Friday, April 15, 2005
Scope Height
I have been moving scopes about on a couple of my rifles this past week. As part of my preping some of my rifles for the shooting season, I made a few trajectory charts for the rifles with their new scopes. It hit me that one of the major factors required for all trajectory programs is the height of the center of the scope from the center of the rifel bore. If this parameter isn't measure accurately all the number the programs spits out are worthless.
To measure the correct scope height, I like to use a set of calipers and measure the different segments of the scope height individulally and then add them togetther. Scope tube, most scopes are 1" or 30mm so you can take that value and divide it by 2 and that is the first measurement. I don't have any 30mm scopes right now so I start with .5" for the scope tube. Another segment is the size of the barrel, which can be measured directly by putting the calipers on it, measuring and again, dividing the value by 2. My R9 barrel measured .625 so the bore center would be 0.3125".
The next segment is the measurement from the bottom of the scope to the top of the barrel. This can be an easy or hard measurement, depending on the rifle. For many pcp rifles the scope extends right out over the bore so the measurement can be made using the calipers to measure the distance from the barrel to the scope. However it can get tricky on piston guns where the scope sets over the receiver and the barrel isn't level with the top of the barrel. I haven't come up with a sure fire way to measure this yet. I many cases I can use my calipers to make a very close measurement of the distance from the top of the barrel to the top of the receiver and then add that to the measurement from the top of the receiver to the bottom of the scope. In other cases I have to use an accurate straight rule to get the measurement.
Once all the values are established they are added together to get the total measurement from the center of the bore to the center of the scope. It will vary from gun to gun and from mount to mount. My R9 with Sportsmatch low mounts measure 1.44" while the same mounts on my TX200HC measured 1.81" This number can then be used to get some trajectory numbers from the computer which are just an estimate of the path of the pellet so always be sure to verify your number by shooting them on the range.
To measure the correct scope height, I like to use a set of calipers and measure the different segments of the scope height individulally and then add them togetther. Scope tube, most scopes are 1" or 30mm so you can take that value and divide it by 2 and that is the first measurement. I don't have any 30mm scopes right now so I start with .5" for the scope tube. Another segment is the size of the barrel, which can be measured directly by putting the calipers on it, measuring and again, dividing the value by 2. My R9 barrel measured .625 so the bore center would be 0.3125".
The next segment is the measurement from the bottom of the scope to the top of the barrel. This can be an easy or hard measurement, depending on the rifle. For many pcp rifles the scope extends right out over the bore so the measurement can be made using the calipers to measure the distance from the barrel to the scope. However it can get tricky on piston guns where the scope sets over the receiver and the barrel isn't level with the top of the barrel. I haven't come up with a sure fire way to measure this yet. I many cases I can use my calipers to make a very close measurement of the distance from the top of the barrel to the top of the receiver and then add that to the measurement from the top of the receiver to the bottom of the scope. In other cases I have to use an accurate straight rule to get the measurement.
Once all the values are established they are added together to get the total measurement from the center of the bore to the center of the scope. It will vary from gun to gun and from mount to mount. My R9 with Sportsmatch low mounts measure 1.44" while the same mounts on my TX200HC measured 1.81" This number can then be used to get some trajectory numbers from the computer which are just an estimate of the path of the pellet so always be sure to verify your number by shooting them on the range.
