Every GM/player stumbles in RL (or on the internet) upon weapons, which should appear somewhere in the 40k Universe.
I, at least, found 2 which intrigued me long enough to convert them into playalbe stats.
1.In the Fane of Westingkrup the artisians specialise in making the deadliest but also simplest tools of murder and slaughter. When a customer once complained about a Carnodon Pattern precision Hand Cannon not having enough punch, he was asked what kind of punch he wanted from a pistol.
As an anwer, the customer pulled a machine gun bullet from one of his pockets and put in on the table. "That would a have a good enough kick" he said.
He was, of course, cursed and thrown out, for such an insulance.
One of the artisans however picked up the bullet, and the challenge.
For years he laboured and tinkered in his workshop, to either find a suitable design that would simply accept a bigger calibre, or commit the atrocity and create something new. In the end, he decied to simply increase an existing weapon in size till it could hold and fire the bullet. He chose a Model 20 'Scalptakter' Stub revolver. The artisans name was Falkner, Hrumingar Falkner.
The product, called the Falkner Zeliska (named after his deceased wife) is a revolver of huge dimensions. It is 550 mm (21,65 inches) long, up to 69 mm (2,71 inches) broad and weights 6,5 kg (14,33 lbs) unloaded alone. The chamber holds 5 bullets, and requires most people to use both hands to fire it. It is so far the most powerful solid projectile hand cannon one can own. And it has its price.
Falkner Zeliska Pistol 45m S/-/- 1d10+6 I Pen: 3 Clip: 5 2 Full 6,5 kg 350 Thrones Very Rare, even on Scintilla.
It only takes heavy weapon ammo, and requires a SB of 5 to be fired as a pistol, otherwise it requires both hands and counts as a basic weapon.
2. On the battlefield an Imperial Commander is often tasked with completing a mission with limited resources. In order to make the most of the often damaged tools of war he has his men in the field workshops must come up with quick solutions that help win the war, even if they have to ignore several rules.
One thing that infantry relying companies often face is a hard hitting, long range offering, but not to heavy weapons. No one can say if it was a struck of genius, or simply the desperate act of a workshop, when they decided to misreat the damaged Autocannons. By reducing the firing mechanism to singe fire, reducing the clip size, and otherwise stripping it of any other part considered unneccessary they came up with the Long Range Reactivated Delivery System, nicknamed 'LoRD' by most troops. It offers a greater range and higher fire power than a long las, while still being carried by a single trooper. The drawbacks, heavier than a long las, smaller clip and lack of RoF compared to a real autocannon are often dismissed as keeping the troops more mobile, and able to knock out heavily armored opponents, from Chaos Marines to light tanks. Field workshops of the IG produce during their campaigns more than enough of these weapons, and several have turned up in the hands of privateers.
LoRD Rifle Heavy 300 S/-/- 4d10+5 I Pen: 4 Clip: 5 2 Full Accurate 20 kg 1200 Very Rare
Feel free to criticise these and/or post your own interpretations of RL guns.