I can get 18 in my G17 mags and they are new. I think all glock mags have a bit of extra space to allow for spring compression.
Maybe it helps the springs last longer, who knows but that is my guess.
I barely remember a thread about this several years ago about this issue.... something like that sometimes its possible to load the extra round, but raise the possibility of feed problems. Wouldn't want to trust my life to going outside of a mag's designed parameters.
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