30mm scopes are old hat anyway.
34mm is where it's at these days.
I doubt there is any difference in performance between a 4x40 from body tube size.
The larger bodies give the designers more options when designing in all the toys like high magnification. Also a stronger thicker walled housing.
Everything else is in the quality of glass, the choice of coatings, and how all the lenses are designed to work together. Some manufacturers are better at putting it all together than others. Some scopes just have it all just right, more so than others in any range. Pure objective size is no guarantee of gathering more performance, especially if the glass isn't much good to start; which happens when trying to match a price to market. Whatever the scope it should match the scale and balance of the rifle. The rifle design and weight should match what its intended purpose is.
A lot of glass works in good light. The better glass works in all light conditions be it bright or at dusk. Quality does count.
I couldn't let this drop, the little Simmons. They aren't that bright. I know why people use them as they give a great sight picture, but thy aren't bright nor particularly good at dusk.
There are a good few 4x40 scopes that are super bright, and the odd 3-9x40. Its all in the glass. The odd Hakko, Tasco, Leupold and Zeiss fixed x4 or x6 can be surprisingly bright. When it comes to air rifle scopes then its often what the manufacturer/brand decided to do that year. Some models in a range just light up, the rest are "also rans". It can be the case that a larger objective lens is so badly matched that it allows less light through to a small sized objective in the same line (Tasco, Simmons). There is more to making a scope than just putting some lenses together.
They all promise the earth. A very few do, or go beyond their price range.
Tube size doesnt have a huge effect on light transmission.
Bigger tube allows more elevation and windage.
Weight can be more down to how many lens are in the tube as much as the tube itself.
I have both and they give good results if enginereed right
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