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I would use the 7.5
depends on the conditions and how fast you get on target
Why?
What conditions make one favorable over the other?
Does getting on target faster favor the 7.5, or 8?
I'm not really looking for "what to do", but more of the why.
Heavy wind influences #8's more than #7 1/2s, also #7 1/2s are better for the longer distance shots, however if you get on target pretty quick, #8s are great. I personally usually shoot #7 1/2s in my 20ga and #8's in my 12 as the 20ga has a shorter barrel, less powder, and a fixed modified choke. I have found that 8's out of my 20ga have a harder time breaking the new "corn meal" clays that have been exposed to the humidity. You'll see a few pellets go through, but the bird manages to stay together.
For trap it does'nt much matter really. Like MisterHappy said trap is a mental game.
Now if you were talking sporting clays there are advantages to #7 1/2 or #8's.
http://shotshell.drundel.com/pelletcount.htm
And each gun will digest a given load differentlt from the next gun in the rack.
As a practical matter, go with what you're comfortable with - the mental certainty of a good load in your shotgun will outweigh the minor differences in retained energy. When you're good enough to note the difference in the quality of the hits with different size shot, THEN it will be important.
I'll let you know when I get there!
Can you expand on this? What's different about sporting clays that makes it matter?
I don't shoot trap, but always thought the way to select shot size was to pattern each with your shotgun and go with the size that gives the most evenly spread pattern (no holes or gaps). I figure, even if the results vary from round to round, doing this once will give you enough confidence in your choice that you won't worry about it anymore. Then you can concentrate on form, and practice practice practice.
That being said, I warn you that I am more of a rifle shooter. I tend to try to aim my shotgun, and so I suck with it on clays.