I’m a little behind the curve on some terminology.
What are we referring to when we say a rail is 20 MOA, or 30 MOA?

That means the rail has 20 MOA of elevation built into it. So all things equal, if you installed a scope on a 20 MOA rail the point of aim would be the same as if you'd dialed in 20 MOA with the reticle.

That gives long range shooters an additional 20 MOA to work with and also keeps the scope working near the center of it's adjustment at long range, where the adjustments tend to be more accurate.

Thanks for the explanation. So then which one you choose depends on the average distance the shooting will be done?

Sorry. Yes, I added to my post while you were posting.

Yes . The rails with built in elevation keep the scope reticle more centered in the tube at very long distances.

Thanks. Any recommended brands? I’ve bought a rifle with a Seekins on it. I need another one for my 308.

Pretty much, most scopes perform best when they are optically centered

You want to look at the adjustment range of the scope and the ranges you'll be shooting and what your drop will be at those ranges. Try to put the most common ranges near the center of the scope range while making sure that everything you want to do is within the adjustment range.

So for example say I got a hankering to shoot my 375 WSM anywhere from point blank to 1000y. It needs 8 MOA of elevation for a MPBR zero. it needs 60 MOA of elevation at 1000y. The scope is a Nightforce NSX copact with adjustment from +50 to -50 MOA. So without a rail I couldn't even get on target at 1000y. An ideal rail would have about 35MOA of elevation built in, so I'd be using from +25 to -27 on the scope. A 30 or 40 MOA rail would also work fine.

Obviously that's an extreme example of a slow, fat cartridge at long range, but it illustrates how you work the problem.

So now we’ve going to Greek!!!!!!
I kinda get it. Most likely I’ll stay less than 500 yards.
Most of the lanes at the family farm are 300-500 yards.
However, there is always the future!!!
I’m thinking 30MOA???

How much elevation adjustment does your scope have??

No clue. I’ll check the manual.
Burris 4.5-14x mil dot.

https://www.murphyprecision.com/Page/What-is-moa

It's a balancing act...
When I set up a new rifle I install a flat rail first, sight in @ 100 yards and see how much vertical adjustment I have left. A .308 Win needs aprox 10 MOA over a 100 yard zero to make it to 500 yards, 34 MOA for 1000 yards. Depends on your total adjustment range, if you put too much MOA into the rail to make long range shots you may have to hold under at shorter ranges (100~300 yards). YMMV...

So then 20 is a good starting point / compromise?
I’ll just be mostly target shooting. Nothing over 500 yards.
Mostly 200-300 yards.

For what you're describing you probably don't need any elevation on the rail. Most scopes can get to 500 easily, I'd be surprised if yours won't. And if your shots are mostly 200-300 yards you'll be optimizing your set-up for shots you'll rarely take and your scope will be centered at long range, but not the distance you do most of your shooting.

I just set up a .300 Weatherby for an elk hunt this fall and I went with a level rail from Warne. Seekins is good stuff, no doubt, but I just got what was in stock at Midway.

I don't believe they had any Seekins rails in stock for the Weatherby rifle, but the Warne rail seems solid.

What he said. ^ ^ ^ 👌

A standard 308 load of mine is about like Lake City match with a 168 grain bullet moving out at 2550 fps +/-.

Relative to a 100 yard zero this load only needs about 13 moa of come up to hit at 500.

A 20 moa rail would be more than sufficient.

Are you rationing powder due to the lack of availability? My loads are a tad faster, 185gr Berger Juggernauts @ 2735 fps.

He ran an algorithm to find the most efficient ratio of fps/grain of powder.

Don’t need no stinking algorithm….
I read Barsness!!!!!

LOL, I was talking about mathman. They don't call him mathman for nothing.

I hear he writes algorithms for Barsness....

Nah, I was loading that way when I was able to buy eight pounders no problem. It's actually pretty simple, I don't need that much juice for what I'm doing. My range only goes to 300 yards, and my hunting camp doesn't have any shots I'd take much past 300 or so either. For example, we do have a pipeline right of way, but the ground undulates and I won't shoot over crests. A 165 grain Ballistic Tip at 2550-2600 has been quite effective for my purposes there. Same same for a 168 Amax at the range.

The Dynamic Duo???

As mentioned, sounds like 0-20 would be a lot more appropriate for the ranges you have indicated. Warne does make a very sturdy set of mounts.

Just a thought but rather then lock yourself into an MOA rail you could use a set of Burris Signature rings with the inserts. You can add or subtract MOA at will and it would cost you next to nothing.