The using a gun breech or barrel liner on the Shilka or GAU-8 doesn't give any significant (>0.1%) increase to sustained DPM.
With the way the overheat mechanic works, the primary limiting factor to sustained DPM with continuous fire (i.e. holding down LMB) is not the RoF, but the cooling rate of the barrels. Every shot that is fired increases the temperature of the barrels by a specific amount, and while the barrels have a non-zero amount of heat they will cool at a set rate. As such, the amount of heat the barrels can disperse in a given period of time determines the sustained rate of fire of the gun system, which in turn determines the sustained DPM.
Because the gun breech and barrel liner retros do not increase the cooling rate of the gun, they do not have any significant effect on the sustained DPM of the Shilka. You're better off using just about any other retro instead.