20 or 40 MOA Nightforce base

[gaps]

I am debating between the 20moa and the 40moa Nightforce base. I am building a custom rifle and I want to shoot 2000 yards plus. It will be a 300RUM shooting 215 or 230 berger hybrids (if I can ever find any to buy!!!). The two scopes I am considering are the Vortex PST 6-24x50ffp or the Vortex Razor HD 5-20x50. The PST has 65moa of travel and the Razor has 125moa. I know with the Razor I could use the 20moa rail and be fine. But with the PST I think I would have trouble being able to dial enough elevation. I was planning on a 200 yard zero, would this be possible with the PST and a 40moa rail? If anyone has a PST with a 40moa rail I would love to hear your experience. And if you don't I would like to hear your advice too! Thanks for any input!

 

[Rockfish Dave]

To figure out the usable range take the vertical travel of the scope and divide by 2; add 5.8 MILS for the additional travel afforded by the 20 MOA base, and it will give you the amount of usable travel for a given scope/base combo.

So for a Vortex 5-20 HD:

Total vertical adjustment: 36 MRADs / 2 = 18 MRADs

Add the Scope base, in that example it's 20 MOA: 5.8 MRADs

Total vertical is: 23.8 MRADs

Then use your ballistic calculator (here is a free one: JBM - Calculations - Trajectory) to determine how much adjustment is needed to reach your desired max range.

The down side is the image quality/fov degrades as the internal adjustment approaches the max (in either direction).

My personal feelings are it is more important to have the internal adjustment bottomed out at the zero, and have the scope closer to the center of the adjustment at range, and hence the clearer image at the distance; Rather than the other way around. I feel it is more important to have the highest quality image the further out, than close in.

 

[MontanaRifleman]

You will find that every scope is a little different when it comes to advertised and actual vertical adjusttment. Example, my NF 5.5-22 has 98 MOA of total travel while my 3.5-15 has 115 MOA, both advertised @ 100 MOA. Also, if you have say 100 MOA of travel it doesn't mean you will have exactly 50 MOA of up or down from center/zero and when you zero your rifle, you will loose a few MOA of up. Aslo, not all bases are EXACTLY 20 or 30 or 40 MOA as advertised and the machining of the receiver might have an affect on that as well.

The safest thing to do is leave a little fudge room. With my 300 RUM I am mounting a 40 MOA rail to use with the 5.5-22. I should have about 80-90 MOA available in total up adjustment from zero.

IMO, a 20 MOA rail might work if you want to shoot the RUM past 2K but you might be be cranking close to the edge of the travel with the Razor.

If you can get the 230's going @ 3200 fps with RL33, which is optimistic, they will be going subsonic in the 2200 - 2500 yd range and you will be needing about 70 -75 MOA of up correction. And more for less MV.

 

[Rockfish Dave]

You will find that every scope is a little different when it comes to advertised adjustment.

The safest thing to do is leave a little fudge room.

IMO, a 20 MOA rail might work if you want to shoot the RUM past 2K but you might be be cranking close to the edge of the travel with the Razor.

Good advice and something I should have included in my simplified example.

 

[gaps]

Thanks for the help and info guys, I appreciate it.

 

[gaps]

@montanarifleman with your rum and 40moa base, what are you zeroed at? I would want to zero at 200 but im not sure if i have the 40moa base if the PST scope would allow that....thanks again for the help.

 

[MontanaRifleman]

It depends on what bullet and velocity I am shooting. For 3400 fps and faster I like to zero @ 300 yds. With less velocity I like to zero @ 200 yds. I still need to work up loads in my new rifles so I haven't decided any yet.

 

[azsugarbear]

You won't be able to use the PST with a 40 MOA rail. Not enough travel. The razor will work because it has more the 40 MOA travel either direction. Most of us have found that scopes do not always track reliably when you are working near the bottom of the travel distance. I like to have at least 5 MOA of cushion underneath when I am zeroed. Don't forget your reticle will most likely provide you with another 10 MOA of holdover as well.

 

[gaps]

Good to know, thanks! I ended up going with the 40 just in case I was going to use the pst but I'm bought the razor instead...I figure that it should be fine and I'll have an extra 20 MOA to dial with when I really stretch her out!