The mags i have for it wouldn't lock the slide back on the last round, they use to, but the plastic followers are wear, were to get new ones? Its a Colt slide and barrel kit.

Marvel Precision make replacement Colt/Metalform magazine followers that have a steel insert in the plastic follower body to activate the slide hold open feature without wearing the follower out. I purchased 2 and they work flawlessly in both of my Colt Conversion Unit magazines. To purchase go to Marvel Precision's website and click on the order form. The followers are located at the bottom of the page.