Real-world criminal science questions.

[acrosome]

This is all for the sake of a modern-day monster hunter campaign where the characters would presumably want to avoid hassles from the authorities for... well, for being modern-day murder-hobos.

Obviously, ballistic matching of a firearm to a fired slug is a thing. How well does this work with shotgun pellets, or for that matter with slugs out of non-rifled shotgun barrels?

Even if you swap out the barrel in, say, an automatic pistol (which is just a drop-in part with modern Browning-style pistols) a fired brass casing can sometimes be matched to the impressions left on the rear of the casing by the action of the gun. How reliable is this? As reliable as matching a slug to a barrel?

One could, of course, use a revolver to possibly avoid leaving casings laying around. But you'd preferably have to get rid of the revolver afterwards. You could, with great effort, change the barrel on a revolver if you really tried, but replacement barrels are not common items so acquiring one without leaving a credit trail will be a challenge. Well, except for revolvers meant to swap barrels easily, such as some of those the Dan Wesson specials, I guess. But those are at least semi-custom and thus, again, straightforward to track.

Has anyone statted the various sizes and calibers of SIG p320? I ask because every part of that gun except a small trigger group is legally a non-serial-numbered "part" that can be bought without ID (in the USA of course) and replaced, thus potentially avoiding even the casing impression matching issue. The authorities would probably be able to identify the gun make and model, but not match to the specific individual gun involved. Right?
Now, it turns out that most of the replacement part kits are bought over the internet directly from SIG USA, using credit cards or PayPal of course, and thus those purchases would show up on a credit check if one were under suspicion. A very few (incredibly unusually large) gun shops do stock them, though, so players could find such a shop, show up in disguise, and buy with cash.

Brass catchers are a thing, but really only practical on long arms, right? The ones for pistols are cumbersome as hell and I, personally, would give skill penalties for using one. Not to mention hard to conceal.

Another option is using the common AR-style rifles, and just buying a new upper every time you use it. Right? Or am I missing something?

Of course, one could just throw a gun away every time it is used. But unless you have underworld contacts and deep pockets that may not work for long. And buying an underworld gun always risks the possibility that there is already a body on it, for which one could be blamed if caught with it.

Anyone have other clever ideas for confounding the fuzz? I know a lot about firearms but almost nothing about criminology.

[Refplace]

Originally Posted by acrosome

Obviously, ballistic matching of a firearm to a fired slug is a thing. How well does this work with shotgun pellets, or for that matter with slugs out of non-rifled shotgun barrels?

Pellets? not at all.
Slugs, a little possible as wear on the barrel can leave indications but seems rather remote.

Originally Posted by acrosome

Even if you swap out the barrel in, say, an automatic pistol (which is just a drop-in part with modern Browning-style pistols) a fired brass casing can sometimes be matched to the impressions left on the rear of the casing by the action of the gun. How reliable is this? As reliable as matching a slug to a barrel?

More a matter of luck. The gun would probably have to have a lot of wear to change from factory standard.
Sometimes though you can lift a fingerprint off a casing.

I dont know if it went anywhere but there was an effort to pit chemical markers so you could track individual lots from chemical residue.
The best defense is to not have anyone looking at you, next is to leave nothing for them to find in anyplace connected to you.

[(E)]

Shell casings can also be checked for the marks made by the firearm's firing pin and mechanical actions.

The rifling marks made on the bullet may change after a number of additional rounds have been fired down the barrel. Though the differences may be just legally different rather than fundamentally different.

Perhaps using a gunsmiths skills to clean, polish and replace the parts of a firearm that come in contact with any part of the bullet. Then use the MOS as a penalty to the forensics roll?

[RogerBW]

Sources: various. I may be out of date. Do not rely on this information in a real-world endeavour.

To a first approximation, no ballistics on shot. Some shotguns do have some barrel rifling (it does make the slug ballistics much better), and there are also saboted slugs that will engage with it (in that case you'd police the sabot rather than the slug when cleaning up the scene, which may be easier). If you don't mind being basically at shot range when using the slugs, you can probably go competely non-rifled.

If you are worried about forensic evidence, put a lot of ammunition through the gun after you fire the criminal shot - rifling microstriations change after 3-5 shots and this can make an evidential match harder.

Firing-pin and ejector marks may be unique, but aren't always. In the cases I've read about, they're used for "this is consistent with the gun we found on the accused" not "only one gun in a million would leave these markings".

Brass catchers are favoured by some special forces troops but they tend to be bulky and inconvenient. Stats in High-Tech p. 161.

Really your best bet is just to use a commonly-available type of gun and be prepared to buy new ones. It probably won't be a lot more expensive than the parts kits you're talking about. Ideally, buy your "use" weapon from somewhere dodgy just before you get into a fight, and dump it afterwards, so that the time it's associated with you is as short as possible; your "reserve" weapon is squeaky-clean with all the paperwork, and is the one you keep ready in case you're attacked unexpectedly.

[PTTG]

If the monsters can hide, the PCs can hide. Maybe there's some kind of standing magical field that makes fights with monsters leave obscured evidence? It could be that killing a monster undoes its magical energy pattern, resulting in it "having never been," so the fight and some of its most recent attacks are undone. People being injured or killed, however, stay dead -- they can't be brought back. They just died by mundane ways, leading to urban legends.

