Can Sylveon ribbons handle guns used by gunners? No
there would not be enough magical energy / mana to be extracted from the appendage to power an effective bullet, it's not because there is not enough surface area to extract mana from, but its the amount of mana flowing to those parts of the body for the gun to use to make an effective bullet. keep in mind a gun user in this do not cast spells in the conventional sense but instead manipulate their mana to alter the properties of their bullets so essentially it kinda comes down to can you cast a spell using a Sylveon ribbon appendage? im gonna have o say no
then there is the existence of mana crystal powered guns that don't use the wielder's mana, those guns are more similar to our own traditional realife firearms , and they tend to have much more kick and recoil to each shot and would require a reload of a new mana crystal once it depletes without adding any more kill-power or special properties, i would say sure a Sylveon appendage could use these types of guns, but it would require two or three to have any kind of stability, handling, and consistent accuracy, also the bullets fired from those guns do not compere to the bullets fired from the vampuric counterparts, and also it would likely be restricted to pistols, and other small arms s anything large the recoil would be too great to handle