I just went through this at the beginning of this year. KAC doesn't typically offer a factory pinned/welded option, so your only option for a factory gun is to buy a factory SBR and have it removed from the registry after having your dealer pin/weld the muzzle device. That, or go the route I did and by a 16" gun and simply replace the upper with a pinned/welded 14.5" upper. I sold the 16" upper off mine as new and bought a new 14.5" upper, pinned/welded the muzzle device, then slapped them together.
Aside from one or two guys who said they were able to get KAC to do the pinning at the factory before shipping, I'm not sure why this isn't an available option. It seems like this question pops up frequently and it would make sense for them to offer non-NFA factory 14.5" guns. There are MANY higher end factory pinned/welded 14.5" guns available today, and while not as popular as the 16" guns, there is still a good market on them.