Point-blank range is the distance at which the shooter can aim directly at the center of the target and hit it with no hold over or under. Target size and bullet trajectory are variables that affect this distance. The closer the center line of a scope to the center line of the bore, the further point blank range will be for a give cartridge, rifle, and target combination.
Scope ring height has a definite impact on point blank range and therefore, accuracy of a rifle. Generally speaking, the closer the center line of the scope to the center line of the bore, the more accurate you will be.
If two rifles are both zeroed at 50 yards and rifle 1 has a scope that is mounted with a center line above the bore of 1.5 inches and rifle 2 is 2.5 inches and the target is the same size for both, rifle 1 will have a further distance where the target can be hit while aiming at the center of the target.
This happens because bullet trajectory varies less from the line of sight when the the line of sight is closer to the center line of the bore. Try a ballistic calculator that uses scope height above the center line of the bore.
Enter all variables for your rifle, cartridge, etc., then change the scope height variable to one that is higher and look at how much your drop charts are affected. Point blank range will be shorter as the height increases. Some say this is not important because it can be compensated for but anything that requires me to be less perfect is good.
What is sight height?
Most ballistics calculators will ask for sight height. This is the distance between the center of the bore to the center of your sight – scope, red dot, iron sights, whatever sight you are using. In our case, the center of our sight will be the center of the scope’s objective lense. The smaller this distance is, the larger your point blank range will be for a given target and therefore, the easier it will be to hit.
How does a lower sight height help us?
See the diagram below. Compare the lower sight line (red) to the higher sight line (black) and note how they both compare to bullet trajectory. On the sight line closer to the center line of the bore, bullet trajectory is closer to the sight line and never deviates as much as the higher sight line (black line).
Point blank range is determined when the bullet trajectory varies more than the diameter of the target from the line of sight. If we assume that the red line of sight is perfect for our target, this makes point blank range around 130 yards because trajectory never got far enough away from the sight line to miss until after 130 yards as gravity began to take affect.
On the black line for a line of sight that is further from the center line of the bore, point blank range would occur around 65 or 70 yards because the variance from that line of sight exceeded the diameter of the target. This of course is just an estimation but is representative of what really happens. The whole concept is a little confusing.
Hopefully I have set it up to make it easier to understand. If you are like me and don’t want to burn anymore brain power than necessary, just trust me.
If you are shooting at an elephant either sight line would be good, but if you are shooting at golf balls at 125 yards the lower sight line is the best choice.. After 125 to 150 yards or so, elevation adjustments will need to be made to stay on target
So how can we sum this up? On smaller targets, with out goal of long range accuracy, we are much better off with the scope being closer to the center line of the barrel to minimize as much as possible the difference in bullet trajectory and line of sight for as far out as possible.
It’s just common sense that the flatter shooting a cartridge is, the more advantage we will have because our point blank range will be further. A 17 hmr will have a long point blank range than a 22 lr and a 22 wmr will have a longer point blank range than a 22lr.
How is cheek weld affected?
Another reason for the scope to be closer to the center line of the bore is it helps to prevent what I call “a loose head”. A strong cheek weld to the rifle stock promotes accuracy and is a component of proper shooting technique that cannot be compromised.
Any contact or rest the shooter has for any part of his body, especially the head allows a more stable shot picture and more focus on the center of the target. You cannot shoot accurately with your head not rested properly on the stock comb.
Your hold on the rifle, hands, cheek, etc. must be conistent on every shot if accuracy is your goal. See my article on how a loose head affects parallax. In addition, a cheek weld adds more opposing force to the rifle to promote stability. If you are shooting without a good cheek weld, you are not shooting at your best because your rifle doesn’t fit you.
If you intentionally mount your scope higher to accommodate accessories or your rifle stock has a low comb, or both, you can overcome the cheek weld issue by installing a cheek riser. I have these on several of my rifles.
Stay away from the fabric style with foam for the cheek rest. These do nothing for improving shooting. They are like soft pillows, which my be good if you want to take a nap, but does nothing for stabilizing you sight picture for consistency Go with the hard plastic variety linked above if you really want to see sustained improvement.
