Using a set of my own plain, cornered, white dice, I rolled out a bunch of test rolls for 7d6. I even went to a website and got some automated, electronic rolls.
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After about 100 rolls I've found that the most common numbers rolled were 22, 27, 24, 25, 21. I didn't make an exact tally, but in that order, those were the most frequent. The lowest number I rolled frequently was 18. The highest number I rolled frequently was 31.