I shot my first round of trap with a full choke because that's what all the old guys said they were using. I think i hit maybe 8 or 9 for the round. Then a DIFFERENT old guy at the club told me to try shooting with a modified choke. I did, and promptly saw my score jump to the mid teens.
Using a more open choke when you're just starting WILL help you because you will hit a couple more shooting a bigger pattern. The ones that don't break because they're out at 40 yards, well, you weren't going to hit those anyway regardless of choke, so it's not such a big deal. Once you top out at around 18-22 for your score for a round, switch back to a full, or if you have one, Improved Modified. You'll still be hitting the close ones that you can get on quickly, because you've gotten better, and the tighter choke will help you hit the ones further out. I shoot an IM for trap now, and usually average about 22-23. I can't bring myself to shoot full just yet.
On a lighter note, one of the guys on my league team (our AA shooter) got his full choke stuck in his gun after dropping it in the drink out hunting or something. Shot for 3/4 of league season with a full choke, and shot more than one clean 25/25 round of skeet with it in :O
Oh, and watch these videos, they're pretty good
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[youtube]os8NqvNxeug[/youtube]
[youtube]2WYPkVOHfOI[/youtube]
[youtube]MnYVkBYNb0s[/youtube]
[youtube]V8dWIXFed2M[/youtube]