Here is a link to a letter from the ATF in which they address cheek saddles (CAA Saddle and MFT Saddle) LINK
MK18 Clone
"Saddle devices designed for "cheek enhancement" generally would not change a pistol's classification to a "shorl-barreled rifle. " FTISB finds that the submitted saddle devices are not designed to support the ARtype pistol in the shoulder of the shooter during firing but, rather, to rest against the shooter's cheek."











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