20 moa bases
- Thread starter maka
- Start date
Scharfschütze
Regular
- Location
- Courtenay B.C.
The base has a 20moa taper. Its kinda like putting shims at the back of a base. It allows you to get more adjustment out of your scope for long range shots.
A scenario I ran into was shooting 800 yards and my scope ran out of elevation clicks. I had topped out. a 20 MOA base is like a wedge shape....which is a gross overstatement, but I would have enough elevation in my scope because the base provides me with an automatic 20MOA, thus leaving me the rest of my scopes adjustments to use.
Only problem you could run into is useing really tall rings and a 20 moa base, and you won't be able to zero your rifle at closer ranges.....100 yards.
Only problem you could run into is useing really tall rings and a 20 moa base, and you won't be able to zero your rifle at closer ranges.....100 yards.
without long range bases you are wasting all that down elevation that you wont really use as you will barely ever click below your zero range, so a 20 MOA base lets you make much better use of your turret click range for long range shots rather than have your zero somewhere in the middle.
Great article by Scott Powers at Sniper Country answers pretty much everything
http://www.snipercountry.com/Articles/ScopeMountSelection.asp
Great article by Scott Powers at Sniper Country answers pretty much everything
http://www.snipercountry.com/Articles/ScopeMountSelection.asp
Anything that attaches to the receiver and isn't straight can distort it. It is just a thin metal tube, really. - dan
This is from a navigation background, but I think it still applies :
360 degrees in a circle, 60 minutes in every degree, 60 seconds in every minute.
So a 20 MOA canted rail will give you 20/60 of a degree or 0.3 degrees. So the rail is wedge shaped, slightly. At first glance you probably wouldn't be able to tell the difference.
360 degrees in a circle, 60 minutes in every degree, 60 seconds in every minute.
So a 20 MOA canted rail will give you 20/60 of a degree or 0.3 degrees. So the rail is wedge shaped, slightly. At first glance you probably wouldn't be able to tell the difference.
). I keep reading the benefits of the single rail and thinking they are the best option.
- Location
- Somewhere on the Hudson Bay Coast
This is where the position of the scope on a 20 MOA base can be tweaked a little to enable the shooter to have a reasonable short range zero and still have the full use of his scope's elevation adjustments. The rifle can be roughly zeroed at 100 yards by bottoming out the scope adjustment then moving the position of the rear ring on the base. This is where many people misunderstand how the 20 MOA base works, confusing the angle of the base with the height of the scope above the bore. While it is true that the base angle is consistent along it's entire length, the height of the scope above the bore increases the further to the rear of the base the rear ring is placed. I found I still had the full elevation adjustment when I moved the rear ring to the third slot and doing so provided me with a 100 yard zero. Usually 30 minutes of elevation from a 100 yard zero is enough to zero the rifle for 1000 yards.
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The rings clamp the scope parallel to the top of the base. How will shifting the position of a ring on the base change the axis of the scope relative to the top plane of the base, or the relative relationship to the bore?
If Burris Signature rings with eccentric inserts are used, the distance between the rings would affect the angle of the scope axis relative to the bore.
The rings clamp the scope parallel to the top of the base. How will shifting the position of a ring on the base change the axis of the scope relative to the top plane of the base, or the relative relationship to the bore?
If Burris Signature rings with eccentric inserts are used, the distance between the rings would affect the angle of the scope axis relative to the bore.
Boomer,
Gotta disagree with you. The distance above the line of bore changed, but the distance but the distance beween the rings also changed, neatly cancelling each other out. Selecting two different points, or 100 different points on a straight line from here to infinity will not change the alignment of that line. Straight is straight.
What you experienced is a crooked rail. If it wasn't crooked before mounting, it became crooked when it was screwed down to a crooked action.
Gotta disagree with you. The distance above the line of bore changed, but the distance but the distance beween the rings also changed, neatly cancelling each other out. Selecting two different points, or 100 different points on a straight line from here to infinity will not change the alignment of that line. Straight is straight.
What you experienced is a crooked rail. If it wasn't crooked before mounting, it became crooked when it was screwed down to a crooked action.
- Location
- Somewhere on the Hudson Bay Coast
It is easy to prove. Clamp your scope in the ring placed at the very rear notch on the base and look though it at the grid on a bore sighter. Loosen the front ring so the scope can slide easily. Now, move the rear ring with the scope still attached a couple of notches forward, or even a single notch forward, and look again through the scope at the bore sighter. It will appear higher relative to the grid, meaning that the bullet will strike lower on the target. The height of the scope has moved relative to the bore.
If you continued to move the scope forward on the rail, the bell would sooner or later hit the barrel, so the scope has moved closer to the barrel, the farther back it moves, the higher from the barrel it gets. This proves that moving the scope on the rail moves the scope relative to the bore.
