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Winchester Scope Mount - Bases

dfletcherdfletcher Member Posts: 8,163 ✭✭✭
edited November 2014 in Ask the Experts
I got much good info regarding a Litschert SpotShot 15X scope I just bought at a gun show, it's going on a 1941 made Winchester 75 Target.

http://forums.GunBroker.com/topic.asp?TOPIC_ID=641724

A question about bases -

I happen to have the "Unertl" style bases referenced - a Lyman "B" (.225) for up front and an unmarked rear that measures .167 from base to top. Since the scope is externally adjusted I know the tube may not be parallel to the rifle barrel, but even with the rear adjustment set in the middle of the rear tower there's quite an dive - the tube angles down alot front to backt. Is this normal?

Barrel diameter at the center of the rear mount is .900 and at the center of the front mount is .850 - give or take a 1/1000 or so. That pretty well matches up with a .058 difference in base height.

Is there "flexibility" with respect to base height? I'd be inclined to center the scope tube in the rear tower, then use a rear base that brings the tube parallel to the bore. It seems to me this would give maximum possible adjustment capability, much like putting the windage turret of a modern scope in the "center clicks" & using the large rear screws to move the ring left & right in the middle Is this viable or is it a matter of "must be .167 and .225"?

If there is some flexibility and I use .175 rear/.225 front, I presume so long as I keep that .050 difference I could use .225 rear/.275 front?

Comments

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    perry shooterperry shooter Member Posts: 17,390
    edited November -1
    remember with the lyman and Unertl rear mount actually moves the rear end of the scope tube Up Down Left Right within the mountthere are two spring loaded buttons to contact the scope tube. then look at the fact the bullet is taking a path as an arc the bore is below the the centerline of the scope tube if you raise the rear of the scope it will raise the POI. I would for one Mount the scope first and then if you cant ZERO the gun decide how to go about either raising or lowering the front or rear base.
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    HerschelHerschel Member Posts: 2,035 ✭✭✭✭✭
    edited November -1
    I have and have used some of the externally adjustable target scopes. I am by no means and expert but it seems to me that a difference in the scope base heights could be adjusted with the elevation knob. The scopes I have seem to have a lot of elevation adjustment in them.
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    XXCrossXXCross Member Posts: 1,379 ✭✭✭
    edited November -1
    If this scope is going on a M-75, it should be close to parallel to the center line of the bore. With the rear adjustment centered, you
    can alter the front or rear bases to give that parallel condition.
    (change both bases if you have to) Height is usually only to accommodate the bolt handle when opening. (as long as it clears the
    scope, you're okay)
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    babunbabun Member Posts: 11,054 ✭✭✭
    edited November -1
    As XX said above.

    It is common on long range rifles to tilt the scope so you do not run of vertical adjustment. A higher rear base or a base with 10,20, 30 moa of slant built in is normal for 600 yard and more ranges.
    I image you are going to be shooting no father than 100 yards with that gun, most likely 50 foot or 50 yards.
    You will have plenty of adjustment.
    Visualize, if you can the straight "line of site" thru the scope crossing the bullet's path.
    trajectory.png

    This illustrates why it is better for hunting rifles which may take shots at all different ranges to have the scope mounted as close {low] to the bore as possible. The two bullet points of cross the site line is
    "flater" allowing less hold over or under.
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    MG1890MG1890 Member Posts: 4,649
    edited November -1
    Your mounts have a slight "muzzle high" condition built in to their heights. This is correct to compensate for trajectory. Shoot it.

    The scope base height bias is .058". Try to maintain a similar bias regardless of overall height.
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    babunbabun Member Posts: 11,054 ✭✭✭
    edited November -1
    You want the rear base higher than the front base, "pushing" the butt stock downward. See how much higher this iron micrometer rear site is than the front globe site.
    Anschutz-1907.jpg
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    dfletcherdfletcher Member Posts: 8,163 ✭✭✭
    edited November -1
    Thanks. I have other bases and will be prepared when I go to the range. Other than dumb luck at having exactly what is supposed to be used, it may be that the bases I found stashed away actually came with the rifle when I bought it some 15 years back.

    It sounds as though a certain amount of fiddling is OK, I'm glad to learn the phrase "height bias" and know there is a methodology to it.
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    richardaricharda Member Posts: 405 ✭✭✭
    edited November -1
    Or, replace FRONT base with the one lower height, the "A".
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    dfletcherdfletcher Member Posts: 8,163 ✭✭✭
    edited November -1
    All went well on the first outing. Bore sighted at 100 yds the old fashioned way and got on paper the first shot. I think I'd like to use a .175" rear base and get a bit more elevation "adjustability" but other than that, this was a heck of alot easier than I thought. Thank you to all who helped me along.
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