Scope Mount Selection
Tapered or Flat?

20 May 2002
By Scott Powers

Tapered or Flat? I am often asked if one should buy a tapered or flat base. The person asking the question often seems to be under the impression that the tapered base will be stronger and more versatile. This is not necessarily the case and often you may be making a better choice going with a flat base. There are both upsides and downsides to every choice and I will try to elaborate on them within the confines of this article. I am sure I will not cover everything, so never stop asking your rifle building gurus for input. A really competent gunsmith is worth his weight in gold. In this case, his opinion is like doubloons.

First and most importantly, get the mounting system you NEED, not the one you think you need or are attracted to because it "seems hardcore". Base your selection on your actual use of the rifle, not some possible dream or fancy. For instance, if you never shoot beyond 500 yards, do not bother with a tapered base at all. There are plenty of high quality tactical mounting systems that, when coupled with a decent telescopic sight, will provide a lifetime's worth of service at any range you hope to exceed in real life. They are plenty strong enough and do not have some of the more serious downsides of a tapered system.

If you only use the rifle for punching paper or taking deer, the odds of you needing the extra elevation a tapered based provides are probably slim. Especially on the east coast where ranges are often short. However, if you do plan on shooting long distance, then you need to assess the next step in the decision-making process: What is your telescopic sight capable of in terms of internal adjustment? In this case, if your scope has more than 85 to 120 moa of internal adjustment, you may still only need a flat base. The scope should have enough internal adjustment to get you to 1000 yards from a 100 yard zero, assuming a typical caliber like the .308 Winchester and a match bullet. Scopes with less internal adjustment will require a tapered base to reach beyond 600 or 700 yards. In addition, rifles with less than stellar factory drillings and barrel alignments sometime cloud the issue. But if you are shooting a more powerful caliber like the .300 Win Mag, then you are back in the ball game since it shoots a bit flatter and requires less moa to reach out to 1000 yards. Again, a flat base may be all you need.

Lets take a quick refresher on mechanical zero for those who may not understand what we are talking about here, and why it applies. If you do, skip ahead.

When first zeroing your rifle it is important to take note of the actual mechanical zero of the sight because this allows you to one, have a solid starting datum and two, shows you just what the sight and rifle are capable of in real terms. Determining this mechanical "zero" also lets you double check the manufacturers' claims for the scope. I am currently reviewing a scope that is claimed to have 100 moa of internal adjustment. The actual count is 115.5 moa, which is great. In this case the manufacturer erred on the conservative side in their claims. I have had, in another instance, a manufacturer claim 60 moa when the scope was only capable of 54 moa. At any rate, determining the mechanical zero is important. It tells you how much of that internal adjustment you have used up when zeroing your rifle for the first time at the range. THAT in turn, lets you know just how far you will be able to shoot with that particular sight, based on what internal adjustment is left after your actual zero is achieved.

To find the mechanical zero of your telescopic sight, carefully turn one turret all the way to its stop. Do NOT force it once it lets you know it does not want to turn any further. You can bend things that way! Now, turn the turret back in the opposite direction while counting clicks. Take care in counting clicks so you do not skip any or jump a series of clicks. Go until you come to the stop at the other end of the adjustment range. Divide the total number of clicks by two and that represents the mechanical zero of the scope or sight. You may want to verify it by clicking back in the other direction and averaging the difference if any is present. Do this for both windage and elevation. Then set the turrets to reflect that number. Write the numbers down.

On a scope represented to have 100 moa of internal adjustment, you should come darn close to 100 moa represented by what-ever incremental division is said to be on the turrets. On a 1/4 moa scope, you are in for a lot of clicking as FOUR clicks equals ONE moa. So, to set the scope at its mechanical zero, from one extreme of the internal adjustment range, click BACK half the number of clicks. On this hypothetical 100 moa scope, you would need to go back 50 moa, or in terms of actual clicks, 200. Perform this operation with EVERY new scope you buy or with every iron sighted rifle you own that allows adjustment (like the AR15 for instance). We are mostly talking about mounting systems for telescopically sighted rifles, so I won't go into iron sights any further. Now back to the story.

