You only really need longer mag for say 75 & 80 Eldm's, you can fit 77 TMKs, SMKs, 73 EDM's, 69 TMK's in tikka factory mag ok as designed for AR mag. The waterman mags are nicely made but you know what, sometimes just easier to grab a bigger golf club out of the safe than trying to stretch the old 223 unless you are more specifically into target shooting and want some more range?
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