The big advantage to 1st Focal Plane vs 2nd Focal plane reticles in long distance shooting is that 1st Focal Plane the image AND the reticle remain in constant corrolation. A really good scope does 3 things:
1 - It lets you see your target as clearly as possible.
2 - It lets you size your target almost regardless of range.
3 - It lets you use holdover marks reliably regardless of the range to the target!
The 1st Focal Plane reticles have become the standard for long range shooters in all branches of the U.S. Military because they allow more precise ranging and holdover with the reticle verse having to adjust the crosshairs as much.
With a 2nd Focal Plane scope you can only use some reticles (epending on type, Mil-Dot/TMR) at the MAXIMUM magnification. With plain crosshairs this isn't as much of an issue.
1 - You can see your target BUT
2 - You can't size your target UNLESS you have the scope at MAXIMUM magnification and depending on the reticle in the scope.
3 - If your scope has any holdover marks they are only accurate with the scope at MAXIMUM magnification.
If your scope is not at maximum magnification the ability to size and range your target will be off as will be any holdover marking on the reticle.
With the Leupold Mark 6 1-6x20mm I use on my long range rifle with the TMR-D here is how the reticle works:
At 1000 Yards the holdover marks are 18" apart.
At 500 Yards the same holdover marks are 9" apart
At 250 Yards they are 4.5" apart.
This is correct at ANY magnification with the 1st Focal Plane Leupold TMR reticle.
If your target is 6' tall at 1000 Yards the target is 2 holdover marks above and below the center crosshairs.
At 500 Yards a 6' Target is 4 holdover marks above and below the center crosshairs.
As long as you know your distance and your approximate bullet drop at those distances with a 1st Focal Plane TMR reticle you can use the holdover marks on the verticle to place your shot WITHOUT adjusting your elevation dial up to the number of holdover marks on the reticle, at longer distances you can also use the elevation dial.
On another rifle I have a Leupold VX-6 3-Gun 1-6x24mm with 2nd Focal Plane SPR reticle. With the Leupold SPR reticle you sight in your rifle at 200 meters at MAXIMUM magnification. At 200 Meters a 6'6" target fills the Circle at 200 meters and the holdover marks are 18" apart in the Reticle.
At 500 Yards the holdover marks would be approx 45" and at 1000 yards approx 90".
Either scope is fine for Zombies at 200 meters/Yards but once you start to reach out and touch a target at greater distance the 2nd Focal Plane Leupold SPR reticle gets left in the dust by the 1st Focal Plane TMR by the time you get to 500 plus yards unless you really like adjusting your elevation dial AND very few scopes have enough verticle crosshair adjustment to dial up more than 500/600 yards of bullet drop when zeroed to 200 yards or meters (depends on what distance the manufacturer recommends you sight your scope at).
Most long distance shooters use a 20-30 MOA slanted scope base to get in the ballpark for long distance shooting even before starting to dial in the elevation on their scopes at whatever range they sight in.
There is a pretty big difference in reticles that are good for Zombies and long range target shooting between reticles and the amount of adjustment, and how fine adjustment between the 2 types of scopes. I'll try to post some pics of the reticles of the scope I use for different purposes.
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Sorry for the crappy pics!
Leupold now has a VX-3I-LRP with the First Focal Plane TMR reticle for $1299 MSRP normally around $999 street price and they were on sale at Sportsman's Warehouse for $899 a couple of weeks ago:
https://www.leupold.com/scopes/rifle-scopes/vx-3i-lrp-8-5-25x50mm-30mm-side-focus-mil-ffp
For the money this is the best entry level long range scope in my humble opinion.