In an ideal world (it obviously does not exist) you would be able to zero your rifle and scope combination at 100 yards at exactly the mechanical zero of the telescopic sight, or, better still, "below" that mechanical zero so that you had more UP elevation left than down elevation. The odds of this are slim of course and no two rifles zero exactly the same way. However, you will occasionally find a combination that gets to with in 3 to 10 moa of mechanical zero either way. So using a typical scenario to illustrate this, lets examine it in more depth and how it relates to the topic that started all this.

You have a .308 caliber brand X rifle and the factory did not totally screw the pooch and mount your barrel off at some odd angle that makes you feel drunk each time you look down the receiver. Your scope mounting holes are actually drilled fairly square to the center line of the bore for a change and when you bore sight the rifle you find that, low and behold, you are only a few moa above or below the mechanical zero of the scope. Well say 5 above for the sake of argument, but you might luck out and have much less "wastage".

On a scope with 85 moa of internal adjustment, you have a mechanical zero of 42.5 moa. You shoot your rifle and find that to achieve your 100 yard zero when actually firing the rifle it took 5 moa from mechanical zero. In this scenario, you had to use up that 5 moa of internal elevation adjustment and you now only have 37.5 moa of UP adjustment left before hitting the stops. This is where you cuss up a storm if you are a truly long range shooter. With your .308 and 175 grain match bullets, you will only be able to get out to 940 yards or so. Hardly worth crying over though unless you actually compete at 1000 yards or are in the military and actually plan on engaging targets at those ranges. Lets keep the scenarios but change the scope. The story becomes more bleak if your scope only has 60 moa of internal adjustment. It now has a mechanical zero of 30 moa. From a 100 yard zero, it takes approximately (it varies with load weights and bullet type) 41+/- moa to reach 1000 yards with a typical .308 caliber rifle. So if you used up that same five moa of internal adjustment, you are now only left with 25 moa of UP elevation, or in real terms, about 740 yards reach, give or take. In this case, a 20 moa tapered base is going to prove necessary if you hope to get on target at 1000 yards. With it, you would then have approximately 45 moa of up elevation. That's four more moa than necessary to dial in at 1000 yards. Not ideal, but it gets you there. Four moa at 1000 yards is 40" so you have room to spare.

One question that may arise in your mind from all this is "why then would I buy a scope with less than 100 moa of internal adjustment." It's a fair question but lets face facts, there may be a scope that particularly suits your desires, or is from a brand you are loyal to that only has 60 to 80 moa of adjustment. It may have features you want that others do not (a variable vs. fixed for instance) or it may be in your budget allowance whereas another scope may not be. It may have a turret style that you insist on (pick one, they all have their proponents!). A good example is the Leupold Vari-X III, Long Range M3. It only has 60 some moa of adjustment, but some prefer it's 1 moa elevation turret for its fast tracking to ranges. If you are using a .308, that scope will not get you to 1000 yards unless you really luck out and do not need to use up any internal adjustment for actual zero. It comes with three turrets marked for .223, .308 and .300 WM. In the case of the .308 turret, it is marked to get you to 1000 yards but it can only rotate so far and if you used up the internal adjustment for your 100 yard zero, a tapered base is in your future if you want to reach 1000 yards. That scope mounted on a 15 moa base will get you beyond 1200 yards. At least on my rifle. So, why not just buy a scope with 85 moa or more? Because something with less moa may be what you wanted for other very valid reasons and that's good enough.

Now, the real question is, do you really need to reach out that far? It's a hard thing to answer. Everyone wants the "most" out of their equipment. Some do it for simple reasons like "I have to have the best", confusing "best" for some goal they will never actually pursue. Some just like to throw money at their projects even though they will never use all the potential built into the system. Some simply do not understand the trade-offs one decision may force upon them. Some see a massive base like a tapered base as being "stronger" than a thinner flat base. Its not necessarily the case and while it may be very strong, that may have its downsides too.

