Anyone ever play Gunman? (Little known Halflife engine game).
It had slider configurable weapons.
The shotgun for instance could have its cone of fire, and number of shells consumed when firing changed.
The basic pistol had like 3 different modes as well.
Being able to say; configure the 3 basic properties of a weapon
DOT: Damage Over Time, a shotgun can fire 1 shell fast-ish or 4 shells once in a blue moon.
--Changes firing speed or number of projectiles created. (low values mean put all damage into as few shots as possible. Also affects recoil and accuracy.
Total Capacity: Bigger Mag = Heavier gun and fewer reserve clips, also more expensive. Less reloading. Won't be running any 1 minute miles there Rambo!
Absolute Damage: How much damage all the ammunition will do added together. More ammo == Weak bullets. Less Ammo == Anti-Tank Slugs.
So for a Rifle lets alter its properties as an example.
(exact values unknown, I'm just guessing)
--Standard Rifle--
DOT; Normal ROF = 500RPM/1 (8.3 bullets/sec)
TC; 30 Mag, 6 Mags total = 180 bullets
AD; 20~, x 180 = 3600 Damage Total.
--Turbo Rifle--
2X | DOT; 2X ROF = 1000RPM/1 (16.6 bullets, second

)
2X | TC; 60 Mag, 4 Mags total = 240 Bullets.
-0.25X | AD; 15~, x 180 = 3600 Total Damage.
3.75X Normal cost.
--Heavy Rifle--
-0.20X | DOT; Normal ROF = 400RPM/1 (6.6 bullets/sec)
-0.25X | TC; 25 Mag, 4 Mags total = 100 bullets
1.8X | AD; 36~, 100x = 3600 Damage Total.
1.35X Normal Cost
--Rambo Special--
2.5X | DOT; 2.5X ROF = 1250RPM (20.8 bullets, sec. !)
2X | TC; 60 mag, 2 Mags total = 120 Bullets
1.5X | AD; 30~, x 120 = 3600
6X Normal Cost.
So with that in mind these are all the 'same' weapons from a balance point of view. Averaged out over time, they will all perform the same.
When the Rifle is being bought, you would have an extra 'customize' UI button that lets you tweak the values with 3 sliders (range of 3X +/-)
This could be very interesting as you could customize the weapon for the situation.