20MOA rail how does it work??

[Daniel Sultanian]

So I’ve done a fair bit of research and the logic doesn’t seem to make sense to me.

If I’ve got a rifle with a 20MOA rail and I mount the optic. I can essentially see lower so I get access to more usable range from the scope adjustment I get that. What I don’t understand is how that zero isn’t completely messed up at let’s say 1000yrds.

Because there’s an angle now…. So at 100yrds cross hairs are at the infliction point barrel but at 200 yrds the cross hairs would be below the infliction point and you’d have to adjust up wouldn’t you? I mean no bullet drop is 20MOA at 200 yrds.

This problem is exasperated the farther out your go is it not???

Ok obviously it’s not otherwise it wouldn’t be a thing but I can’t for the life of me understand how my understanding is flawed.

 

[shlouf]

I could be wrong, but it sounds to me like you are thinking about it in the terms of the bottom part of this graphic.

 

[Daniel Sultanian]

I could be wrong, but it sounds to me like you are thinking about it in the terms of the bottom part of this graphic.

I’ve got the worst drawing lol but in the 0 MOA diagram the bullet and cross hairs are parallel so until the bullet starts to drop (I know it starts dropping instantly but it’s less noticeable sub 300 yrds as it is after 600yrds (arbitrary numbers to help illustrate))

Where as the 20MOA the cross hairs will show up below the bullet. You can adjust it up so it’s zeroed at 100 but it holds that 20MOA angle all the way does it not?

At 100 yrds your dead center will then at 200 yrds (100yrds more x 20MOA your scope crosshairs are 20” lower.) The bullet wouldn’t have dropped 20” yet… does that make sense?

 

[shlouf]

With 0 moa, the scope is parallel, but the angle is still there. Think about the scope eyepiece as the “rear sight” and the reticle in the scope as the “front sight” in the top part of the graphic I posted.

 

[shlouf]

On your drawing, your 0moa scope line is correct in terms of being parallel with the bore, but the look angle is still like what you have on the 20MOA drawing.

 

[UpSideDown]

Your scope has internal angle adjustment by using the elevation and windage dials. You have to dial your scope up/down to match the crosshairs to meet the bullet path at whatever your zero distance is.

As you go farther out and the bullet drops more you need to adjust the scope lower and lower (or, imagine the barrel raising higher and higher). Your scope only has so much adjustment available. To shoot really far you can augment the amount of angle your scope can adjust by canting it 20MOA from the beginning so your initial adjustment starts above-center rather than near-center of the scopes adjustment range. It’s free adjustment basically.

 

[Trigger Monkey]

I thought I'd add my $.02 to clarify some of the nomenclature because while it appears you guys are on the right path there are some misunderstandings about how the riflescope works.

The sighting system as we're referencing it here is the scope base (0 or 20 MOA), scope rings, and riflescope. Inside the riflescope moving from the front to the rear is the objective lens assembly, field lens/parallax lens, turret globe where the turrets contact the erector, the erector assembly, and the ocular lens assembly. There's a lot more going on inside but those are the basics.

When the optic is mounted on a 0 MOA base the scope's main tube and bore centerline are parallel. Internally, the erector is adjusted so that there is an intersection point at the rifle's zero distance and it is angled slightly inside the scope's main tube. What the reticle actually sees inside the scope is upside down compared to what the shooter sees, the ocular flips it for us, so when you're dialing in elevation and it appears that the reticle is moving down, internally the erector is rising towards the elevation turret.

In the 0 MOA scope base scenario, because the bullet starts falling as soon as it leaves the barrel, it hits low on the target and the barrel needs to be elevated. Dialing in elevation, pushes the reticle down to where the impacts are as we see it but raises the erector internally above mechanical center, which reduces the overall amount of elevation available after getting zeroed.

In the 20 MOA scope base scenario, the scope and bore centerline are no longer parallel with each other. If you pointed the crosshairs at the target and then looked through the bore, the bore would be significantly higher than what the reticle is pointed at. This means the elevation needs to be dialed down to raise the reticle up to where the bore is pointed at while internally the erector is being pushed down, below the scope's mechanical center. This is what adds to the available internal adjustment range of the scope, allowing the shooter to dial out farther over an optic mounted on a 0 MOA base.

As the amount of cant is increased, it's pushing the erector farther down inside the optic, increasing the amount of room available for it to travel up. Keep in mind that the total distance that the erector is moving is really only a fraction of an inch. I'm going off of memory from way back when but the S&B PMII 5-25 erector moved maybe 1/8" or so inside the tube for its total adjustment range, not a whole lot.

