[Lazer]

"A .25 caliber round carries a LOT more powder than a .22, and thus usually much louder. However, if she had pulled the slug, dumped most of the powder, and re-seated the slug (not a very difficult task) that would eliminate much of the noise and still provide enough penetration."

The problem with this is that all of the .25 cartridges I know are chambered in semi-autos and a reduced load would fail to cycle the firearm. What assassin is going to want this?

Someone mentioned removing the rifling inside the barrel. From an investigators view, have at it, he's now looking for a weapon that's been modified, and undoubtedly the act of drilling out the rifling inside the barrel is going to leave milling marks the forensic examiner is going to have a hay-day with when it comes to linking the projectile to the weapon.

Blowback causing an auto to jam? Not in the real world, but this is fiction.