Re: Ring Height Info - How to Measure Correctly
Just wanted to add my method for figuring scope ring height because I didn't see it here. First let me say I typically deal with flat top stuff, so this equation: [rail height + ring height] - [bell diameter x 0.5] doesn't fully apply, and leaves something to be desired.
Also, my way is a little easier, as we typically choose a rifle, then a scope, a scope base, and then the rings to bring it all together. First you take the largest diameter (typically objective lens diameter), minus your tube diameter, and the take that number divided by 2. The reason you divide by two is simple. There is only half of that objective diameter that will be below the scope's center line. For example, on a scope with a 50mm objective with 30mm tube, it would look like:
50mm-30mm=20mm/2=10mm.
This means that you would need at minimum a 10mm ring height to clear your flat top rail. On conventional scope designs (those with an ocular bell that is slightly larger than the tube diameter, but still smaller than the objective bell), you do not have to worry about the ocular bell's clearance unless you have BUIS or fixed sights in the way. Just remember the numbers advertised are the lens measurements, and the actual outside diameter of the objective bell will be several millimeters more. To make it easy we will just use some common, even numbers, but you should have the scope in your hands, or at the very least call the manufacturer to find out true outside dimensions before ordering rings.
In the even that you are using a bolt action rifle, or some other setup where you need to take ring height and base height into account to bring the bottom of your objective bell as close to the top of the barrel as possible, this equation is helpful:
[Bell Diameter-Tube Diameter/ 2]- base height = ring height needed
We will pretend our base height is 5mm installed. So using our example again:
50mm-30mm=20mm/2=10mm - 5mm (base height) = 5mm (our minimum ring height needed is 5mm to make base and bell clear/touch), so in the real world, you would use a 1/4" or low ring and be good to go.
When figuring for fixed sights or BUIS, it's almost as easy as before, but you do not subtract base height.
Example: Figure BUIS is on the same rail as the rings, like on a flat top AR:
50mm bell
30mm tube
5mm base height
10mm BUIS height (folded)
50mm-30mm=20mm/2=10mm of bell radius below center line. Then you go:
10mm bell radius+ 10mm folded BUIS height=20mm ring needed to clear BUIS.
NOW, the only caveat to that is typically your Objective bell is not going to be sitting over your BUIS. Your ocular bell is. SO, if you are going to be fitting buis, it is important to know this dimension and figure it in when trying to clear BUIS. The easy part to that is once you know your ocular bell diameter it is the same math as figuring the difference between your tube and objective diameter, only the difference should be a lot less.
For example, let's say you want to clear the same 10mm tall BUIS with the same scope. Lets say the scope's ocular bell is 38mm. Well, that's 38mm-30mm=8mm/2=4mm, meaning your ocular bell lower radius is 4mm. If you know your buis is 10mm tall, you can simply add the two 10mm+4mm=14mm, which means with 14mm rings your BUIS and ocular bell would be touching, so you would go to a 15-16mm or 5/8" set of rings and clear just fine.
I hope this helps.
Also, I wanted to compile this for you guys really quick. Know first that one inch is equal to 25.4mm. Knowing that, you can almost do a plug and play with the above formulas as such. This is simple subtraction, but:
For 1" scope tubes:
Objective Diameters|Difference between tube and objective. I also did the division by 2 for the heck of it.
32mm|6.6mm/2=3.3mm
40mm|14.6mm/2=7.3mm
42mm|16.6mm/2=8.3mm
44mm|18.6mm/2=9.3mm
50mm|24.6mm/2=12.3mm
56mm|30.6mm/2=15.3mm
For 30mm scope tubes
32mm|2mm/2=1mm
40mm|10mm/2=5mm
42mm|12mm/2=6mm
44mm|14mm/2=7mm
50mm|20mm/2=10mm
56mm|26mm/2=13mm
I also wanted to say that there is nothing wrong with using rings that are measured in inches or mm to get the desired height, as long as you choose the proper ring/ tube diameter. As you can see above, I try to figure everything in MM as far as measurements go for easy numbers, and then pick the closest thing in ring height. A 1/4" ring is 6.35mm. A 10mm ring is close to 3/8" (.39370" to be exact), 1/2" and 13mm may as well be interchangeable, as are 5/8" to 16mm, and 19mm to 3/4". They are so close it is a wash. The nice thing in metric ring heights though, if you need say, 12mm for the barrel and objective to touch, and don't want to jump clear to 5/8"\16mm or 3/4"\19mm, you can usually find rings that are just a few mm taller than touching and be good to go. Just make sure you allow enough clearance for rifle flex during recoil.
