Seen a lot of posts asking the difference on the two, so I figured I would throw my .02 in and share my recent experience. I've shot 50mm bells exclusively up to recently, when I put a Leupold 4-12x40 on my 6mm Remington. I can honestly say that I have bought my first and last 40 mm objective scope. It can not compare to my 50mm's in low light. I was sitting there during "magic hour" last night thinking, "why did I buy this?" So for the next post that askes the difference, and what to buy, my heart felt opinion is to buy the bigger bell. Carry the extra few ounces to make the shot with confidence.