First off, please put anything final fantasy related on the backburner of your mind for a moment. I'd prefer people come at this idea from both a real life historical angle, and some of the older fluff in the 40k universe.
Now for those not familiar with the idea of a gunblade it is pretty much what it sounds like. It's a gun, that also has a melee weapon of some kind attached to it. In RL there were attempts from the earliest days of gunpowder tubes all the way up to modern times to create just such a weapon. Furthremore in 40k we see examples of this going back as far as the earliest days, and still occasionally popping up in the modern fluff.
Two 40k examples are the old nemesis force halberds of the Grey Knights and Adeptus Custodes which were halberds with a built in storm bolter, and the gauntlets used by Marneus Calgar. Eldar, orks, and Necrons also posess similar weaponry so it's not a foreign concept to the universe. in lue of the expanse there's also mention of a combination power glaive, hellgun.
So I've been plotting a simple way in which to create these items 'on the fly' for those characters wanting a little something extra and willing to pay for it. Thus I'm going to try and present some simple rules for their creation.
HOW TO MAKE A GUNBLADE FOR ROGUE TRADER
1) Select the two weapons you wish to combine and make note of which is the bigger one, and which of the two has the higher rarity. Increase the total rarity by one step. Only non unique components can be used for regular acquisitions, though the final item can end up as unique.
2)Two handed and or unwieldy weapons are always larger than one handed weapons. They may mount one compact basic or normal sized pistol weapon
3)One handed weapons can only mount compact pistol sized weapons.
4)A heavy ranged weapon can mount a one handed melee weapon but cannot mount unwieldy weapons, and gains the unwieldy trait.
5) Any weapon mounted to a melee weapon must have the compact upgrade unless it's a pistol on a two handed or unwieldy weapon. Furthermore the melee weapon loses the balanced ability, but retains any other bonuses to parry it may have.
EXAMPLE 1: A player wishes to mount a shuriken pistol to a chainsword the chainsword loses blanced and the shuriken pistol is now a compact shuriken pistol.
EXAMPLE 2: A player wishes to mount a bolt pistol onto a power fist. The bolt pistol does not have to be made compact. Were you to mount a bolter or lasgun instead, it would need to receive the compact upgrade.
EXAMPLE 3: Someone wants to mount a chainsword to a heavy bolter. The chainsword loses balanced and instead the weapon become unwieldy.
6)Take the weight of the heaviest weapon, and then apply half the weight (after any upgrades) of the lighter weapon. Note that this is the heaviest weapon, whether it's the melee part, or the ranged component.
7) Quality works slightly differently for combination weapons such as these.
POOR: Both weapons gain the unreliable, inaccurate, and imbalance properties. If it also has the unwieldy property the weapon is at -5 to hit.
COMMON: Normal rules apply
GOOD: Pick two of the following: Weapon regains the balanced property, may be a non compact weapon, or may have a +5 to hit in melee.
Best: Apply all three above, and add one to melee damage as normal. Furthermore one other modification can be applied to each weapon in the combo.
IE a best quality shuriken pistol, chainsword can now have a non compact shuriken pistol with the twin linked ability, The chainsword regains balanced, and is now +5 to hit in melee.
For those who play normal 40k, that makes the final example the equivalent of an Eldar Striking Scorpion's chain sabre.
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So, any thoughts?