Scope clearance, how low can I go?

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13 years 8 months ago #7551 by jtallen83
Scope clearance, how low can I go? was created by jtallen83
I just mounted a new Bushnell Elite 4200 2.5-10 on my AR10A4. I ended up with about 1/64 clearance between the front lens and the handguard, will this cause any problems? It feels and looks just right as far as eye releif and cheek position go. Will the distance from centerline of bore to centerline of scope change any numbers for sighting it in? I've noticed most pictures of AR-10's with similar sized scopes have them mounted much higher, I could most likely adjust to this if needed but prefer the slimer profile of the low mount. Any advice would be greatly appreciated.

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13 years 8 months ago #7555 by JustMe
Replied by JustMe on topic Re: Scope clearance, how low can I go?
The only problem I can think of for mounting your scope low is trying to clean under your bell.

The reason you see most AR scopes mounted high is because of the straight AR butt stock. Most bolt guns and shot guns have stocks that drop. Consequently, to get a comfortable alignment of your eye with an AR scope, the scope must be mounted higher just like the AR open sights/handle is mounted much higher than a bolt gun or shotgun open sight. If your low mount is comfortable to you and you don't feel like you are looking down at the scope or crunching your head/neck down to get a full scope view, then it's OK for you. Don't select a low mount because that's what you like on your bolt guns. Be open minded. Try higher mounts on your AR. You might find you'll like it better, it's much faster to get eye alignment, and much more comfortable to shoot.

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13 years 8 months ago #7556 by jtallen83
Replied by jtallen83 on topic Re: Scope clearance, how low can I go?
I think I'll try it. I've noticed there are some cheaper mounts available so I may try one and then if I like it I can invest in something more substantial. I already know what brand I'll try first, if I like the way it feels then what brands should I be looking at for tactical reliability? Thanks for the advice, Jim

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13 years 8 months ago #7557 by JustMe
Replied by JustMe on topic Re: Scope clearance, how low can I go?
I'm sure there are a lot of good mounts out there. I used to use Leopold mounts and have never had a problem with them even shooting large magnum calibers. Lately I've been using Burris mounts because they look more substantial with 3 scews instead of 2 screws. I don't have any test results showing one is better than another. Maybe you can find some comparisons on Google, etc. Some on this forum like LaRue mounts. I don't have any experience with LaRues.

After trying a couple other heights, I ended up mounting my scope very high. It's 3 inches above bore and about 1.9 inches above the picatinny rail. Much higher than I like my bolt guns which I like mounted low. I've noticed a lot of others went this high also on their AR-10s.

Good luck, Jer
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13 years 8 months ago #7563 by BUILDING MY SASS
Replied by BUILDING MY SASS on topic Re: Scope clearance, how low can I go?

jtallen83 wrote: I just mounted a new Bushnell Elite 4200 2.5-10 on my AR10A4. I ended up with about 1/64 clearance between the front lens and the handguard, will this cause any problems? It feels and looks just right as far as eye releif and cheek position go. Will the distance from centerline of bore to centerline of scope change any numbers for sighting it in? I've noticed most pictures of AR-10's with similar sized scopes have them mounted much higher, I could most likely adjust to this if needed but prefer the slimer profile of the low mount. Any advice would be greatly appreciated.


In the "IMMORTAL" words of a curtain Female Dr....
If It Feels Good...DO IT....
Or as I like to say...It is all a matter of "Personal Preference".
What feel right to you, may not to others...so go with it, if it works..For You.
As far as the NUMBERS...Once you sight it in..IT'S YOURS...Numbers Don't Matter..
As long as it is not "TOUCHING" the rail...All Is Cool...Go With it...IT IS YOUR RIFLE...AND YOU SHOOT IT ...Not anyone else...Get It... ;)
BMS

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13 years 4 months ago #8841 by txlongshotb4
Replied by txlongshotb4 on topic Re: Scope clearance, how low can I go?
jtallen,
I know I am late to this thread, but I'd like to put in my two cents.

