I have received quite a bit of mail over the past several months concerning scope magnification. Many readers wonder why the military chose the 10x scope, while others wonder what power, if they plan on building a tactical rifle, they should plan on using.
First let me discuss the most important topic - your reason for building the rifle in the first place. Law Enforcement? Military? Civilian competition? Tactical competition? Plinking with the hottest technology on the market? Some of these pursuits will overlap but more often than not, the glass requirements are different.
Law enforcement is better served by low power. A 6X scope is pushing the limit. "What?!" you say? Think about it. When (in the history of law enforcement) has a rifleman been authorized to take a shot beyond 100 yards? Most are taken at 70 yards or less. Sometimes much less. A 10x scope will give you a totally blind and narrow field of view at these ranges. Think more in the range of 4 to 6 power. Less, if you operate in a totally urban environment. Also consider that during WWII, some of the most effective snipers in the German army were using 4x scopes. Very few had 6x scopes. These shooters were making shots well over 600 meters with these supposedly limited optics. Shooting a terrorist or hostage taker at 60 yards does not beg for a 20x scope! The odds of needing high magnification are slim, particularly if you have the opportunity to move closer to the target which, due to the urban environment you probably serve in, is not all that difficult. Problematic, yes, but impossible? No, more than likely the very terrain you operate in will dictate a shot at very close range.
Presently, military needs are different. Take the standard 10x scope the military is so enamored with. This has been considered an ideal magnification for those needing to make body shots on targets at medium to long range. It is best used from 250 yards on out to 1000. Under this range, the 10x becomes somewhat of a liability as your field of view is so reduced that if the subject of your attention moves laterally -- even slightly -- he could walk right out of your viewing area. Obviously, the 10x scope is not ideal for law enforcement. It is quite good, on the other hand, as a compromise for ranges beyond those most often experienced by law enforcement personnel. It offers a reasonable field of view at medium range, gives the shooter a fair amount of light transmission (for a higher power), and it is not so large that it makes the weapon system cumbersome. In a fixed power, the 10x gives the operator a strong and reliable aiming device. Lastly, at long range, the magnification is not high enough that it makes the effects of mirage severe. A 20x scope on a hot day could literally make a target disappear in the boiling mirage -- or bounce so badly that it is impossible to hit. The lower the power, the less this effect. A 10x scope is a good compromise.
For the competitor, things are different. If you are not a duty-slotted sniper, but enjoy participating in the various tactical shoots that are run each year, you might be very happy with a 10x, 4x or 6x. In other words, a variable makes a lot of sense. You will not be under the same level of stress as is experienced by law enforcement or the military, nor will lives depend on your ability to range a target. Field of view, while always important, will not be as critical during a competition as during a hostage situation. Shooting a cardboard moving target at 100 yards with a 10x is possible, but taking on a living, breathing, and MOVING human -- with evil in his heart -- is another matter entirely. The competitor can settle for just about any magnification as long as is it not extreme. A variable from 3x to 10x makes the most sense, unless the shooter expects to compete at very long distances.
The part-time shooter/collector presents another need. His interest is in collecting sniper weapons for historical value and occasionally shooting them at paper or at competitions. Maybe even hunting with them when the mood hits. He can build his rig with whatever power scope is standard for the weapon system in question. A friend of mine has an AT1-M24 topped with a 10x scope. This weapon is set up like the U.S. Army's current M24 SWS. This gentleman will never have to take a shot on a human target at close range, so this high power is totally acceptable for the intended use. He can compete, attend various military based shooting schools, and do fine with static targets. He will not suffer for this magnification until asked to shoot a running target at close range. Believe me, I tried it at 100 yards -- perhaps it was even less -- with this very rifle, and field of view was a major problem. The target would leap in and out of my field of vision and I had to be fast to stay with it. Not good for law enforcement, but just fine for a couple of guys having fun at the range!
The next issue you must consider is fixed versus variable power. The military shooter in this country currently uses a fixed power scope, and for good reason. He lives in an extremely stressful environment with a lot of data flowing about in his head. He must range targets from HERE to infinity. He must deal with ballistic tables, long-range windage deflection, targets appearing and disappearing constantly, noise, confusion, and the "fog" of war. The last thing this fellow needs is the extra hassle of remembering at which power his scope is currently set! His mil-dot reticle will only be accurate at one power, usually the highest. If he needed the wider field of view for a short range engagement, and had his scope dialed down, he may very well forget to dial up before ranging the next long-range target! This could prove very wasteful and frustrating. Not to mention a little dangerous. A recent magazine article mentioned that the Army is considering a variable scope because they are currently being expected to fight more and more in urban terrain, but until this becomes SOP, I would not agree. The current troop of "snuffies" already has a lot to deal with. There is a lot to be said for the good old "Keep it simple" principle. If and when the Army adopts variable power, I am sure the training criteria will change to reflect this. A better alternative might be a fixed power scope of lesser magnification. (Again, recall the Germans of WWII.) As the military sniper mission is generally one of body hits, a 6x makes a lot of sense, although a 4x might actually be better for an urban environment. A case has been made that even less power is better. This could raise an issue of having two separate optical systems -- urban and battlefield. These developments shall prove interesting and educational. I still balk at the idea of a variable on the battlefield. Call me old fashioned, but let's just say I have seen troops break that which was considered "unbreakable." A variable presents the grunt with one more thing to break, abuse, and misuse -- all in a regular day afield!
