I do believe in fact that in an article he wrote for Double Gun Journal that Bill Wise specifically wrote it out as "Peeper" for the pronunciation. Don't recall which issue, but was a very informative article on the guns. That was the first time I knew how to pronounce it. I have slowly gleaned over the yrs, & I know there are exceptions to all rules, but most double vowel words are usually correctly pronounced by using the second vowel, as in tis case "Pieper", pronounce the E.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra