There's probably no way lube on the bolt could damage the rifle, but it could cause firing pin issues.
Any lube inside the bolt/gas piston either gets blown out or burnt up in fairly short order.
So it might be:
1) harmless if blown out
2) eye hazard if blown out
3) negative if the burnt up lube residue is sticky around the bolt tail
4) beneficial by softening carbon if used after firing as a pre-cleaning treatment
For reference, HK also advises against any lube in the gas piston area on the HK416.
Our practical experience so far has been things run just fine with no lube in that particular area, hence the "not productive" statement.
As long as the rails and cam pin are addressed periodically with something like SLIP 2000 EWL, things keep running.
BTW, for a variety of reasons, a lightweight grease is probably the best product to use on the rails and cam pin area.
We haven't had enough experience with it yet to confirm, but the science behind it is sound.
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