When I was in law school, I interned at the US Attorney's Office in the Southern District of Ohio. It is pretty much par for the course when there is an administration change, especially when it's a party change as well, for all of the US Attorneys to be replaced. Nothing nefarious in this move by Biden. Who gets appointed in large part depends on the state. In some states, an attorney with the right political connections will get suggested by the state party. In other states, they will look for someone with actual experience as a prosecutor, again with ties to the party in power. The rank and file aren't purged, but new hires do tend to fall along the party lines of the current US Attorney. As for gun grabbing, I'll say this... I was an intern when Clinton was elected the first time. There was no change in policy as to what charges were filed in federal court. With the existing case load and backlog of cases in Federal Court, I can't see this change resulting in a sudden deluge of antigun criminal cases in federal court.