The XS AR Upper & Lower Receiver Block is designed to allow the user to hold an AR-15/M4" AR-10" or SR-25 receiver in a bench mounted vise for assembly" modification and maintenance. This armorer's block will allow the gunsmith or enthusiast to assemble" modify" or maintain almost every AR pattern modern sporting rifle. While most blocks will only fit one specific style" the XS block provides the versatility needed to work on any AR pattern upper and lower receiver: AR-15" AR-10" SR25" or DMPS GII. Includes pins to lock in the receiver. This versatility" plus the reliability of an XS Sights Made in the USA 6061 T6 aluminum product" is a one size fits all solution. To allow bracing into the mag well of AR10 lowers" XS has a 3D printed adaptor plate for purchase or home-printing. Will not work for ARs in .300 Win Mag or 12 ga.