The trolls in my story have a very thick and nigh impenetrable skin. In the time before guns, you would need a very sharp high-grade steel blade to breach the skin and, even then, after a few hits, the blade would be dulled significantly.
The trolls are very thick-skinned and the skin resembles a stone like material like thick callouses. It is covered in layers of moss and lichen. The trolls are turned to stone during the day and they don't really care about how clean they are. The trolls will be roughly 4-5 meters high (10-13 feet)
I did some research and the breaking point of a claymore is 2624 newtons.
A 9mm bullet weighs 7.45 grams and will travel roughly at 310 meters per second. Then 0.00745 kg * 310m/s gives us an impact force of 358 ish newtons.
A baseball hitter can hit hit a baseball with much greater forces than the breaking point of a claymore.
So let's put a guess at penetration at roughly 2200 newtons needed with a sharp edge.
So a sword seems a lot more effective than a 9mm gunshot. Would a skin that can withstand ordinary sword slash and jab attacks from a skilled swordsman be able to resist bullets from modern day firearms? My basic lookups seem to indicate so. If so, what caliber would be needed for penetration?