bit off. You think you have aimed the crossbow at your target but you haven't. You've actually aimed just off to the side or up or down slightly.
TO ADJUST PARALLAX
With the crossbow in a firm rest get aim it at your target. Now take your hands off. Get behind the scope still not touching anything. Do the "bobble head" routine watching to see if the crosshair moves around on the target. If it does, take your left hand and slightly move the setting on the parallax dial. Bobble head again to see if it's improved. Turn the dial again, do bobble head, turn dial, etc.
Until the error is just gone. Make note of what yardage number is showing at this first distance Write it down.
Last edited by Len Backus; 08-07-2011 at 07:12 PM.