I'll go ahead and contribute to this since this is a Cole-creation. Before I start, though, let me just remind you all: Cole is an engineer, not a smith. He engineers armor more so than he smiths it.
The armor relies on several inset sheets of steel (but of course, one can have this custom-tailored on request), layered over one another in sets from top to bottom of the armor, rested against, pretty much, a mud-filled sponge, the mud being composed of sand and some water rather than dirt and water. It's contained and sealed in a leather mesh with the more sensitive parts toward the inside. At the right ratio, moisture can strengthen a body of soil or sand by filling the empty spaces between molecules, and anything more than that will weaken the overall strength.
The armor holds up well against kinetic force and piercing attacks by distributing force, and weighs less than chain-mail or scale-mail with similar capabilities, but has the same drawbacks to other attacks as most leather armor. It's defensive features are also chiefly in the front and back. The sides are more susceptible to attacks of any kind. In addition, if the mesh is cut open or the seals are melted off, or for whatever reason the mesh is opened, etc, and the inside gets exposed to excess water, it would ruin the armor indefinitely, and Piotr would have to have to give it to Cole for serious maintenance.
What Piotr is referring to is Cole's somewhat famous 'Needler' round, the grapeshot that shoots 16 small needles rather than mini bb's for a devastating short-range attack. It makes full use of the shot's forcelike a shotgun's grapeshot, but has the neat feature of piercing or exploiting even the smallest cracks of someone's armor, and generally works on everything but plate.
To put it into perspective for -this- armor, the armor manages to stop it through various layers of force absorption and force distribution, though (of course, it'll still look really beat up afterward.) Note that this is only in the case of an attack directly from the front or back; the chance of surviving such an attack from the sides is far less likely.
The standard price for this armor is 100 gold when made with the cheapest materials. Cole made a special exception with Piotr in bartering for the potion.