uscra112 wrote on Jul 31^st, 2015 at 1:42pm:


FWIW, based on my research into vintage scope, I definitely agree

Wayne

When considering a vintage or classic style externally adjustable scope there are several considerations. Longer scopes require wider mount spacing for adequate support.  For externally adjustable scopes the total adjustment range is in inverse proportion to the spacing between the front and rear mounts. In other words, as the mount spacing is increased the overall adjustable range is reduced but the adjustment resolution is increased. E.g. doubling the spacing decreases the total adjustable range by a factor of 2, but increases the adjustment resolution by a factor of 2, a tradeoff that must be considered when determining the length of the scope and mount spacing. 

Therefore, prior to purchasing and installing a Malcolm-style scope and mounts, it’s very important to consider several factors.
1. The length of the scope. A longer scope generally requires a wider mount spacing to properly support it, which, as noted, reduces the total adjustment range.  But a shorter scope can run into problems if the shooting range is extensive.  For example, a 23” or shorter scope set up to handle Creedmoor competition out to 1000 yards will likely not work using normal height scope blocks. Montana Vintage Arms’ website indicates that their long range Creedmoor Mount is not recommended for their 23” scope on a 30” or longer barrel. Further stating that at the adjustment top of travel the scope will be pointed at the barrel, thus obstructing the view. The combination of the short length and the scope tilt necessary for 1000 yd shooting would result in the scope image being partially or completely blocked by the front portion of the barrel. This can be corrected by using higher front and rear scope blocks.
2. The maximum amount of bullet drop expected over the shortest to longest target range should be less than the rear mount total adjustable range.
3. The adjustment resolution of the rear mount as designed by the manufacturer.
4. Finally, the best mount spacing considering the above factors.

The simple formula is mount spacing in inches = rear mount total adjustable range in inches X 3600 ÷ maximum expected bullet drop in MOA. Be sure to build some margin into the calculation. For example, if the maximum expected bullet drop is 60 MOA from 200 meters to 500 meters, set the mount spacing to handle a bullet drop of 80 MOA. Then use the correct scope blocks heights to “zero” the scope in at 200 meters with the rear mount set at 10 MOA. Easier said than done.

Wayne

So on a 1,000 yard rifle, rear base height compared to front base height and distance between centers of bases are the two most important factors for mounting a scope with external adjustment mounts.  Plus the overall height of the axis of the scope over the axis of the bore would be another factor. 

Val, the MVA mounts have a lot of adjustment.  For 500m I use only a small amount of the vertical adjustment range on my 23" scope for my .40-65 and .45-70.  1000 yards would not be a problem as far as the scope is concerned.  With a 30" barrel the scope won't be pointing at the end of the barrel so it's good in that respect too. 

Chris.

What distance on centers are the bases for your 28" scope?

I know what you mean.  A lot of the repro stuff is high quality and in the right spirit, but there is something special about original rifles and equipment.

Chris.