For £400 its a no brainer,second hand big Nikko 10-50X60.A rail won't improve your range finding,what it will allow is for your head to be in a better position and it will flatten your trajectory out at the further distances,hence giving more margin for error,but as I said it won't improve range finding.Of course the closer ranges will need more dialling and you will find yourself having to go more than 1 turn on the turret which can itself cause problems.
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