So, what is the real difference between a flat base and a tapered base? Both have advantages. One has serious drawbacks, but these are usually easily dealt with. One has a drawback that may not be so friendly and we will discuss that below.

The FLAT Base.

A flat base affords you the absolute BEST cheek weld you can have on a scoped rifle. It does not force your head up off the stock. This in turn keeps your cheek down, pressed on the stock where it belongs, not hanging precariously above it in mid-air. It requires no neck strain to hold yourself in place as you acquire a sight picture because you are not supporting your head with neck muscle alone. It can be very strong in terms of mating the base to the rifle and the rings to the base, yet it is much lighter than the average tapered base, which on the extreme end can approach the weight of a full sized combat handgun. It "may" flex more if your action screw holes in the receiver are drilled out of alignment, but that is not necessarily a BAD thing! What say you? How can that be? Think about it; Would you rather have to lap-in a scope ring that is slightly out of alignment due to a base with a slight twist -- caused by the rifle manufacturer's off-axis drilling -- or would you rather your mount not flex at all? Think about this: Instead of the mount flexing, the rifle's receiver is forced to flex to meet the scope mount. This is one serious issue with the tapered base when mated to an out-of-true rifle receiver.

While you can usually expect no lapping to be necessary on a scope ring set mounted to a high quality tapered base, in actuality, that base may be distorting your action to the point of affecting accuracy via upsetting the bedding of the receiver in the stock. Bet that one blindsided you. I use both types of bases and this is the one thing that makes me choose a tapered base as a last resort and only if needed. You really can twist your receiver using a solid one piece tapered base. So, the flat base has a lot of advantages, which only fall short if you just have to reach beyond 600 to 700 yards, but do not have a scope capable of it on its own merit. Now, having said that, there is one area where a flat base does fall short. If you are one of those innumerable people who insist on buying a telescopic sight with a big headlamp of a objective lens, you may not have enough clearance for the objective bell with a flat base. You will certainly be forced to go to high rings, which have their own set of problems (loss of cheek weld primarily) just to clear your rifle barrel and objective bell. A tapered base, while it has the "bending receiver" potential, is often tall enough that you can mount that headlight of a scope on without resorting to taller rings… to a point. Keeping in mind that the tapered bases slope forward, you might just run into problems anyway. Big scopes may require tall rings no matter what. And in both cases, your cheek is forced up off the stock as a result.

The Tapered Base.

Not all is doom and gloom with a tapered base, however, I would be derelict if I did not point out the obvious short-comings upfront. Cheek weld. Anything over a 10 moa taper is going to require you to raise your head up off the stock. Most tapered bases come in 15 to 20 moa tapered with 20 moa being the norm as it appears to be most useful on the surface. I do not agree wholeheartedly with this theory, but can see why some believe it. I have two tapered bases on my current rigs. One, the IOR, is said to be a 20 moa but in actuality I believe it is 15 moa, which makes me VERY happy. The other is definitely 15moa and came from north of the border. Both are represented as 20 moa bases here in the States and in the second one's case, it probably is 20 moa. Guess it all comes down to how you run the math. At any rate, both of these bases are manageable in terms of cheek weld. But both also bring my head up off my ideal cheek weld enough that I could, if I so desired, add a bit of material to the top of my stock to compensate for this. Its borderline right now so I live with it and have not really found much reason to complain. However, I recall years ago playing with a true 20 moa base (no longer in production) that was so high that without an adjustable cheek rest you were not using the stock to support your head at all and "cheek rest" was an oxymoron. You either had to affix something to the stock as a field modified cheek rest or buy a stock with a fully adjustable rest like one of the HS Precision Tactical models. The second downside to a tapered base is the one mentioned above. It really can bend your action. I am familiar with several very high quality tapered bases (I sell the IOR and the Badger units on the PX for instance) and none of them are going to flex when you mount them to your rifle. They are very rigid and that is a selling point as well as a curse. RAM tuff you could say. Hardcore. Rock solid. Yep. All of the above. But if your rifle's receiver is drilled by a drunken sailor -- like some of the factory rifles I have seen in recent years -- you may have a problem. Now this does not mean you WILL have this problem. Check the rifle out BEFORE ordering your base. If the action is fairly true, and the scope base mounting holes are drilled along the axis of the bore and not off center, AND inline with one another, you are going to be fine. But you MUST check it first before mounting one of these kick-ass systems on your rig. You could use any one of these bases as a fighting instrument all on its own. They are that tuff. As a blunt instrument, you can do serious bodily harm to an opponent with one. So just remember that you must verify that your rifle is not ate-up right from the factory before mounting one of these things up, because the mount ain't gonna give. The rifle will.

