“It seems that every time I go to the range with friends I’m the only one who has one of these, and everyone is always “borrowing” mine! This device is very useful for loading new magazines with very stiff springs, or loading using gloves in cold weather. I have tested it with Beretta, Sig-Sauer, Springfield, and Ruger magazines. Others have printed and tested it with Smith & Wesson, Steyr, and FN/Browning magazines. It loads both single and double-stack magazines, and should work with any caliber which is dimensionally similar to 9x19mm.
OPERATION: The speed loader is placed on top of the magazine, and pressed downwards. A small boss depresses the top round in the magazine and in turn compresses the magazine spring. Another round is then loaded, from the front, and pressed backward against the boss. The speed loader is then lifted slightly so the new round can be fully seated. This process is repeated until the magazine is fully loaded.
CAUTION: Firearms, ammunition, and magazines should only be handled by trained adults. This design is provided “as is” without any warranty.”
-https://www.thingiverse.com/thing:209583 (Original creator)