What the name says. A dummy magazine to function as a bolt hold open for ranges or competitions that may prefer a rifle have a bolt hold open on empty. So you can reload all your mags or keep them all loaded while you still have your bolt locked open. It's not spring loaded or anything so you have to do this little song and dance where you hold the bolt open THEN insert this dummy mag. It would go in all the way and latch if the bolt is in battery, what ever, you'll figure it out.

~.3mm tolerance all around, should work with just about any setting.

Works great on an Ender 3 V2 with the following: 3 walls. 10% infill, grid. PLA.