Enough doom and gloom. There is obviously a real market for tapered bases and a real advantage to a tapered base or they would not sell so well. And that advantage is obvious. You have your rifle and scope and have been shooting it merrily for years as is. Then you discovered Sniper Country, the "sport" of tactical competition, or some other venue for long range marksmanship and got the bug to really reach out and ding steel. Hitting the longest range you can find, you suddenly find that your scope can only get you to 700 yards and dammit, that just won't do! A tapered base only cost $100 to $160 compared to that $800 piece of glass on your rifle so the answer is obvious. Replace the old base for a new one and keep the glass. Walla. Problem solved. Now you have enough moa to get to maybe 1200 yards, well beyond your ability to hit squat, but who cares? Its there if you need it! As compromises go, a tapered base offers you the ability to keep your existing glass. Glass that may have every other feature you desire. The tapered mount may give you the ability to shoot beyond any reasonable range with ANY scope combination you can think of, and it is for all intents and purposes, RAM tuff, as Dodge likes to say. I even use one for "Short" range stuff like varminting. Why? Because my varmint scope only has 36 moa TOTAL and its 1/8 moa clicks make for very short range shooting without a tapered base. Sell the scope? Not a chance. I love that thing for popping prairie dogs.

There you have it. A "short" summary of both types of mounting systems. I seem to have gone three pages over my original intent but what the heck. It was either this or wash the laundry. Think wife. Think Rolling Pin. See, I am doing it all for YOU. I hope I have given you something to think about. Both types of base are plenty good in terms of long range shooting and hardcore field work, particularly the newer M1913 spec bases so prevalent today. Both flat and tapered get the job done. You must decide which actually fits your needs based on your true use of the rifle system you are assembling. If you really are not going to be competing or shooting regularly at 1000 yards, don't sweat that your rifle can not quite reach that far with the current glass. You can always use hold overs if necessary, which when combined with a mil-dot reticle, are easily within your grasp to 1000 and beyond, at least on large targets. Forget coke cans...

...and then there is some serious self assessment to be done. Are you even capable of hitting at those ranges? Many people are not. While it is not rocket science, it still has some science to it. Even if you master the range estimation and elevation necessary, you still have to master the wind, which is the great equalizer of all long range shooters. Learning come-ups or keeping a log is simple. But wind estimation is a perishable skill so unless you really intend on keeping that skill fresh and well practiced, you may find yourself very frustrated when tossing lead waaaay down range. The point of this is that you need to self assess and base your purchasing on your actual requirements, not some Walter Mitty dream. If you are a police officer, slotted as a team sniper, who does not go to long range matches (shame on you!) you won't need a tapered base, period. Its only useful for training (again, shame on you if you are not shooting max range as it really does improve your close range ability!) and you will never use it for "real" on the job. At least not statistically. I'd rather see you with the most consistent cheek weld for obvious reasons.

If you are a civilian you also need to assess your end game. There are a lot of reasons to shoot long range. But if you never intent to or do not have the ability to, or only hunt, don't bother with a tapered base at all. If I see you shooting at a deer at 800 yards I am going to kick yer ass on principle unless I know you to be an excellent marksman AND know you are using a caliber that can cleanly drop a deer at that range. An irresponsible rump or leg hit will net you a serious head smacking and days long tracking session if I am anywhere near you when it happens. If you compete, it's a different story. But always build what you NEED as opposed to some vague belief.

So. Which to buy? ONLY YOU CAN FIGURE IT OUT! So stop asking me! Hehe.



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