weapon bame-blunderbus
basically a very primitive gun that was used during the middle ages
not sure of the stats for this weapon there I will need help.
I think your post is in the wrong forum.
This sounds like it should be in Northern Crown.
Due to the extreme lack of blackpowder weapons in 1220 AD Ars Magica ,
there will be no stats for such a weapon.
Lack of agreement with the settings and rules never stopped abe before. Like Zaratustra to Good and Evil, he is beyond such trivialities. 
Northern Crown is a great recomendation. I would also suggest looking at the Video Game Arcanum, a steam punk game. Inspiration can be drawn from there and stats from you best judgement.
Something else to keep in mind is that all the early firearms tended to be more dangerous to the person using it than the target. I would say any "0" that gets rolled, botch or not, means the weapon has blown up in your face, inflicting it's damage on you, sans (without a) soak roll. That is more than enough reason for me to use one, at least until House Verditious comes up with the Musket, the first reliable firearm.
Black powder isn't very available in the early 13th century. Maybe House Flambeau has it; Doisseteppe was mentioned to have 'ignem sand', brought back from the crusades. Who would want a weapon that took a year's income to fire once, because one had to finance an expedition to the Levant to get the black powder?
Most 13th/14th century plate armors were proof against the firearms of thier day. It was the footsoldier, in greater numbers, armed with crossbows and things like the godentag that rendered the armored knight a moot point, not the gun. The Musket drove the last nails into the knight's coffin, though.
Steve
Steve makes a great point about the armor being able to withstand the 'bullets' of those days.
Read about Ned Kelly
en.wikipedia.org/wiki/Ned_Kelly
And if you can't read, watch the movie.
thanks for the input & yes I was thinking of gunpowder weapons of this era.