We get questions now and again regarding bayonet lugs on M14 rifles. As I'm sure you're aware, several states have laws that regulate the presence or absence of bayonet lugs on semiautomatic rifles with detachable high capacity feeding devices. We won't address those here, as we are neither lawyers nor residents of such states. What we will address are the practical issues involved with bayonets and the M14.
Affixing a bayonet to the M14 is physically possible only for rifles with full-length barrels. With 18" barrels, there's not enough room behind the bayonet lug for the bayonet handle to clear the gas plug. Thus, a bayonet lug on a short-barreled M14 is unuseable.
When we examine other muzzle devices, e.g., Vortex flash suppressors and Smith muzzle brakes, two other considerations come into play. In these cases, the body of the device may be too large for the bayonet's muzzle ring to slide over, and/or too short for the muzzle ring to even reach the device.
That's why you won't find bayonet lugs on aftermarket muzzle devices, and really can't use a bayonet lug on a short-barreled M14-type rifle.
Hope this helps!