This is a model of a 35mm solenoid intended for use as a nerf pusher, originally found by /u/thejesusfreak on reddit. Should be similar to this one on banggood, probably also available on eBay/taobao/etc. if you google it. This is a somewhat rough model but the key dimensions should be there for clearancing around the moving parts and anchoring. Has not been tested against a printed project using this measured model as reference.
080918 UPDATE: Oops! Accidentally drew the model in inches. New source file should be correct scale.