Depending on the power of magnification you intend to hunt with you may not need any more than 32mm. You see the human eye can only absorb so much light through the pupil. A young eye can go to 5mm, maximum. As you get older that reduces with time, 4mm and even less. The formula is simple, the capability of the eye X magnification = needed objective size. An older eye with a 8X scope can only utilize 32mm of objective (4x8=32), any larger is useless. A younger eye could take advantage of a 40mm bell at 8X, but how often does one hunt at max power. My biggest scope is a 2.5x8 Weaver and it doesn't spend a whole lot of time at 8x. FWIW, Rod.