Designed to fit under a 20mm diameter magazine tube, ie 20 gauge shotgun. Interior diameter is a little bigger to accomodate a thin layer of cork board material to prevent slippage. Use medium nylon zip ties to secure to the magazine tube. Thanks to Hinfa for the "Crosman 1377 Picatinny rail" which I used to get the base rail dimensions from.Example shown was made from PLA on a Printrbot simple. 1.75mm fllament. The bridge between the two side is a bit of a challenge, monitor closely and make sure you tweek the cooling fan and temperatures as needed to help bridge. Expect some slight clean up with a dremel. Medium sized zip ties should pass through the part snugly, but without need for additional work. Other total materials include: printed part 2 nylon zip ties, medium (~4mm width) small rectangular piece of 1mm thin cork material Of course, temperatures may cause some issue in the field. Eventually I am looking to start printing nylon, and hope to print these out of nylon for a more durable part. These haven't been field tested yet, but I will update if I notice any issues this season.