I've never seen a...

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With the spring I'm guessing that it's some sort of shock absorbing mounting system?
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The scope adjustments were external. Those scopes were very good scopes for the age they were manufactured in. I would love to have one.
 
Lyman, Fecker, Litchert and Di Simone all made external adjusted scopes along with Unertl. The best ones made optically were Unertl and Di Simone. Lymans were a close second.

When real good match bullets were finally available in the 1960's, 'twas learned that those scopes' front base on the barrel degraded accuracy a little bit. Internally adjusted scopes mounted on receiver bases were these old scope's death knell.

That spring on the tube was used to push the scope backwards into "battery" after it slid forward as the rifle recoiled under it. Most folks used the spring with smallbore rifles but that spring was removed on high power rifles. Eye relief was only about 2 inches. It slid forward from recoil then was pulled back and turned one way so it returned to battery perfectly. With a 7.2" mount spacing, each click moved the scope .0005" to make a 1/4 MOA change.
 
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Those were popular for some log range shooters, and the Marines used those (or similar unertyls) on Winchester model 70 sniper rifles through Korea and VN.
 
Some folks shortened base spacing to 6.8755" so adjustments would be in trigonometric MOA units; 1.0472... inch per hundred yards.

They couldn't adapt to the decades old standard of one inch per hundred yards.
 
Bart- shooter's MOA (SMOA) vs true MOA (TMOA) results in less than 1 MOA deviation at 800 yards (or further- can't remember)
 
Or space the bases to suit your taste in adjustments:
5.4" = 1/3" Per Click.
7.2" = 1/4" Per Click.
9.0" = 1/5" Per Click.
10.8" = 1/6" Per Click.
12.6" = 1/7" Per Click.
14.4" = 1/8" Per Click.

I don't know why anybody would want a 1/7 minute adjustment, but the others make sense for some uses.
 
Bart- shooter's MOA (SMOA) vs true MOA (TMOA) results in less than 1 MOA deviation at 800 yards (or further- can't remember)
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In MOA the error won't change since the angle is the same regardless of distance. So it will make a deviation of less than 0.05 MOA, regardless of the distance. It's an error of about -4.5%

It's exactly the same error as if you approximate the value of pi (3.1415926535897932384626433832795028841971693993751058209....) as 3

At 1000 yards, it will make a little under half an inch of difference--less than 0.05MOA.
 
Both the English speaking countries and European ones first based their rifle sight adjustments based on ranges used and target scoring ring sizes; inches per hundred yards or centimeters per hundred meters. That made it easy for every one to use and make sights in their local environments.

Then someone decided that was trigonometrically wrong. It now has an antagonizing history. A 5% approximate error between shooter's MOA and the trigonometric one was/is too much for some people to deal with.
 
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Then someone decided that was trigonometrically wrong.
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There's nothing trigonometrically wrong (or wrong at all, for that matter) with basing sight adjustments and target measurements on inches.

It is, however, incorrect to state that 1 MOA is equal to 1" at 100 yards. It's only approximately equal to 1". It's incorrect in exactly the same way it would be incorrect to claim that 1" is equal to 1.047198... It's incorrect in more or less the same way as it would be incorrect to say that 100 yards is equal to 100 meters--they're pretty close to each other--but not equal.

1MOA is simply not the same thing as 1" at 100 yards. It's reasonable to say that the two things are close enough for practical purposes that it's acceptable to approximate 1MOA as 1" in most cases. It would even be reasonable to use that approximation as a standard in the shooting sports--although I know of no official statement of such a standard.

It's not that any one suddenly decided something about minutes of angle or inches. An MOA (minute of angle) has a definition that was established long before someone decided to express shot groups in terms of minutes of angle. Just as the inch measurement had a definition before the first European shooter ever measured a shot group in inches.

So sure, it's fine to approximate 1 MOA as 1" at 100 yards for practical purposes. Just as it's fine to approximate pi as 3 for some applications--in fact, the two approximations are identical. Just as it's fine, in many cases, to approximate 100 yards as 100 meters. But that doesn't mean pi IS actually 3, or that 100 yards is actually equal to 100 meters, and it doesn't mean that 1 MOA is actually 1" at 100 yards. It's just close enough that many folks don't care about the difference.
 
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Prior to the M1903 and it’s .30 caliber cartridges with 172-gr machine gun (match) bullets, popular rear sights were graduated in “points.” At the standard range of 200 yards; the range most matches were shot at, a point was 12 inches across the target for the Krag rifle’s sights and 8 inches for the M1903. The sights were moved in fractional points; ½, 1/3 1/4 and even 1/5 point depending on the shooter's needs.

https://books.google.com/books?id=7...epage&q=1/2 point 12 inches 200 yards&f=false

When Lyman and others made receiver sights for the M1903 and more accurate ammo with the 172-gr. bullet was possible, the leade screws on those sights had the same pitch as those on scope sight external mounts; 40 tpi. That's the standard followed by virtually makers of aperture sights used on high power match rifles. With the typical sight radius at 30 inches used, 1/3 turn comprising 4 clicks moved the aperture .008333 inch which moved bullet impact 1 inch at 100 yards. One click moved 1/4 MOA or 1/4 inch per hundred yards. Of course, with longer sight radiuses on longer barrels, it's going to move less per click.

The National Shooting Sports Foundation standard is 1 MOA moves shots 1 inch for every hundred yards of range:

http://www.nssfblog.com/firstshotsnews/terms-to-know-minute-of-angle/
 
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And don't get started on the current fad of mil scope adjustments and reticle features. There is the infantry mil and the artillery mil and the mil as used in various foreign lands. Hardly anybody uses the geometric milliradian, 1/1000 of a radian where 2 Pi radians = a revolution.
 
The National Shooting Sports Foundation standard is 1 MOA moves shots 1 inch for every hundred yards of range:
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From the provided link:

"When discussing MOA, we typically depict 1 MOA as an “impact difference” of one inch at 100 yards. To be really specific, the actual measurement is 1.047 inches, but at 100 yards the .047-inch is so minimal we simply round it off to one inch for practical use."​

The video in the link is also pretty good.
 
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