Posted: 8/14/2017 9:21:55 AM EDT
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I don't know why this finally occurred to me after more than half a century of shooting but here goes.
When I mount a scope on a bolt gun I generally use mounts which allow for gross windage adjustment by means of opposing screws on the back of the mount. I make sure the reticle is centered horizontally (generally by the counting clicks method), mount the scope and adjust the mount until the point of impact is centered windage wise. I then lock down the mount and make any further fine adjustments with the scope. With a MSR or other rail system I purchase a good scope mount and hope for the best. So far I have had decent luck. As to the elevation... I adjust it for the desired point of impact and had never given it any more thought. Until yesterday... I have a new Leupold VX6 7-42X - 56 mm scope which Leupold sent me in exchange for (originally) two FX target scopes which had a design deficiency which they have been unable to resolve. That was the story of another thread. The ultimate home of this scope will be a Fulton Armory FAR-308 PVR-L rifle. In the interim I mounted the scope (using a Mk 6 IMS also provided by Leupold) on a .223 AR platform rifle so that I could get some experience with the scope using more economical ammo and to do some development on my home made "ergonomic-ized" A2 stock. I also want to see what the 16" Lothar Walther barrel on the rifle is capable of. It normally sports a 1.5 4X mark AR scope. Not exactly the thing for shooting groups. To continue... I found that the elevation adjustment on the MK6 was turned all the way down as it came from the factory. It has a micrometer type scale ranging from 0 - 570 which represents 570 1/8 minute clicks or a little over 71 MOA of elevation adjustment. Interestingly the Leupold web site says the scope has 55 MOA of elevation adjustment - I will have to ask my contact which is true. With the scope regulated to 100 yds I find that the elevation is at 367 or about 10 MOA above half way. the Mk AR was also set somewhat above mid way as far as elevation. Which got me wondering... When I set an A2 iron sight I start with the rear sight a few clicks off bottom the adjust the front sight to give me a 6 O'clock hold on a 5" bull at 100 yds. This allows me to back down to point of impact = point of aim and/or to adjust for ammunition variances. It is the UP adjustment which will be most useful when shooting from a longer range. Except for the two situations described I would not go BELOW my starting point and certainly not by much. All of which makes me wonder... Why would I not want my scope similarly set. With a LITTLE downward adjustment and MOST of the available adjustment UP? TIA, Ken p.s. At the moment my range is limited to ~300 yards so this is really an academic/theoretical question. Still, my inquiring mind would like to know 
p.p.s. An update on my math  It was late last evening when I was studying the MK6. Actually the micrometer readings are base 8. 1 = 8 clicks, not 10. So the max of 57 is nominally 57 minutes of adjustment - closer to the 55 listed on the Leupold site. That said - the question stands. What is the benefit of giving up half of my potential elevation adjustment?
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This is one of the major reasons for the popularity of canted mounts, to get the closest zero off the bottom of the adjustment range, and closer to a useful range of windage adjustment.
Your quick fix is Burris Signature Rings that use nylon bushings to adjust elevation. The also protect the scope finish and can be used to tweak windage, although that would be a headache to set up. I don't know how far you plan to shoot, but you probably bought the wrong scope, i.e., one with eighth minute clicks. 1/8th minute clicks are not "base 8", and you will hunt a long time for a scope that uses 1/10th minute clicks; I've never seen one and there isn't enough difference to matter. I would challenge anyone to hold off 0.025 inches at 100 yards. What is desirable is a scope with 10 minute per revolution adjustments, not some awful odd number. http://www.burrisoptics.com/mounting-systems/rings/xtr-signature-rings 
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Thanks AeroE,
I am aware of the canted bases. The MK6 IMS is I believe available in a 20 minute slope. For my needs I do not think this is necessary. Unless I find a much longer place to shoot. As it is I will just use smaller targets
I actually did not buy the scope. I had purchased two FX3 fixed power target scopes. Like most Leupold scopes these days they use a multi-start thread on the eye shell. It seems that AR platform rifles produce a vibration which resonates with the eye shell and its lock ring. And they come loose. I have two .223 rifles with free float pencil barrels which will unlock the eye shell in 5 shots - consistently - no matter how tight the eye shell is locked against the lock ring (short of using a couple of pipe wrenches.) These scopes went back to Leupold several times and were exchanged for two VX3i scopes. Same problem. They offered me two VX6 3-18X scopes which have a different ocular adjustment mechanism. I had no use for the lower powers in these scopes, nor did I like some of the bells and whistles they came with. I did some negotiating and ended up with one top-of-the-heap VX6 competition scope. The 1/8 minute elevation adjustment is by design. The windage is 1/4 minute. Both at the request of Class F shooters I am told. For an initial investment of about a grand and a couple of years of aggravation I now have a $2700 MSRP scope and a $460 MSRP mount 
As to "base 8" 1, 2, 3, 4, 5, 6, 7, 10 is the way the scale on the adjustment goes. To quote the signature line of a frequent contributor to a computer forum I frequent "there are 10 kinds of people - those who inderstand binary and those who do not" 
Thanks samuse, As I said above, I am familiar with those bases. I do like the looks of the rifle in your sig. Just needs some triangular handguards! Ken |
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Thanks again AeroE,
I do not think I will change the mount. As I mentioned I only have a 300 yard range. Once I get used to the scope and have done enough testing with the .223 I will install the scope on the Fulton .308 which is its intended home. My early indications are that the rifle is capable of some fine accuracy. Last fall when I finally discovered the shaking loose issue with the FX3 scopes I installed my ancient VariX 6.5 - 20 EFR scope. I managed 7 of 10 shots in a hole 3/8" max center to center at 100 yds. The other 3 were outside that hole - no doubt due to operator needed more practice. And that was with Hornady 150 grain Ball bullets loaded on my Dillon 550. Not match bullets and not really meticulously loaded. My goal is to be able to produce 1" groups at 300. Then I will be happy. Back to the original question... Almost if not every other scope I have ever purchased has come from the factory with the windage and elevation roughly centered. I never had one where I had to crank the elevation up and up and up to get to the paper. So I guess I was looking for an opinion (or factual representation) that a scope is more accurate with the reticle centered in the field of view. I have no idea if there is any merit to that idea. Considering the quality of the glass and the SIZE of this scope I would not expect a few milimeters of movement of the reticle would make any difference. Anyhow, I will ask my Leupold contact next time I speak with him. He has talked about sending me back the original FX3-12X to test a new locking ring which he and an engineer have designed. It is supposed to prevent the vibrations of an MSR from shaking the eye shell loose. Ken |