Maybe really a change to add damage to Called Shots by DoS already would do the job...alongside any buff for accurate.

As the weapon stats for low RoF weapons often are already better.

The single-shot weakness is, as Nimsim says, the relative lack of RF, specifically for instant kills.

Against Novices, nobody cares - novices die fast anyway, and the damage of single-shot weapons already has a good chance of ending them.

Against Masters, nobody cares - sure, critical hits are good, but they're not that important. The higher chance of single-shot weapons to pass through Defence makes up for this.

Against Elites... there's the problem. Higher damage won't help - single-shot weapons are already pretty steady in passing defence. A higher wound result could be good, but compared to killing them outright, it's not a big deal.

The most obvious solution would be to allow slow weapons to gain (or improve pre-existing) Vengeful qualities in certain cases.

Possible solution (that incorporates another proposed change):

For every weapon that has an RoF of 1/2, change RoF to 1 and give them "Slow(2)".

For every weapon that has an RoF lower than 1/2, change RoF to 1 and give them "Slow(1)".

Slow(X)

This weapon's RoA can never exceed X.

If this weapon is currently benefiting from a Called Shot action, it counts as having the Vengeful(9) quality. If the weapon already has Vengeful, improve the quality by 1 (ie. Vengeful(8) becomes Vengeful(7)).

You could make doubles on the D100 a jam for automatics, or define a critical failure zone that's larger for automatics than single shot weapons. 1 in 10 chance instead of 1 in 20 for example.

You could also make rolling doubles on damage dice jam the weapon as well.