Midwest Industries Stock Tube Adaptor, Fits Picatinny, Black MI-STAP The Midwest Industries Stock Tube Adapter fits a Picatinny rail interface" enabling the user to install an AR-15 collapsible buttstock to an alternative rifle platform. The tube has 6 different positions for length of pull" and contains a hollow" water resistant internal compartment for the storage of small parts or batteries. Does not function as a buffer tube. Midwest Industries is a proven leader in the firearm industry. Made in the USA" focusing on the small details to make your rifle platform better.