POA/POI shift between 100yds and 200yds

TheBelly · 07-31-2011, 20:27

OK here's the question and it may be a bit long/complicated:

All others thing being equal, and using a 0 MOA base for a .308 Win, will the POA/POI shift between 100 and 200 yds be different? if so, which way?

Will this difference be magnified using a 20 MOA base?

@ 100 yds, the POA and POI are the same, will the POI be higher, lower, or the same at 200 yds? how will the difference be magnified using a 20 MOA base?

Zak Smith · 07-31-2011, 23:56

The 20 MOA base, provided the scope center over bore center axis is the same, does not change anything about the ballistics. All it does is offset the erector assembly when you have your zero so it has more "up" travel before it "bottoms" (or "tops") out in the scope tube as you turn the turret.

If you have a 100 yard zero, you will typically be 0.4 - 0.5 mils low at 200, depending on your load.

The inclined base does nothing to this.

TheBelly · 08-06-2011, 16:44

If I zero a scope for the 0 MOA base, but then switch to a 20 MOA base, the POI will be a lot higher, is that correct?

I didn't think that the .308 started dropping between 100 and 200 yards. Or, more accurately perhaps, I didn't realize that 100 yards was the actual highest point of the path of the bullet.

I understand your point about a 20 MOA base giving more 'drop' in elevation to be able to reach out and get the longer distances. I just thought that, given a 20 MOA head start in the upward angle (if the line of sight was parallel to the ground), that the bullet wasn't going to be heading upward still between the 100 and 200 yard marker....

does that make sense?

Zak Smith · 08-07-2011, 11:28

Quote:

If I zero a scope for the 0 MOA base, but then switch to a 20 MOA base, the POI will be a lot higher, is that correct?

If the mounts were otherwise identical and no other error was introduced-- and you didn't adjust the scope knobs-- yes.

Quote:

I understand your point about a 20 MOA base giving more 'drop' in elevation to be able to reach out and get the longer distances. I just thought that, given a 20 MOA head start in the upward angle (if the line of sight was parallel to the ground), that the bullet wasn't going to be heading upward still between the 100 and 200 yard marker....

does that make sense?

No. There is no head start. It just changes where the erector assembly is physically in the scope. If you have a 100 yard zero with a 0 moa base and a 100 yard zero with a 20 moa base, you have the same zero and the trajectory is identical.