Everything seems to have some sort of meaning, but the peacemaker just feels random.
1 Answer
In my opinion it is kind of memorabilia which Marty is leaving behind for Seamus.
We can also interpret it as a symbol of courage which Marty stood up for and defeated Tannen.
Since generation of McFly has always been bullied by that of Tannen's, it was a symbolic message from Marty to Seamus for being courageous so that coming generation of McFly can stand up for themselves.
Also because Seamus didn't have a gun and Marty didn't need it back in 1985.