Why in the hell...

THAT is a great question! I wondered the same thing...

I think it probably has to do with making their guns difficult to trace.

Every criminal knows you never use the same gun twice even if you didn't fire it. New job, new gun. That's the rule.

They want "clean"[untraceable to themselves] guns that they can throw away after the job[s] .

Scalise (Alex Rocco) explains it in the scene at the trailer. They dump the guns in the river after each heist.

you dump the guns and stat fresh


~I see a little silhouette of a man, Scaramouche, Scaramouche, will you do the Fandango.

Police forensics can link bullets that have been fired to not only types of guns, but the individual gun used. Witnesses can describe what guns were used, and cameras can record them (if there are any). There are doubtless many other reasons why you would not use the same firearm to commit more than one crime.

Don't have time to read through the whole thread, but in case somebody hasn't said so already, it's a combination of factors, but one primary factor is that untraceable guns are typically dumped after a crime where they're used at all.