One reason for the 25M zero is that some military bases do not have appropriate range facilities for the full distance.
The quick answer is that the arc of the bullet passes the 25 M target on the rise of it's arc and the 300M target on the declining side of the arc. This is not a perfect situation but is enough for man size targets. It is possible to set up a rifle at 25 yards and have a high probability of hitting paper at 1000 yards. This all depends on factors including bullet weight, velocity,caliber,ETC. The best zero is shooting the actual distance. Keeping a data book on sight settings for various distances can be very helpful.
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