Pin and welding permanently attaches a muzzle device to the muzzle of the barrel. 

Pin and welding can be done to make shorter barrels longer, to reach the 16” minimum length set by the ATF for rifles and not need a Short Barreled Rifle tax stamp. 

It can also be done to meet various state requirements that prevent threaded barrels. 

Once a muzzle device is pin and welded, the muzzle device can NOT be removed.  

If pin and weld is added to a cart, the muzzle device on the firearm or upper in the same order will be permanently attached.