"Do the small bores use the standard Mauser locking lugs or just the safety lug?"
Safety lug only. Remember, the locking lug recesses are removed.
"but in the case of the shotguns where the front lugs weren't useable the headspace was set with the safety lug contacting it's shoulder."
Correct. One of the reasons the clumsy bolthead was made so large in the first place was to ensure the safety lug locked up. As I said earlier, the lug can stand up to repeated batterings of 15,000 PSI. However, keep in mind most rifle cartridges are up over 35,000 PSI-40,000 PSI; hence why it would be a "one-time-use-only" thing in a rifle just to deaden the force of impact, but it'd be a servicible locking lug in a shotgun.
I suspect a lot of why people think the locking lug is in danger of sheering off is because of the huge disparity between rifle and shotgun pressures. Even a perceivedly wimpy round such as the .22 Hornet has a pressure topping off around 40,000 PSI (I could be off by a few thousand, but it's around that). And that .22 Hornet would blow the lug right off if the barrel were somehow sleeved. But a 12 Ga. shotshell that is 2 3/4" inches long maxes out around 12,000 PSI (with 16 Ga. and 20 Ga. decreasing). Since pressures are really the only thing that matters about the gun's structural safety when it comes to the bolt supposedly flying out, it's well within the safe and normal range for a shotgun. I think a lot of people miss that disparity in PSI and either figure the pressures are similar or just look at it after having shot both rifles and shotguns for an entire lifetime and say to themselves "this can't be right." Well, it is right. One of many "strange but true" things about shooting.
