I should think that if they were burning atium, they could definitely steelpush a bullet. Hell, I imagine they could Neo their way around the shots. They could also push/pull against the barrel of the gun (almost certainly metal) or the ball inside before it was fired.
If it the Allomancer were taken by surprise or didn't actively target the ball before it was fired, I would think it'd likely travel across intervening space faster than they could react with a push. If they were pushing on it actively when
it was fired, I'm not sure what would happen... without an anchor, somone's probably gonna fly, and the gun would almost certainly explode (if the push holds the ball inside and effectively plugs the barrel).
As for Greatest Feat of Pewter-Strength...
I'm inclined to think it's the end of book 2, but I'm not positive. t's worth remembering that when Vin was pewter-dragging that koloss sword, she also then leapt through the damn sky like classic golden-age Superman, and brought that same sword down through a full-grown, presumably armored man, and his horse.
That's a neat trick.
But I don't know if it would count as the most powerful example of a pewter-burn in the books. Have to re-read them all for that.