First, let's remember that unlike the M1, the semiautomatic M14-type rifle is to a great extent an aftermarket proposition. That is, some components, to include flash suppressors and other muzzle devices, must be made at the top end of the USGI tolerance spec to ensure that they will work with othe components, both USGI and commercial. In the case of M14-type muzzle devices, the the fit of the device on the barrel grooves is not critical to function or accuracy. Why is that?
The answer is simple. In the case of the M14 muzzle device, when the device is fully installed and the lock nut fully tightened, the muzzle of the rifle is hard up against an internal shoulder in the device. Consequently, the muzzle device will not move, even if the splines and grroves aren't as tight as you might expect.
Hope this helps!