How is sight height measured?
Why would you want to know what your sight height is? You really need to know your approximate sight height before you order or purchase rings. After you purchase your scope and scope base, the rings are your only control to make sure site height is correct.
For our purposes, correct is defined as close to the center line of the bore as possible without any part, usually the objective end, touching anywhere. You really don’t need to concern yourself with a measurement from the center line of the bore. As stated above our main objective is to get our scope as low as possible without touching anything.
Measure the largest diameter on your scope, which is usually the object lense end. Measure from outside to outside the the tube at it’s widest point. Next you must know the diameter of your scope tube. Usually they are 1 inch or 30mm which is slightly larger at 1.18 inches. Some higher end scopes can have a larger tube but the same process is used to determine sight height.
Once you know these numbers subtract the scope tube diameter from the largest diameter of the scope, , then divide by 2, then add 0.1. Then subtract the height or thickness of your base. The number you get is dimension C on the illustration below.
With this number you should be able to order or buy the rings that will work. When the scope is mounted and everything is locked down, the largest diameter on your scope should be 0.1 of an inch above the barrel. If you want to get a little higher away from the barrel or just want to make sure you allow for error in your measurements you can increase the 0.1 that is added to give you extra clearance.
Whatever number you use will be how high from the barrel the largest diameter on the scope will be. Finding the right rings with the C dimension shouldn’t be a problem. Most rings have a similar drawing to the one above in their material. If you are having trouble finding the right rings
Why doesn’t everyone mount their scope lower?
There are several reason why sometimes you just can’t go any lower. Some accessories installed on a rifle, like an AR15 might interfere with the scope being lower. The design of the stock itself could have a high comb. The objective lense of the scope might be so large it has to be mounted up to stay out of contact with the rifle.
Some people do it simply because they like their scope higher or don’t realize how it will affect their zeros and point blank range. Mounting your scope higher than you need to is not a deal breaker. It’s not that you won’t be able to shoot well, but you will give yourself a slight advantage by mounting it as low as possible. I don’t know about you, but I’ll take every advantage I can get.
What is the drawback to having a long point blank range?
Having a longer point blank range as we have discussed is a good thing to a point. One drawback though is if you are shooting at long enough range that you are bottoming the adjustments on your scope out, you are at a disadvantage. Your elevation adjustments will bottom out faster for a given scope.
As a shooter you must realize that these two things are a trade off, almost a direct trade off. To have one you sacrifice the other. You could install a scope base with built in moa or a slight tilt designed into it for the purpose of getting 20 more moa out of your scope but by doing so you will be giving up point blank range.
So what is the best alternative for us long range rimfire shooters. We should take advantage of the point blank range improvement by keeping the scope as low as possible but search for scopes that maximize the amount of adjustment.
Typically scopes with extra adjustment are more expensive. Having the right combination of cost, amount of adjustment and point blank range is what it’s all about. Every shooter will have his own personal preferences as to what combination is the best for him or her.
Use a base with more MOA built in
If you decide to use a scope base with an extra 10 or 20 moa built into it, you are essential buying a base that is tilted slightly. There is nothing wrong with doing this, I have them on some of my rifles. The scope rings will fit just as well as if the base were flat.
The rounded portion of the the rings where the scope contacts the rings is won’t be tilted. Most rings are designed for a perfectly flat base mount unless it is specified differently. In this case, I suggest lapping the rings once installed on the base just to ensure that the scope will have a secure fit and proper fit.
If you are not familiar with the process of lapping scope rings it is basically using a round straight tool that matches the diameter of your scope tube to remove tiny bits of material from the rings to ensure the scope is aligned properly and there are no high points to prevent the scope contacting the entire surface of the ring. See the image below.
Most picky long range shooter who are really serious about accuracy do this to their rings whether there is additional MOA built into the base or not.
Just an extra step that could eliminate problems after everything is together one to have to start over. You can have a good gunsmith do this for you are you can easily do it yourself with the right tools. Just take off enough material so you are sure the scope is in full contact with both rings.