When I changed my scope to the S&B, and I used the rear notch on the 20 minute rail, IIRC I was more than 2' high at 100, but moving the rear ring forward to the third notch provided me with a 100 yard zero.
If you continued to move the scope forward on the rail, the bell would sooner or later hit the barrel, so the scope has moved closer to the barrel, the farther back it moves, the higher from the barrel it gets. This proves that moving the scope on the rail moves the scope relative to the bore.
When I changed my scope to the S&B, and I used the rear notch on the 20 minute rail, IIRC I was more than 2' high at 100, but moving the rear ring forward to the third notch provided me with a 100 yard zero.
If you are moving the entire scope foreward or rearward on an inclined base, the scope bell will get closer to the barrel as it is moved foreward. The axis of the scope mount, and the axis of the bore are not parallel; they are at a slight angle - 1/3 degree to each other. The scope and the bore axes will converge at some point in front of the rifle.
Draw a long skinny triangle on a piece of paper. Pick one of them as the line of sight. Note that the line points at the little pointy part at the end of the triangle. You can take any segment of that line, any 2 points along that line, irregardless of the distance between those points, draw a line between those points and it will still point exactly at the same point. It has to, its a line. For it to point somewhere else it has to be a curveor a different line.
- Location
- Somewhere on the Hudson Bay Coast
I observed the same thing using two rings with my old scope and with the one piece rings that came on my S&B. The relationship between the rings on the one piece ring set doesn't change. Move the scope tube higher above the bore and the bullet impact on the target rises.
Think of using a sloped base as being similar to moving the rear sight when shooting with iron sights. The higher the rear sight is moved, the greater the angle to the front sight and the higher the muzzle must rise before the front sight covers the target. When we put a sloped base on a rifle we create the required angle to raise the muzzle, but rather than having a front sight and rear sight 30" apart, you have an ocular and objective a foot apart and the reticle is roughly centered in between them somewhere along that 20 MOA base. The angle of the base requires that we raise the muzzle in order to see the target through the scope.
We know that the height a scope is mounted over the bore effects the POI on the target at any given range. Now, when we move the scope along the length of the base we raise and lower the height of the sight relative to the bore, which is separate from the angle of the base, and displaces the reticle relative to the bore, not unlike adjusting the elevation turret. The angle of the base causes the shooter to raise the muzzle before he can see the target through the scope, but changing the height of the scope above the bore, moves the POI on the target.
Think of using a sloped base as being similar to moving the rear sight when shooting with iron sights. The higher the rear sight is moved, the greater the angle to the front sight and the higher the muzzle must rise before the front sight covers the target. When we put a sloped base on a rifle we create the required angle to raise the muzzle, but rather than having a front sight and rear sight 30" apart, you have an ocular and objective a foot apart and the reticle is roughly centered in between them somewhere along that 20 MOA base. The angle of the base requires that we raise the muzzle in order to see the target through the scope.
We know that the height a scope is mounted over the bore effects the POI on the target at any given range. Now, when we move the scope along the length of the base we raise and lower the height of the sight relative to the bore, which is separate from the angle of the base, and displaces the reticle relative to the bore, not unlike adjusting the elevation turret. The angle of the base causes the shooter to raise the muzzle before he can see the target through the scope, but changing the height of the scope above the bore, moves the POI on the target.
I have always wondered about 20 MOA bases. Consider the following Bullet drops for a 308
Yards Drop in inches
100 0
200 -4.2
300 -15.1
With a 20 moa base, does it change they way you aim? Do you still have to hold 4.2 inches higher than you aim at 200 yards?
Yards Drop in inches
100 0
200 -4.2
300 -15.1
With a 20 moa base, does it change they way you aim? Do you still have to hold 4.2 inches higher than you aim at 200 yards?
Boomer, Isn't it more important to have the correct eye relief? I would think that getting the best sight picture when you bring your gun up is the more important factor as opposed to sliding the scope along the base to sight in at 100 as opposed to 200 yards. You don't want to have to adjust your sight picture by moving your head back or forth after you bring your gun up.
- Location
- Somewhere on the Hudson Bay Coast
Gun5tuff, correct eye relief is very important, and sliding the scope ahead on the rail will impact this. Target quality variable scopes often have very long eye pieces, so it might actually benefit the shooter to have the rear ring slightly farther ahead on the rail. But many things effect eye relief, including the clothing you need to wear on the day you're shooting. This is one reason to choose a stock that is adjustable for LOP. If your stock is not adjustable, you may be able to re-position the rings along the scope tube to improve your eye relief. But if you have chosen a 20 MOA base that has you shooting 2' high at 100 yards when the ring is in the rear notch of the base and the elevation turret is adjusted for minimum range, you need to change something if you are ever going to use that rifle at close range. The easiest thing to change is to move the rear ring forward, then if you need to, loosen the ring cap and slide the scope rearward a little to correct the eye relief.
