djandj,
13 - 15 rds isn't bad with the ZB39. You can get 20 rounds reliably with a helper spring. You may get 25 rds (mag capacity with 8x56r) but it won't be consistently.
Check this link for a detailed discussion of the ZB39 mags. & 7.62x54R:
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The pic shows how I set up mine.
Attachment 46793
Joe