To answer your question, yes. The height of your optic above the bore line does adversely effect your ballistic data as it applies towards the amount of elevation you will have to dial on to your optic in order to achieve point of aim/point of impact on the same vertical plane. But, to put that into practical application, it has a lot more relevance in certain situations than it does in most of the common applications.
Specific applications such as shooting short range-like close quarters combat drills-are heavily effected by line of sight/line of bore differences. An AR-15 carbine with an optic mounted, say, 1.5 inches above bore line-sighted at 100 meters- is going to hit substantially low at 15 meters when clearing a room. While I cannot give you exact deviants, the shift of impact will vary over a matter of an inch or more from 15 to 100 meters with this type of set up. Another application in which height of bore can be critical is in the practice of bench rest competition, where deviations of fractions on minutes of angle make the difference between winning and losing.
For everything else I can think of, it's a moot point. In fact, mathematics dictate that the further out you are shooting, the less this factor will effect the shot. This is a question of geometry. Imagine the target as one point of a triangle, the line of sight through your optic as another point, and the line of your bore as the third. By logic, the further the target, the smaller the deviation of the angle between bore line/ sight line. That said, the height above bore will indeed effect the amount of elevation you will need to "dial on" at given ranges, but the deviations will be both negligible and repeatable-granted you have a good solid mount and reliable optics.
HOWEVER- This only applies to the VERTICAL data-and in this case, the horizontal data might be the Achilles heel...I say this, because mounting an optic on an AR-10 as low to bore line as you can get it might produce the effect of causing the shooter to have to contort stock weld and body position into an un-natural shooting position in order to obtain a sight picture. To make a long story short, if you have to force yourself down onto the gun, you are inviting a whole array of issues that can and will force your shots to stray. A huge issue with the AR type rifles is cant...due to the largely vertical ergonomics of the platform, it is very easy to pull or push the rifle into a cant...in other words, your cross hairs are not straight up and down, they are "leaning" one way or the other. This issue creates deviations in your horizontal shot grouping, and this is directly associated with height above bore line of your optics...to boot, this effect greatly increases over range.
There are a ton of other things that could be effected by having to push yourself into a lowered shooting position due to a low scope mount like eye relief, shadowing, poor body position, body fatigue, and "turkey necking" wherein you'd have to stretch your neck out in order to find a good sight picture. Thus, it's a better deal to bring your optics up and find the spot where they fit your body and the gun feels natural...Line of sight/line of bore deviation is a lot easier to work with than a gun that is to small to fit the shooter.
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13 years 4 months ago #8852 by jtallen83
Replied by jtallen83 on topic Re: Scope clearance, how low can I go?
Thanks for the info, the clearest explanation I've seen to date! So far taller has felt better. I plan on giving it a go on the low side again but want to wait till after Christmas, maybe santa will bring me one of those fancy magpul sniper stocks. I have noticed that body position and cheek weld are a bigger issue now that age has stiffened me some, at 18 I would have fired my M-16 hanging from a tree by my legs and not given it a second thought, now I need a bigger bipod so I can shoot sitting, prone just doesn't cut it for me anymore. Thanks again for sharing your knowledge, Jim

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13 years 4 months ago - 13 years 4 months ago #8855 by AR-Freak
Replied by AR-Freak on topic Re: Scope clearance, how low can I go?
I have the same concern, the only issues ive had so far is when i tried to use a friends laser bore sight i had only like 20 yds to do it and the scope capped out on elevation, need to site in like 100-200yds. Also for lens cap covers i had to dremel a flat spot in the front cover for clearance. cheek weld is no issue.
Last edit: 13 years 4 months ago by AR-Freak.

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13 years 4 months ago - 13 years 4 months ago #8856 by BUILDING MY SASS
Replied by BUILDING MY SASS on topic Re: Scope clearance, how low can I go?
OK..in reply..I didn't realize it was a 50mm..but anyway...Personally that would be "too Low" for me...especially with the the 10...15 maybe..you might consider the ADM mounts..they are Quick Detach..hold "return to ZERO".. and are great...I have the Recon H..with the offset and height ...but the standard recon might be to your liking..
Check them out...
BMS..

www.americandefensemanufacturing.com/
Last edit: 13 years 4 months ago by BUILDING MY SASS.

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