Originally Posted by REDHEAD
Ok , if I have a standard AR platform and I purchase a 6.8 spc upper - does anything need to be done with the recoil buffer ? 450 , 6.5 ?
The purpose of the spring and buffer in an AR-15 is not to control recoil. Their function is to control the velocity of the bolt carrier and to store energy during the extraction, ejection, cocking the hammer, (and locking open on the last round) and to return that stored energy at a controlled rate to strip the next round off the magazine, chamber the round and lock the bolt. How powerful the cartridge was makes almost no difference since the barrel designer will give the gas port the correct position and diameter to give the correct gas impulse to the bolt carrier. Unless you're cutting your own barrels or using very non-standard loads you don't have to worry about it. The buffer and spring have a small effect on felt recoil, but felt recoil should be controlled with a muzzle brake, shoulder pad, by added weight, or usually just ignoring it.
Messing with the spring rate and buffer weight can "tune" an AR-15's timing, but that is rarely needed or desirable.
I have AR-15 uppers in many cartridges including 6.5 Grendel, 458 SOCOM, 50 Beowulf, and 300 OSSM (necked up 25 WSSM) and they all use standard lowers with standard springs and buffers (some rifle and some carbine). My 300 WSSM has a Stag 223 marked lower with a standard 223 buffer & spring and it works just find. The only special parts in a 300 WSSM are the barrel, the barrel extension, and the bolt which are larger both for strength and to fit the case head. The bolt carrier is standard. The only thing special about the upper receiver is the ejection port has been widened.
Even 50BMG single shot uppers don't put exceptional force on an AR-15 lower unless you restrain the lower against some solid object. A human shoulder is not a solid object.