Yeah, I figured people would have all sorta zany ideas for how tech, magic, and the planes would interact. Here're a few I've thought up:
Firstly, some trial rules for firearms on the planes:
Firearms are odd weapons, seen from a planar perspective. In places that are fueled entirely by blind belief, weapons based on logic do not function too well, mostly because physics work slightly differently on all the planes. Consequently, firearms take a -1 on to hit and damage rolls on outer planes(except Logos). When transported to a new plane, they take an additional penalty of -1 per plane they were moved(like magical items). This last penalty can be removed by recalibrating the weapon, an action taking one hour per plane removed from the last calibration. Each hour this goes on, the recalibrator must make a DC 30 Craft(mechanical) check. If this check fails, the hour is wasted.
The Snail Gun: Invented by the bytopian branch of Snail Outfitters(it's speculated that the gnomes were directly responsible for its creation), the Snail Gun is pretty much a legend on the outers for its extreme customisability. The gun is basically an assembly set, and Snail Outfitters provides several upgrades, addons, and styles(these last have been added only after the guns' release onto the planes as a whole.). These range from different barrel lengths and calibres, to two- three- four- or six- fingered grips, to barrels with integral and attachable silencers and red-point scopes, as well as drum-magazine and belt-fed magazines(of several different sizes, of course). Due to the extreme ease of assembly, the time a calibration takes is halved.
In addition, a character may make a Craft(mechanical) check DC20 which takes an hour, and if successful the Snail Gun counts as mastercrafted(this doesn't let it be enchanted, though) for a number of days(24 hours each) equal to the amount the check result exceeds the DC.
Initial stats for an unmodified(these are rare among professionals) Snail gun are as follows:
Damage: 2D6 ballistic
Critical: 20
Range: 40ft
ROF: Single
Mag: 15 box 9mm
Size: Small
Weight: 3 pounds
The Sodkiller:
This bullpup style SMG is standard issue to the Sodkillers' special forces teams. It incorporates a pump-action grenade launcher, flashlight, scope, and twin magazine feeds. Enchanted specimen aren't exactly rare amongst officers.
Damage: 2D8 ballistic
Critical: 20
Range: 40ft
ROF: Single, auto
Mag: two 80 box 10mm
Size: Small
Weight: 8 pounds
Näïlo:
These Olympus-manufactered rifles are regarded as prizes amongst gun collectors across the multiverse, and rightly so: Each is said to have been made over several years, and only five are known to exist. The origin of their name is unknown: some speculate that it refers to their creator(as "Nightbreeze" is actually a pretty common name amongst elves), while others believe that it refers to the near total lack of sound they make when fired. Despite them being totally nonmagical, the rifles are said to posess several unusual abilities, amongst them being:
Lack of any penalty for being on the planes.
Always counting as using subsonic ammo(without the penalties), even when they aren't.
Being integrally silenced(this, combined with the subsonic rule, means that it takes a DC 15 listen check just to hear the shot)
Näïlo rifles are always mastercrafted, but can not be modified to use a scope without losing all properties besides being integrally silenced(treat it then as with other longarms).
Damage: 2D12 ballistic
Critical: 18-20
Range: 200ft
ROF: Single,
Mag: 1 internal .50
Size: Huge
Weight: 8 pounds
If you thought hackers were annoying before just imagine hackers with the Magical Laptop with 1337, AweXome, and RoXXoR enchantments.
If you are going to go with the penalties for firearms like that, I really don't think the penalty should apply on either of the three strongly lawful aligned planes at all (the Arcadia, Mechanus, and Archeon equivalents, whatever their names end up being), logic and order reign in those areas fairly strongly. I'd make it for every step away from one of those three planes, making it really damn difficult to get it to work on Limbo. I'm not a fan of the idea to begin with though, it's just a needless complication, but I was never a big fan of the magic item movement penalty system to begin with because it just encourages just keeping massive amounts of weapons with a Pokemon "Gotta Catch Em All" type of attitude (which is already prevalent in 3.5 thanks to needing 75 billion different combinations of weapons to be prepared to fight various damage reduction enemies), and needless book keeping.
I can just see it:
Okay, I need a holy gun for fighting evil guys. But I need one calibrated to the Abyss if I plan to shoot there (just sitting around for hours calibrating it is asking for trouble if you are already in the abyss). Oh, and I'll need one pre-calibrated for Baator . . . and every other plane. Never know when you'll have to go to a plane and won't have time to calibrate that evil slaying gun.
I need an unholy gun for fighting good guys . . .
Fire gun for . . .
Yar! I be festooned with pistols like a scurvy dog!