Depending on the optic though, too much cant in the scope base means that the erector will bottom out or the turrets won't have enough travel to allow the scope to get zeroed at 100 yards, or whatever the desired zero distance is. The typical complaint will be something like "I'm running a 40 MOA base but I'm still 3" high and can't dial down anymore". In that situation the shooter needs to get a different scope base with less cant or a scope with more internal adjustment.

Hope that helps

 

[FuhQ]

How does a 20MOA base work? It’s simple, really…

You remove it from the package, use the supplied screws, apply some VibraTite VC3 to them, let it sit for about 10 minutes, then install the base and torque the screws to spec… And now you suddenly have 20MOA more elevation adjustment than you had before.

That’s how they work.

 

[Team RCBS & Bushnell]

This may be helpful: https://www.bushnell.com/bu-blog-precision-shooting/scopemountingfordistancetakinga22lr450yards.html

I've got a cutaway scope showing the relation changes between scope body, erector assembly, and then how it changes with a base relative to bore line.

 

[Rocketvapor]

Take some tape and cover any MOA markings on the scope base.
You now won't know what the base is, or where your particular new scope will zero.
Zero the scope at your favorite zero yardage, say 100 yards.
Then DIAL in drop for 1000 yds.

A scope with limited vertical might be tight @ 100yds with a 20MOA base but have plenty of short range adjustment at short ranges with a zero MOA base.
A scope that runs out of long range adjustment with a zero MOA mount will gain plenty of clicks with a 20 MOA base.

Just zero with the mount you have. Then remove the tape and find out what you have.

 

[Gemsbok]

I like pictures. Nevermind how it works for now. The important stuff is the right edge of the two figures below.
The what and why is that a canted base allows us to dial the scope at distances where the projectile has fallen a long ways. It doesn't usually matter inside of (guesing here) 500 yards. But if you want to lob a 150 grain 30 cal at 2800 fps (30-06) out to 2000 yards you've got to be able to dial about 42 mil (250 feet) and most scopes don't have that much available. (My appologies that the scale in the figures below is weird (150x vertical exaggeration) and doesn't extent to 1000 yards, but I think you can imagine where the blue trajectory line extends beyond 100 yards.) sht too much text again.

 

[sinister]

For legacy 3.5 - 10X scopes (generally-accepted mid- to long-range scopes from around 1985-2000) you wanted the additional slope angle so at the most-used range your scope elevator put the reticle in the middle of your scope tube. This put the view through the crosshairs not just through the middle of the tube (for maximum left-right travel on your windage knob) but also in the middle of your lens stack (less aberration or distortion at the edges of your lenses).

Let's use 175-grain 308 Winchester ammunition in a 24-inch barrel rifle as our example. Typically Zero is at 100 yards. Let's keep elevation changes simple, so use whole Minutes of Angle. We'll add elevation correction to get to our general longest working range, so:

100 yards ------------Zero
2 minutes up to 200 yards -- cumulative now 2 MOA (18 MOA lower than scope center)
3 MOA to 300 ------ 5 MOA
3 more to 400 ------ 8 MOA
4 more to 500 ----- 12 MOA
4 more to 600 ----- 16 MOA
4 more to 700 ----- 20 MOA (crosshairs ideally centered in the middle of the scope, 20 MOA up from Zero)
5 more to 800 ----- 25 MOA
6 more to 900 ----- 31 MOA
7 more to 1,000 --- 38 MOA (crosshairs 18 MOA higher than scope central axis)

The Army generally stated with a 10X Leupold M3 or the 3.5 -10X M3 LR/T the maximum effective range was 800 Meters or just short of 900 yards -- but most work, doctrinally (for Army instruction and use), was 300 - 700 Meters.

With all the talk of "Modernizing" (now that everyone seems to have swung to Mils and Christmas tree reticles) scope bases are still generally sold in 20-, 30-, and 40- or 45-minute slopes (vice Mils).

 

[Daniel Sultanian]

This may be helpful: https://www.bushnell.com/bu-blog-precision-shooting/scopemountingfordistancetakinga22lr450yards.html

I've got a cutaway scope showing the relation changes between scope body, erector assembly, and then how it changes with a base relative to bore line.

This was massively helpful. I was under the impression that the crosshairs themselves were being adjusted not your actual field of view and that even on a 20 MOA base the scope itself may be pointed significantly downward relative to the barrel but there’s actually a tube inside that has its own angle to compensate at lower ranges. Appreciate it! Thanks