malcolm scope mount spacing

[appleseedgunsmith]

Why do places advise using 17" for spacing the mount blocks on a malcolm scope? This doesnt correspond to the unertl chart. 18" would seem better. I know how fussy some people get when it comes to mounting 7.2" vs. 7.25". Ive read this in at least 4 places, two of which are manufacturers. What am I missing?

[Don McDowell]

Mostly depends on how many minutes adjustment you want out of your mounts, and the length of the scope.

[appleseedgunsmith]

But why 17" instead of 18"?
3600/17= 211.8(1/211.8= .0047 inch /moa)
3600/18= 200 (1/200= .005 inch/moa)

[Don McDowell]

Well put it the distance you want. When you can't get enough elevation to reach the furthest target, you'll soon figure out how valuable the .003 was..

[appleseedgunsmith]

Its not that I dont have adequate elevation. Im wondering about perfect divisions.
The unertl spacings:
5.4
7.2
9.0
10.8
12.6
14.4
16.2
18.0
Please advise.....what gives?

[appleseedgunsmith]

Got the answer. You cant use 3600 for accuracy. The formula is a trig problem:
elev= tan (1/60)× spacing
the unertl chart appears based on the rounded figure ( 1/60*60)

[Doc Highwall]

The movement of the shot is dependent upon both the spacing of the mounts and by the thread pitch of the sight itself. The Unertl click value is .0005" per click.

For example my Warner sight uses a 40 thread pitch so 1 divided by 40 = .025" per turn, and the sight has 12 clicks per turn, so .025" divided by 12 = .002083333" movement per click. On my Anschutz small-bore prone rifle with the bloop tube the front and rear sights are 37.5" apart, so 3600" in a 100 yards divided by 37.5" = a 1:96 ratio, now take the 96 ratio times the click value of .00208333 = .200" per click at 100 yards.

[Doc Highwall]

You can also figure it out a different way, say you want .250" click values at 100 yards. Example take the Warner sight again with the 40 threads per inch and 12 clicks per revolution that equal .002083333" per click. Now take the click value you want of .250" per click and divide it by .002083333" and you get a ratio of 1:120, now divide 3600" in a 100 yards by 120 and you get 30". So now with your front and rear sights 30" apart you will now have your desired .250" per click .value

[Lead pot]

Wow Doc. :) I'm a little math challenged and my Unertl mounts don't have clicks now what do I do? :)

[Doc Highwall]

That's the trouble when you don't have clicks. I have one of the Malcolm in 3X mounted on a 1897 Marlin Cowboy and there are reference marks on the adjustments, but without knowing the thread pitch I do not know what the adjustments are. If I knew for example they were a 32 thread pitch with 8 divisions I could say each reference mark was .00390625" or rounded to .004"

The threads I suspect would be metric, but with the sloppy machining it would be hard to get a precise measurement.

[Doc Highwall]

Lead pot, even though you do not have clicks, you do have reference marks to use and record your sight settings.

[country gent]

Starret and several others makes thread pitch gages for determining the pitch. leaves with a thread down one side you match into thread with good light behind them and then read the leaves ID and you know what pitch. They come in Metric, Acme and standard National fine coarse sets amybe others. Draw back is you have to be able to get to the thread meaning the mount might have to be disassembled. I have the MVA malcomn scopes with long range creedmoor mounts on 17" spacings. These mounts dont have clicks either.

[Lead pot]

Doc.

I put a DZ scope on my .44-75 mounted at 7.2” spacing and this gives me 1/2 minute per line (.001") and this is right on so far at the distance I have shot at using this scope.

Kurt

[appleseedgunsmith]

Doc, if you want to find minutes instead of inches, it turns out at 37.5 in spacing that your adjustments are more like .191 moa per click. Please check my math on that.
Tan (1/60)*37.5 =.011
.011/.0021=5.24
1/5.24= .191moa per click

[Doc Highwall]

Lead pot, it sounds like you have a 40 thread pitch of .025" travel per turn.

appleseedgunsmith, the reason I did this in inches is because the 300 meter target that is reduced for 100 yard small-bore shooting has .600" increment scoring rings and makes wind corrections easier. This target is the A-33 which appears the same as the 50 yard A-51, and 50 foot A-36 to the naked eye and the scoring rings are in proportion to one another.

Depending what gun I am shooting and at what target, I have to think in both minutes of angle or inches.
My scope I have mounted on my Anschutz is a Weaver T-36 with 1/8th inch clicks, but I also shoot a M1A that has 1/2 minute sights and AR that has 1/4 minute sights. As long as I keep good notes I can stay out of trouble, never underestimate the importance of good record keeping....YOU WILL FORGET.

[appleseedgunsmith]

Unertl block spacing chart....updated for MOA elevation
feel free to check my math on this...:

5.16" = .0015" per moa
6.88" = .002" per moa
8.60" = .0025" per moa
10.32" = .003" per moa
12.04" = .0035" per moa
13.76" = .004" per moa
15.47" = .0045" per moa
17.19" = .005" per moa

[Doc Highwall]

This is the math used to find a minute of angle.

2 × π × Rad ÷ (360 x 60) = 1.047197551” rounds up to 1.0472” @ 100 yards
(The 360 is the number of degrees in a circle, and the 60 is the number of minutes in 1 degree)

1.047197551” ÷ 4 = .261799388 rounds up to .262”
So you can see that there is only .012” difference between ¼” and ¼ minute of angle at 100 yards.