I built an open 9, researched mags to death and was all set to send then away to tune. Bought 140mm STI 38 mags and decided to try it 'as is' out of the packaging. A year later, 6 mags (need many in canada) the completely stock 38s mags work 100%. No spacer, no tuning, nothing. Never one mag related issue.
I load to 1.170 oal.