Well, remember it is -1 on the attack, then if successful +3 effect.
If you just wanted to give effect on a gun (which are usually in the range of Weapon:2 or Weapon:3 to begin with), I would make it just +1 effect (with no attack penalty).
Or +2 under a specific circumstance.
Although, if you want to be technical, Double Tap (in real life) isn't about more damage, but a better chance of hitting at least once (the shock of getting hit tends to disable most amateur gunmen). So you could say double tap lets you do Guns + 1, but it uses two bullets and can only be attempted with an autoloader gun (revolves, breach loader, and bolt action weapons need not apply). But I would also say it can't be combined with a spray attack (if you had such a stunt or were using an automatic capable weapon in automatic mode).
Using 2 bullets doesn't seem like much of a cost for +1 on Guns, but it doesn't seem any less strict than Sanya's "I use Russian guns at +1" stunt.
You may say "What if both bullets hit?" Well that could just be a really good fudge roll (+2 or more on 4dF could be both bullets hitting). Excess Effort (a.k.a. Effect) converts to physical stress.