Fire mode switching doesn't work for pistols with the auto-fire mechanism installed, though burst fire will still work if you manually rebind "burst-fire wielded item".
The volume increase for caliber retools seems a little high. Adding a .45 caliber retool to a S&W 619 increases the volume from 2 to 4. The revolver doubling in size just because I swapped out some parts doesn't make much sense to me. It also further marginalizes the use of retools.
Also, with the addition of spare magazines, I think revolvers need a little something to balance it out.
Speedloaders and the ability to
extend their barrels for increased damage and accuracy would be nice.
I've also noticed that it is possible to attach more than one under barrel attachment to a gun. While this is cool, it doesn't make a lot of sense.

This is a rifle with an attached grenade launcher, crossbow, shotgun, and spare magazine.