You're correct, the M7 and M9 bayonets for the M16/M4 rifles are designed for the 14.5" and 20" barrels, but that's only because of the location of the standard front sight base which has the bayonet mount on it. The distance from the flash hider to the bayonet mount on both rifles as the same. The 14.5" M4 has a carbine gas system which is also shorter. Bayonets don't work on 16" barreled AR15's with a carbine gas system because the distance is too long. Some people have resorted to getting bayonets with extended handles. Another solution has been the 16" AR15's with a "mid-length" gas system, which puts the gas port (and standard front sight) about 1.5" forward compared to the carbine gas system and allows the use of a bayonet

I just measured my 16" middy AR15 which has a standard front sight and from the back of the bayonet lug to the 2nd ring from the muzzle on the flash hider is 4 15/16".

On my Smithy 15-22 A1, the distance from the front face of the stock handguard to the same location is 5 1/16".

So regarding the bayonet lug I linked to, it may not fit because of it coming into contact with the handguard. It may still work though if you take the front handguard cap off.

Another option is the barrel mounted bayonet lug from an IMI Galil. Apparently it can accept the US M7 bayonet. The Galil and 15-22 have similar barrel diameters, .719 for the Galil and .700 for the 15-22.
https://www.apexgunparts.com/product...oducts_id/2331