They are basically just tubes sticking out the end of the frame with an end cap. Same as a shroud on other guns but we call them frame extenders because the design of AF guns already incorporates a shroud so these just extend whats already there.
They allow the gun to be made much quieter, they protect barrels that stick out past the frame and some people just like how they look I guess.
You can make a real simple one using 1″ Outside Diameter tubing and just inserting it into the frame and then figuring out how to make a new end cap like this below (A lathe makes it real easy)
I made this one for a .25 Condor to make it quiet (I dont sell them)
You can also buy them from –