I have heard of a few other people squeezing in 18. With the gray followers there is barely enough room to squeeze in 18. However it is an EXTREMELY tight fight. I just use them at 17.
The extra room is likely to prevent the magazine spring from going past its yield.
Every time a spring is taken past its yield, it has less spring pressure. Eventually you will run into a myriad of magazine related problems. Mine have all started with not holding the slide back following the ejection of the last round, but many just go straight to feed problems.
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