They are not skilled elite assassins, they are skill-less thugs willing to
take a terrible but well paid job.
The blunderbuss is intended to be easy to use and lethal at short range. From the script:
The Looper script includes the following dialogue. I don't remember if it was actually in the film.
You know why they call that pea
shooter a blunderbuss? Cuz it’s
impossible to hit anything farther