Alternatively, the heroes could use "magic bullets," which are made from magically enchanted things so that they degrade quickly.

Or maybe it's more active; the PCs are friends with gnomes or pixies that are able to magic away some consequences of fighting.

[Varyon]

If by monster hunters you mean Monster Hunters (the line of GURPS books), I’m pretty certain a rather simple Lesser Transform Matter ritual would be able to make subtle changes to your weapon so that, for the duration (or as long as you continue to renew it), your weapon leaves different impressions on cases/bullets than normal. Just keep that ritual going (or have your witch friend do it; it would have a really low energy footprint) until you have to use the weapon, then dismiss or let it end. If the cops come for it, you can hand it over without fear, as it’ll test clean (so long as you’ve never used it unaltered). If they don’t, just have the ritual cast again (it should rerandomize the markings). Eventually something will go screwy and you’ll need to use the weapon in unaltered form; because of this you will want to stop using it (altered or otherwise) if the cops ever test it (and obviously you need to ditch it if you ever do use it unaltered).

Without magic, you probably need to frequently swap out components or outright ditch your weapons, which could become very expensive very quickly. Of course, if monster hunting is a rather lucrative profession, this may not be a problem, so long as the characters can secure a source.

[David Johnston2]

Originally Posted by acrosome

This is all for the sake of a modern-day monster hunter campaign where the characters would presumably want to avoid hassles from the authorities for... well, for being modern-day murder-hobos.

Obviously, ballistic matching of a firearm to a fired slug is a thing. How well does this work with shotgun pellets, or for that matter with slugs out of non-rifled shotgun barrels?

It doesn't. There's no way to trace shotgun pellets.

Even if you swap out the barrel in, say, an automatic pistol (which is just a drop-in part with modern Browning-style pistols) a fired brass casing can sometimes be matched to the impressions left on the rear of the casing by the action of the gun. How reliable is this? As reliable as matching a slug to a barrel?

Police your brass!

[acrosome]

General Commentary:

OK, good to know about the shotgun pellets.

I was unaware that rifling striations wear so quickly that you could just dump a mag through afterwards and be set. Good to know.

I'm aware of the fingerprint danger on ejected casings. Thanks, though, for mentioning it.

Originally Posted by PTTG

If the monsters can hide, the PCs can hide. Maybe there's some kind of standing magical field that makes fights with monsters leave obscured evidence? It could be that killing a monster undoes its magical energy pattern, resulting in it "having never been," so the fight and some of its most recent attacks are undone.

Well, there will always be human minions involved, which leave durable corpses that may not always be easily disposed of in a timely manner. Especially if, for instance, one is running from the fight.

Originally Posted by (E)

Shell casings can also be checked for the marks made by the firearm's firing pin and mechanical actions.

Yes, I thought I had mentioned that?

Originally Posted by Varyon

If by monster hunters you mean Monster Hunters...

I do not. That was non-capitalized intentionally. :) Sorry for confusion.

Originally Posted by David Johnston2

Police your brass!

Much easier said than done- experience speaking, there. The damned things seem to just disappear sometimes and end up in the damnedest places. In vents, behind refrigerators, tiny holes in artwork, eaten by pets, stuck to a piece of gum behind the headboard, in a corpse's pharynx, etc. I would NEVER rely upon it. Ever. Seriously.

[Varyon]

Originally Posted by acrosome

Much easier said than done- experience speaking, there. The damned things seem to just disappear sometimes and end up in the damnedest places. In vents, behind refrigerators, tiny holes in artwork, eaten by pets, stuck to a piece of gum behind the headboard, in a corpse's pharynx, etc. I would NEVER rely upon it. Ever. Seriously.

I’m now inclined to believe you did indeed get that grant from your signature, but lost the goats to a pack of chupacabras before you could start testing. Enraged, you hunted them down, and in so doing discovered more powerful monsters behind them, hence this thread. I’m on to you!

But, yeah, I’d imagine the tiny things would be hard to track down in a post-chaotic-shootout mess, even if you do have a moment to try and gather them up (which is by no means guaranteed). “Burner” weapons may indeed be necessary if you expect to attract police attention (granted, you want to avoid that anyway - they can still deliver you to their secret masters arrest and charge you even without a murder weapon, but there’s no reason to make their jobs easier). Hopefully you’re doing a good job looting vampire crypts and what-not, and have a fence who is good at turning your curios into new weapons.

[Anthony]

Originally Posted by acrosome

Obviously, ballistic matching of a firearm to a fired slug is a thing. How well does this work with shotgun pellets, or for that matter with slugs out of non-rifled shotgun barrels?

It doesn't even work very well for rifled bullet, though monster hunters are prone to using weird ammunition (silver, etc), and that would certainly be distinctive even without evidence from rifling.

Originally Posted by acrosome

Anyone have other clever ideas for confounding the fuzz? I know a lot about firearms but almost nothing about criminology.

Reality is, most people get caught by way more basic errors than this, and the most likely way for monster hunters to get caught will involve eye witnesses, cameras, or them leaving blood and other genetic material at crime scenes.