Help figuring MOA of a rail base

Discussion in 'Gunsmithing' started by Broz, Jan 5, 2010.

  1. Broz

    Broz Well-Known Member

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    Working on a 700 LA Rem. I have a picatini rail base and I am not sure of the Manufacturer. All it says is [ REMINGTON 700 LONG ACTION 35 MOA]

    I used this base with my IOR Tactical 6x24 and should have been close to bottom of adjustment @ 200. I was closer to the middle like a 0 moa base.

    At the back screw of the base it is .430" thick. At the front screw, roughly 5.950" forward, it is .305" thick. Now I know the height of the receiver comes into play. But I am trying to figure where I lost 30 to 40 moa. Oh, rings are a set of 35mm IOR's and I have measured them and they are of equal height.

    Thanks for any help.

    Jeff
     
  2. geargrinder

    geargrinder Well-Known Member

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    Are you sure you're at the middle of the scopes travel? Have you counted it from end to end?

    To calc the moa of the base you'll need one more piece of information. The height difference between the front and rear bridges of the action.

    (inverse tan((.430-Height)-.305)/5.950)/60=base moa
     

  3. Broz

    Broz Well-Known Member

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    Yes, I have 65 moa total in the scope, and I have counted them. From 100 yard zero I topped out at 27 moa.

    I guess I could measure the action weights from a cleaning rod, but fear it would not be accurate enough. I figured since a 700 action is so widely used some one would have these numbers. Or, if someone had a similar 35 or 40 moa base to measure and compair or even a zero cant base to measure.

    Thanks.

    Jeff
     
  4. Broz

    Broz Well-Known Member

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    OK, the height difference front to back is roughly .110" My calculator does not have inverse Tangent...

    I am searching thr net but no luck so far.

    Jeff
     
  5. Broz

    Broz Well-Known Member

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    Well, I never did get the math figured out, with out a calculator I am done. But unless I am wrong I found the problem. I put this base on the action. Then measured from the top of the bolt and also from the action rails (same thing) to the top of this base both front and back. She is parallel to the bore,,,:rolleyes: Yes, stamped 35 MOA but in reality it is 0 moa. Am I just a lucky sob or what??? :D

    [​IMG]


    [​IMG]
     
  6. geargrinder

    geargrinder Well-Known Member

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    Hmmm...that number doesn't add up. It should be some around .06".

    that and my last number should be *6 not /6.

    It's also called arc tan.



    That definitely looks like a heavily tapered base. Looks like an EGW.
     
  7. Broz

    Broz Well-Known Member

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    Well, am I missing something here? If I mount the base and measure from the bolt or action rails up to the top of the base. Souldn't it be lower at the front of the base? I did this several times and it always came up with in a few thou of being parallel to the bolt.

    Jeff
     
  8. geargrinder

    geargrinder Well-Known Member

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    Everything that you are saying sound like it is a 0moa.

    The reason I say it looks tapered is that the base at the red circles look thicker than were the blue circles are at. Could just be a trick of the shadows.

    [​IMG]
     
  9. NesikaChad

    NesikaChad Well-Known Member

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    Here's what I did, check my "math" to see if I'm right.

    I drew a 3600" long line in CAD. I then put a series of arcs at the end of the line. 10, 20, 30, 40 inches in diameter.

    I then drew tangent lines from the arcs to my start point.

    According to my other notes the scope screw hole length on a Std length Remmy 700 is 5.108" center to center. So I put up two vertical lines that bisect all the others spaced this far apart.

    If this was all done right (and I'm thinking it's accurate) my software says there is a height difference of .0071" for every 5 minutes of angle for every 5.108" of distance. (I drew my arcs on center with my original line so you have to take the radius for the MOA change) This works out to .0014" taper per inch for 5 MOA. Multiply by 2 to get 10 minutes, etc. . .

    Assuming I'm not smoking Hoppes (again) all ya got a do is measure the difference in height from front to rear, subtract to get your value, and then measure the OAL of the base and divide again to get your taper per inch. It will hopefully come out very close to these numbers so that I don't look like a schmuck!

    5MOA: .0014"tpi
    10MOA: .0028"tpi
    15MOA: .0042"tpi
    20MOA: .0056"tpi
    etc. . .


    If I'm missing something feel free to crack my skull but I think I got this right. My trig sucks these days as I've gotten so dependent upon software.

    Hope this helped.

    Chad
     
  10. Broz

    Broz Well-Known Member

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    Laying the base on a flat surface and measuring down. The difference between front and back is .110" This is the same number I got when I measured from the top of bolt to top of receiver front and back. The base is roughly 7.120" long.

    Chad, does your calculation take into consideration the 700 receiver is .110 higher at the front mount pad than the back?

    Jeff
     
  11. Broz

    Broz Well-Known Member

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    You are correct, there is a 10 to 12 thousands taper in the length of each mount pad.
     
  12. NesikaChad

    NesikaChad Well-Known Member

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    Ya' got me! I hadn't thought of the changes in height from the receiver ring to the rear bridge.

    Dern it.

    I think the math is the same though, one just needs to know the differences in height. It can vary a bit, but nominally it's .125". I've measured some though that were quite different and this would have a significant influence.


    One way around this would be to stick the bolt in and measure from the top of the bolt to the top of the base in the rear. A guy could calculate from bore center this way. The front would be easy, just add the ring OD and split it.
     
  13. Broz

    Broz Well-Known Member

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    Guys I appreciate all the help. This has been driving me nuts. When removed the base appears and measures to have decline cant. But when bolted to the receiver it does not. Well here is another pic with the base tightened down. It seems the geometry of the base does not match the receiver. When you are snugging down the screws you can actually see the base change shape and the front of the base changes angles. An arch is formed in the rail. After tightened I put a straight edge on top the rail and it is way crooked. The straight edge shows the front of the base is high in the screw area and this is taking away the cant that it had. You can even see this in the pic. I am wrighting it off to a cheap base and will buy a good one. I wish I knew who manufactured it so I could avoid them in the future.

    Thanks!

    Jeff

    [​IMG]
     
  14. Eaglet

    Eaglet Well-Known Member

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    Base on the above information, your base would have 53 MOA.

    Now, using your last picture posted, I measured and came up with 56 MOA, that of course can only be kind of close. From 35 MOA to 53 there's a long ways... something isn't right.

    Check the base on a real flat surface and make sure it's strait, all sides, if it is give precise dimensions from bottom of MOA to top of "Groove" for both ends and the horizontal distance between the points measured. Just like you've been doing.

    I was having similar problems with my pica-tinny base and had to glass bed it, it was
    king of turning to the right as it was moving from rear to front.

    Check if you can, to see if the back of the action, where the base goes is flat.

    Also, tighten the bolts on one end only and see if the base is lifted, if not the take the bolts our of the one end you had tighten and put then on the other end. if there's room under neath you'll have to glass bed it. Let us know.