Over on the Antique Pattern Library is a scan of a book called "Knitting and Sewing, How to Make Seventy Useful Articles for Men in the Army and Navy" from 1918. It has many interesting glove patterns for the different tasks military men found themselves doing (including "Rifle Mittens" and "Mine Sweeper's Gloves").
It's the third item from the bottom on this page:
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's quite a large download (11.5MB and it's served rather slowly) buta fascinating document. It doesn't have exactly what you want but youmay find it interesting. Of course you could knit a regular glove pattern but treat the trigger finger as you would on a fingerless glove pattern (just a few rows of ribbing instead of a full finger). You could easily adapt the "Cigar" pattern from Knitty too I think:
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