[apoc527]

Even for those of us who are fine with the standard GURPS damage tables for melee weapons, the sling at ST 14 generates 2d pi damage. Without a reasonable AD, such as the aforementioned (.5), I think that creates a bit of an issue with suspension of disbelief. Sure, ST 14 is very strong, but that damage is 7 points of penetration...that'll get through all chainmail ever made. I just don't see that happening. With the (.5) AD tacked on, this doesn't happen. I have less issue with the sling going through leather, though even that is probably unlikely.

But, at the end of the day, either these weapons were useful or they weren't. We know they were used. We also know armor was used. And more importantly, in play we know that PCs and enemies will wear armor (perhaps even ahistorical levels of armor), and there's frankly no good gameplay reason to nerf muscle powered weapons down to no effective damage.

Edit: Here's a thought, maybe rather than a straight up (.5) AD, we apply the "edge protection" rule to sling bullets. Basically, if they can't penetrate, they can still do crushing damage, which I find much more believable.