Post by GLSHOOTER on Jun 19, 2015 15:38:03 GMT -8
It's kind of mish-mash effect in engineering interaction. One causes the other problem and that causes another. Separating the issues is like trying to un-mix a Bloody Mary. I'll give it a shot and, yes, that is a pun.
Starting with the loose key issue you will experience as a given gas leakage. This will impede reliable functioning and once it starts getting loose it tends to continue to increase. It doesn’t just “wear in” like mating parts via direct usage or lapping them together. As it changes the gas leakage changes resulting in inconsistent speed of rearward travel and inconsistent bolt seating along with the cartridge. Imagine having a slightly oversized case and letting the bolt resize the case with the chamber. Variation from one to the next with more or less seating thrust while miniscule is still coming into play.
The gas tube in a well set up AR platform will slide up inside the key without interference. AS the key loosens it will tend to pivot up and down or left and right on the two bolts. If it hits one side or the other then torque is applied to the tube transmitted to the barrel as stress. Stress variation causes an alteration in harmonics. Changes mean down range it just won’t put them all in the same place. It will as a side effect also wear out the little button area on the end of the gas tube and this will cause lessened seal of the gas going back in the key. Once again this can induce changes on bolt carrier acceleration speed and degree. . Do you see a trend here?
The parts of a DI system are much more interrelated than many realize. An interference fit here can make a big difference over there. Not only do we deal with the reciprocation bolt but we must address the uppers. As an illustration of this we talk of “squaring” or lapping” an AR receiver. This basically allows for the barrel extension, hoping it’s straight, to mount up on the upper with the center of the bore being centered with the long axis of the upper. Not up, left, right, down or drifting up or down angularly. In other words we want it straight down range. Given everything is square and mounting our glass or irons we should need less adjustment or deviation from the center of the adjustment available. If we fail or if something else is causing a mis-match then it is going to require a lathe, files or shims to get everything looking in the same direction. If we are “out of square: not only will the groups be off but a potential exists of the bolt, almost certain BTW, not locking up squarely and in a worst scene scenario lug breakage due to unequal stress may occur. This last is more common than many realize.
So in summation: Loose key: inconsistent gassing and bolt movement and impingement on the gas tube causing degradation in accuracy due to interference between the key and the gas tube. This subject is covered in a series of books I have by Zediker and I highly recommend everyone buy his missives on building, shooting and reloading for the AR. To me they are while not always the be all to end all a great jumping off point just like the PO Ackley, Elmer Keith and Townsend Whelen's works provide a perspective of the interactions in all aspects of the platform.
Greg