Leapers, Inc. - UTG UTG PRO Receiver Extension Tube, 6 Position, Mil-Spec, Black Finish TLU001 The UTG Pro Receiver Extension Tube is a 6-position Mil-spec diameter receiver extension tube for AR-15 rifles. It is CNC Machined from 6061-T6 Aircraft Grade Aluminum with Precisely Rolled Threads for Durable Thread Engagement with Lower Receiver.