Variable power does, on the other hand, have a very legitimate purpose in law enforcement and civilian use. If restricted to a reasonable power, the variable can be quite useful. I say "if restricted," because against all common sense, one tends to dial up to the max, both in hunting and in law enforcement. Most deer hunters, particularly beginners, will hit the woods with their spanking new 3-9x cranked up to 9x. Then they will blithely miss their first deer when it wraiths across their field of view for one small millisecond. Police have the same problem. At 10x your field of view is a couple of feet at 100 yards. Worse as you move in. The typical deer is shot at 25 to 40 yards and I am told that the typical bad guy is nailed between 50 and 75 yards. Think about it. What on Earth do you need a 9x scope for at 60 yards? Or even 100 yards? You are far better served with a lower power and a wider field of view. A variable is great as you still have the option to dial up if the range dictates, but for the most part, you can leave the power down where your field is optimum. The ranging feature of the military scope is also of little use under 100 yards. If you are zeroed at 100 yards, your bullet will seldom impact far from the center of the reticle when shooting at lesser ranges. Mine is within a quarter of an inch at 50 yards. Common sense will pretty much dictate that you can spend your hard-earned cash on a scope of superior optics and design quality, and forget some of the unnecessary do-dads you find on long-range sniper scopes.
The only area with which I might disagree with that last statement is in a long-range varmint scope. If you regularly plink prairie dogs, I can easily see where you might want a mil-dot reticle and variable power scope. One comes immediately to mind: A 6-20x40mm with mil-dots and a boat load of elevation adjustment. I recently ran out of elevation on a long-range shoot and had to estimate the range, as well as the "hold over." I would have killed for a long-range, high power scope with mil-dots. When on the prairie, 300 yards can look like 600! It can be that tough to tell! There are no range indicators, buildings, trees, or any other objects upon which to base an estimation. Just flat, flat, barren terrain.
Last subject. Objective size. What is reasonable, usable, or just plain hoaky? I will offer my opinion, one I am sure will garnish some argument. I do not believe there is any use for anything larger than 40mm, or 42mm at the most. In a good quality scope, one in fact going to be used for sniping, competition, or collecting, a large objective bell is only a hindrance, no matter what the current hype. Consider first the major disadvantage to a 50mm or larger bell. These large objectives force the shooter's head up so high that, on an unmodified stock, he can get no reasonable or repeatable cheek weld. Think of firing an AR15A2 with a scope. You just about have to use your chin on top of the stock to see through the scope. Until you mount a high-rise cheek piece, you will never be consistent. This is not acceptable on a sniper rifle or, for that matter, any firearm used for hunting.
Your best accuracy is going to be found by mounting the scope as low as possible to the axis of the bore. Why start off on the wrong foot by building in an inherent disability into your weapon system? For more clarity, you say? HA! This is where the industry really loses me. Many companies offer very large objectives claiming that they will transmit more light, be brighter, and cause less eye strain. All of this may be true, but your eye can only accept so much light. About four to seven millimeters at the exit pupil. A good quality scope with a smaller objective is already capable of this, so why pay for something you cannot actually use? Also, consider that most of these 50mm (and larger) designs came about to assist European hunters who shoot at night. If you are not a poacher, why would you need whatever extra light gathering ability these behemoths might offer? If you are a police officer, chances are that the situation you are in is going to be well-lighted by klieg lights, idiotic reporters, or ambient street light. You may even have night vision of one sort or another, depending upon your department's policy.
If you are a civilian, and a hunter, there are many scopes on the market that offer excellent low-light clarity with less than 40mm lenses. This is another advantage to low power. The lower the power, the more light is transmitted. A small 1.5-5x 32mm will transmit more light than a 10x 50mm. So the question begs: Why spend all your money on objective size, when quality of glass is far more important?
To sum it all up, if you truly need a long-range scope, and do not expect to engage targets regularly under 200 yards, a fixed 10x with a ton of vertical adjustment is the way to go. The same can be said for a varmint hunter or tactical competitor. If on the other hand you are a hunter or police officer, a lower-magnification scope will do fine. In this case the need for 120 MOA of elevation is unnecessary. Chances are, once zeroed, you will never need to move the turrets more than a few inches either way.
I will leave you with a question once posed to me by a sniper instructor. "What is the windage adjustment for an 80 yard medulla shot in a 20 mile per hour wind?" The answer? "Zero! It (the amount of corrected deflection that would be indicated) is so little that it is not worth worrying about!" Make your purchases wisely -- and do not buy the hype!