it all depends on HOW MUCH you want to spend on your mount - your arms 18s tend to run a couple of hundred, and the smiths, sadlaks, and others are probably more- this is for steel, which as the same expansion ratio as your receiver, and not ALUMINUM which expands differently- as to the scope itself, i've found that anything bigger than 40mm tends to violate the tunnel-with the bottom of the bell unless you use high rings- then that mandates you need a higher riser- on my q&d folder i have a 32, which has no riser , and allows me to use both the tunnel and the irons as i see fit( excuse the pun) - i also have it in 4-12x. not 3-9
if you look at the picture, you'll see he cannot use his tunnel, and that;s a 40mm objective- i used a couple of pieces of hot water pipe insulation and duct tape to make my riser- each piece will give you about a half -3/4 rise - and it's cheap as heck
oh, btw, the higher you mount your scope, the more it will twist out of your hands- we found this out when we mounted the ans/pv series