Part 4: Sloped Scope Bases & Mounts

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Rifle Sighting Systems: Sloped Scope Bases & Mounts
Written By - Greg Dykstra, 3/3/2015

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You have likely heard a reference to people talking about a "20MOA rail" or a sloped base at some point if you've spent any time around forums or other shooters.  The question comes up very frequently and there are a multitude of answers given.  However, most often no one takes the time to explain to the user how to choose the correct slope for themselves.  In this article we will describe exactly what a sloped scope base or mount can do for us, and how we can choose the correct amount of slope for our given rifle scope and application.  Next time someone asks this question on a forum, you can just direct them to this article! 

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So why do you need a sloped scope base in the first place? The short answer is that without a sloped base, you may run out of elevation in your scope before you reach the distance you are trying to shoot. This is especially true when shooting cartridges such as a .223rem or 308win at distances approaching 1000yds. Both of those cartridges have a pretty unimpressive trajectory to the 1000yd mark, and require quite a bit of elevation in the rifle to get there. A 308win running 175SMK's at 2700fps will require about 10 mils to get there, with the average being between 10 and 11 mils for most shooters rifles. If you shrink the barrel down from 26" to 18" or so, the velocity will be down around 2550fps instead of 2700, and now you'll be needing 12+ mils to get there. Considering this, if you have a scope which has 18 mils (63 moa or minutes of angle) of total travel available, with a flat base that has no slope, you'll only have approximately 9 mils of elevation available from your 100yd zero, and you won't be able to get to 1000yds with your turret. Lets examine how I arrived at these numbers.  

The first thing that must be identified before you choose how much slope you put in your mount or scope base, if any, is how much internal elevation travel your optic has. This is something that nearly all optic manufacturers will list on their product specifications page on their website. The second thing that must be considered is what the trajectory of your particular cartridge is going to look like at the distances you intend on shooting. So lets say that we are going to choose a Tangent Theta TT315M which has a total available elevation travel of 18 mils. That optic is going to be put on an 18" 308. I input all of the information about that rifle and the ammo I intend to shoot into a ballistic calculator in my phone such as Shooter or Applied Ballistics, and generate a drop chart out to 1000yds, which is the maximum distance that I intend to shoot the rifle. Based on the rifle data, ammo data, and environmental conditions I input, Shooter calculated a 1000yd firing solution of 12 mils. Now I know how much travel is available in my chosen optic, and how much of that travel is going to be used getting to the range that I intend on shooting with that specific rifle and cartridge.  

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Have  a look at the graphic above and  you'll see that with no slope, the mechanical center line of the optic is directly parallel to the center line of the rifle's bore.  Obviously if we zeroed the rifle at 100yds, the blue line would be angled down slightly to intersect the red line at 100yds.  Clearly the straight line of the bore does not reflect any bullet drop either, so don't read into the imagery too much.  For the purposes of this article, we will assume that a 100yd zero has been achieved with the optics crosshair centered perfectly in the middle of it's available travel, with the optic perfectly parallel to the bore.  In this condition, the erector assembly on the optic is centered within its available travel.  So if the optic has a total of 18 mils of available adjustment, the erector is sitting with 9 mils "up" available, and 9 mils "down" available.  We determined earlier that I was going to need 12 mils of adjustment to get from my 100yd zero to 1000yds with this particular rifle, so the available come up of 9 mils will not be adequate to get me there.  Notice all that excess travel in the bottom half of the scope's adjustment that I'm not using in this example?  The 9 mils beneath my 100yd zero is being completely wasted.   

If we angle the scope body in relation to the bore, the erector assembly in the scope can be re-adjusted to take advantage of that wasted travel.  Next we can move on to determining just how much slope we need to get to our desired distance via adjustments in our turret. Through reading the previous parts of this series on rifle sighting systems, you know that 1 mil is equal to 3.5 moa.  Most sloped scope bases and mounts are sold with a angle of slope indicated in moa, with 20, 30, and 40moa being the most common.  If your scope is delineated in mils, you'll need to do that conversion to see just how much travel you will pick up.  Lets consider 20moa first.  20moa is equal to  roughly 5.7 mils.  So if we angle the scope body 5.7 mils, we can add 5.7 mils to the 9 mils that we originally had available to come up from our 100yd zero.  This gives us 14.7 mils of adjustment up from our 100yd zero.  We only needed 12 to accomplish our goal, so a 20moa rail will do the job nicely.  What if we wanted to have more adjustment available?  If we consider 30moa sloped mount, that will give us 8.5 mils of additional travel on top of the 9 we had originally with a flat mount, for a total of 17.5 mils.  If you recall, the scope only has 18 mils of travel available, so we've maxed out our theoretical limit of available travel with almost no room to spare.  

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Is there such a thing as too much slope?  In the image above, you'll see the effect of adding a 40moa sloped mount or base to your optic.  You'll notice that by adding 40MOA of slope, which is equal to 11.4 mils, we sent our zero point 40moa beneath our 100yd zero.  So if we added a 11.4 mils of slope, we would need to come down 11.4 mils on our turret in order to maintain our 100yd zero.  With the scope we have been using in our examples here, that would be impossible.  We established that with this scope we had a total of 18 mils available, and with no slope, that gave us 9 up and 9 down.  If we add 11.4 mils to 9 mils, we get a total of 20.4 mils, which is more travel than this scope has available.  In this instance, we would not be able to achieve a 100yd zero.  We could run the scope's turret all the way down to the very bottom of its travel, and our rounds would still be hitting 2.4 mils high at 100yds.  This is not important if you are going to zero the rifle at a farther distance, but would be detrimental if you intended to run a 100yd zero as most do.  

In the above example, the scope's adjustment range was prohibiting us from using a 40moa base.  If we intend to be able to get more distance out of that particular rifle with the 18" 308 barrel, we will need to switch to a scope which has more internal adjustment.  Any optic that has at least 22 mils (77moa) or so of total elevation travel would be suitable for use with a 40moa sloped base or mount and still be able to achieve a 100yd zero.  Obviously there are exceptions to every rule, and it will be reliant upon your dealer to make appropriate recommendations regarding what slope to choose for the optic you are considering.  Nothing trumps first hand experience, and if you are asking us about an optic we sell, I guarantee you we'll have the right answer for you!  

Too much slope can be just as bad or worse than not having enough.  Just because an optic can adjust far enough to give you a 100yd zero with a 40moa or steeper sloped mount, doesn't mean it will function properly at the extremities of its adjustment range.  The farther off from center you get the erector lens assembly which contains your reticle and several lenses, the higher probability that you'll have mechanical and optical issues with the scope.  This can come in the form of something rather insignificant like a little bit of shadowing or shading around the outer perimeter of the viewable image through the scope.  It can also come in the form of something catastrophic as complete internal failure or refusal to maintain zero.  Every scope will be different in this regard, but as a rule, the farther away you get from center, the worse performance will get.  Again, there is no substitute for first hand experience when considering these types of issues.  

Another issue that can surface is the inability to achieve a good cheek weld.   As you tilt the scope body forward, the rear ocular assembly gets lifted up.  This means that to look through the optic, you'll need to have your head higher on the rifle than if it had a flat mount with no slope.  With the modern stocks that have adjustable cheek rests built into them, this is less of an issue now than ever before, but it is still noteworthy.   

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