Here are some ideas I've been toying with. see what you think.

Custom ammunition:
Custom ammunition is specially crafted to work perfectly with a specific firearm. When an attack from a firearm uses only custom ammunition designed for that specific firearm it gains +1 accuracy. Availability +2 cost x 3.

High velocity ammunition:
High velocity ammunition uses a special propellant with a higher velocity explosion to propel the ammunition, resulting in a higher exit velocity than standard ammunition. Unfortunately this propellant lacks the kick of its conventional counterparts and only provides a noticeable advantage to light-weight projectiles, which naturally reduces stopping power. They are still popular with sportsmen and others who require accuracy at range over killing power.
HV rounds: Damage -1 P, AP -, Availability 6 R, 60 *

HVLF ammunition:
High Velocity Low Fragmentation ammunition uses rigid titanium-steel composites in place of the solid brass of regular HV rounds. The result of this tweak is a vastly superior performance against armoured targets at extreme range, though they pay a heavy price by inflicting even less tissue damage than conventional HV ammunition.
HVLF rounds: Damage -2 P, AP -4, Availability 14 F, 150 *

*HV and HVLF rounds reduce range penalties by 1 band, for example an extreme range shot only takes the penalties of long range. this is cumulative with other methods of reducing range penalties. HV tracers use regular tracer prices.

Thoughts?

I would never use HV or HVLF rounds, but that is just me. Custom Rounds are interesting, though.

In order: lolwut, lolwut, and... lolwut.

Ammo of a given caliber is interchangeable; that's kind of the whole point. A gun chambered in, for example, .45 ACP will fire all .45 ACP cartridges. The only noticeable differences from one brand to another are generally in reliability, but Shadowrun doesn't model that.

Higher velocity, but less damage? This violates basic physics. A projectile of a given mass, accelerated to a higher velocity, will impart more energy at impact. F=ma, and all that.

Titanium projectiles instead of brass? The brass is in the casing, the slug is made of lead (sometimes with a copper jacket).

Well, there is such a thing as overpenetration. a projectile moving at high speed without tumbling or pulping up will simply make a neat tiny little hole.
if it's not something important, there will not be enough energy imparted to do any meaningfull damage at all usually.
which is the whole point of hollow point and frag rounds. stop over penetration, do more damage in the hit target.

this is wihout going into shock etc.

HV rounds are made of lightweight metals like brass rather than lead, so Force = Mass times velocity having less force at higher velocity works if you drop the mass. Rounds that don't fragment get better penetration but don't break up in the target, meaning that they have less stopping power (deal less tissue damage).

Lead slugs, to the best of my knowledge, don't normally fragment unless they hit something hard (concrete, steel, etc.) or unless they're designed to. Moreover, a lighter slug will generally have worse penetration, not better, due to having less inertia: it will hit a vest and stop, where a heavier projectile would have the momentum to get through. A lightweight projectile cancels out the benefit of a higher velocity. If you really want maximum armor penetration, you're talking about a very dense flechette: the best ones are made of depleted uranium. Plenty of mass, coupled with a very small cross-section to put all that energy in one very small point.

While it's true a lead slug won't *fragment* all the time, it DOES tend to deform on contact and transfer more energy to a soft target than a much harder slug would.

A harder slug will NOT deform and as a result will continue travelling, minus the amount of energy it took to penetrate.

The "HV" slugs that FuelDrop is showing work off the principle that a lighter slug (brass) with similar or higher amount of propellant than a normal slug, will accelerate the bullet to a higher initial velocity and result in lower ballistic drop. The flipside is obviously less damage due to less mass, slightly offset by a higher acceleration due to impact speed. This is why HV slugs are -1P.

The HVLF use both principle of “lighter ammo, higher initial acceleration” and “Harder ammo, less force transferred on impact, better penetration”. The point of both is less the damage output (hence why both actually are WORSE off than normal rounds) and more for accuracy over longer range (due to straighter flight profiles).

Yes, Depleted uranium is/was used as an armor penetrator due to having close to the same weight/density as gold, having a natural tendency to fragment into sharp edges, the fact that it has a tendency to ignite on impact and the fact that some people feel like they have to get rid of all that waste uranium SOMHOW. However those pretty much fall under APDS rounds already (since most depleted uranium rounds were discarding sabot) and in modern times is mostly replaced by the harder, denser, more-expensive-but-less-questionable Tungsten rounds.

So, any other ideas? What other cool or otherwise ammunition types do you think would be useful?
Ultra-cheap low-grade ammunition that fares worse against armour and penalizes accuracy but is so cheap and easy to get that it's attractive to street-level games when you want to spray and pray without a massive price tag? How about some incendiaries or acid-filled specialist rounds for dealing with spirits? Smart self-dividing bullets that can hit two targets with one shot?

For ideas on many, MANY, MAAAAAANY types of ammo, may I suggest GUPRS High-Tech? Even if you won't use the rules, it describes a lot of different kinds of ammo and how they work.
Hell, they even have rules for wooden bullets

I'd replace the '(custom) rounds designation with something lke accurized or match-grade ammo. Basuically round manufactured with tighter tolérances and a more reliable propellant, combining to reduce the dispersion (better controled amounts of propellant and more costant comosition = less variation in bullet speed, and a better quality bullet that means a more accurate flight path). Though maybe give a bonus to the limit rather than the dice pool - you'll need accurate aim to exploit the improved acuracy.

At least it's not a F.G.M.P.
(Fusion Gun, Man Portable)

"[The FGMP] fires what amounts to a directed nuclear explosion. Those without radiation protection who are nearby when a FGMP is fired will suffer a lethal dose of radiation – each firing of an FGMP emits 2d6 x 20 rads, which will affect everyone within the immediate vicinity."
(Oh, Traveller...)
((Not to mention that the PGMP and FGMP were the only guns to have rules for missed shots...including overpenetration*))

*The World's Most Durable Man wearing the World's Most Durable Armor had a 50% chance of surviving a single shot. Not including the radiation poisoning.

I love that gun. The only contender for its place in my heart is the ARMP (Armour Rifle, Man Portable) which is basically a sniper rifle on every drug ever.

Also, in the edition I have the FGMP and PGMP are both full auto capable and have effectively unlimited ammunition. When does the fun ever stop?

Sounds a lot like the FA Plasma Ejector in Space Master...