This tool aligns an AR style receiver extension (buffer tube) to the receiver so that the nut can be tightened.
There are two models. A "small" that works on small frame ARs like the AR15 or AR9 and a "large" for large frame like the LR308/DPMS and possibly AR10. The large also works on small frames.
LBRY Tags: ar15; lr308; 3dprint; 3d2a
Picatinny mounted folding carbine buffer tube