There are multiple wear / failure modes to consider and discuss with various factors feeding into each wear / failure mode. The "wear" points include the rifling in the bore of the barrel, in particular just in front of the chamber; wear of the trunnion, bolt, rollers (should be hard) and locking piece; any movement or shifting of the barrel (will skip this since Jeff worked on it, checked/replaced a trunnion, made sure the barrel was in and pin correctly etc.); build up of copper and carbon in the bore which effects accuracy; and a range of additional long term and/or low order failure modes that can pop up, but are best left to repair as they appear or long term reliability and end of life studies for HK (i.e. how many barrel changes before time to retire the receiver by caliber ;-).
Barrel Bore Wear - Loss of Acceptable Accuracy vs Loss of Function
- First failure mode of bore wear is loss of acceptable accuracy and on some bolt action higher intensity rounds with sub MOA "needs" can occur in a few thousand rounds of slow fire. For a 5.56 and the lower expected accuracy (i.e. not many if any C93's are 3/8 MOA rifles where the five round group nearly always fits under a dime vs having one wallet group with three rounds touching ...) one can expect many cases of ammo before having to worry. For discussion lets say it was a only a dozen cases of ammo (6000 rounds) when you started to see a larger group (again this maybe low for semi auto), one has to realize that you just spent over two grand on ammo and now have slightly less accurate rifle that was never a tack drive (side note at one friend's we use tack's for targets and try to drive nails, they tend to bend easily ...).
- One can also loose accuracy with a build up of carbon (the brass brush and kroil works well) and/or copper from the bullet jacket. You may wish to include in your cleaning a de-coppering phase every so often. I have been using alternate cycles of kroil until the carbon layers are out and then shooter's choice until the copper is out .... then repeat (the build up in the bore tends to be many layers of carbon and copper mixed together). While cleaning a good HK bore guide to clean from the breach end helps. Along with the correct brushes (i.e. I pass on the ones with sheared steel cores and the sharp center wire). For best accuracy a little more aggressive cleaning may give you better accuracy, but if you are not shooting prone and/or with a scope and/or with better ammo, the increased accuracy may never be noticed for a long time (i.e. what you are doing maybe fine and if some day you loose accuracy enough to hit your targets then de-copper the bore).
- One can loose accuracy from corrosion of the bore, kroil is a good cleaner and probably short term rust preventive in moderate conditions. For long term and/or higher moisture conditions have a storage oil in the bore is always a consideration.
- One major factor is how hot the gun gets when you are shooting. Once the barrel temperature gets high enough (common on full auto use, such as with a sear host) then barrel wears much faster and one can get "nice white sparks" which are the barrel metal. Thus getting the barrel very very hot and continue the shooting will greatly reduce bore life.
- Once you go past the point of lost accuracy, sooner or latter can get to the point where the reduced resistance from the worn rifling starts to effect reliability. I am thinking you may never see this, as this tends to be more the full auto world (close friend has been known to shoot 223's in the M16 until the rifling was no longer visible .... and stopped when there was not sufficient resistance to light tracers which is required for the night shoot at KCR ;-). An example that is probably worst than what you will ever do, is several of the guys (local and on line) have belt fed 223 HKs: one has three barrel, rotates and last I heard was well over 60K and going strong (or 20K plus per barrel); another friend has one barrel, likes shooting model air planes at the shoot and thus his barrel was used harder. At ten thousand rounds was well worn, at twenty thousand rounds did not work with some ammo (limited resistance) and is getting worst (several second to change barrels so no issue).
Reliability Issues Due to Excessive Bolt Gap:
- Once you get into the head space / bolt gap issues, you are in a combination of wear of the locking piece (easy to replace and depends on hardness, if was reground by Century etc.), trunnion (again depends on hardness), rollers (like you said can keep going up up in size and/or replacing other parts), bolt head hardness / construction (I have one 308 FMP bolt head that the roller window started to open up, assume on soft end) and what you are using for lube (HK recommended at one time CLP or Ballistoil ?).
- You may see a reliability issue some day with build up of carbon in the locking lugs and bolt face that will need a little more aggressive cleaning (again in the full auto world thinks tend to get dirty faster ;-) which in part can be addressed by a chamber face brush and/or a M16 chamber brush (using the stainless steel brush on the bolt face was an interesting experience the first time, little like cleaning a 1919A4 booster with heavier tools that one would more often use on farm equipment ;-).
