Manufacturing a working firearm is relatively easy in 2100. Suitable ammunition may prove to be more of an obstacle. Convicts and others isolated from the local gun store have reloaded fired or homemade casings with compositions such as shaved match heads. How practical is this in a world where most ammunition is caseless? Military caseless ammo may be difficult to adapt to zip gun use and too powerful. Cased 9mm MAX ammunition is widely available from the blackmarket but may be too powerful for some homemade guns. Modern zip guns often use .22 rimfire rounds but how likely is it that these will be still in use in 2100? Hunting will probably be a less common activity by then. Target shooting, pest control and plinking is likely to done with easily rechargeable lasers and gauss guns. Immersive simulations will also partially substitute for these activities.
It is up to the GM as to whether rounds such as the .22LR are still widely in use in 2100. The round may have been superseded by the Caseless .20 Short round (GURPs Cyberpunk (3e) p.43)). Such a round could probably be used in a zip gun although there may be some leakage of gases at the breech. Damage 1d pi-; Range 50/200; Rcl 1. Alternately the .22 rimfires may have been replaced by a cased centrefire round that can be more readily reloaded. The 5mm Craig and Russian 5.45x18mm are possibilities ( GURPS Covert Ops(3e) p.72: treat as 1d(2) pi-; 75/1,100; Rcl 1).
Shotgun rounds are a possible alternative load for larger zip guns. Being large, low pressure loadings cartridges are relatively easy to manufacture, either by minifac or more traditional means. Various objects might be adapted to being used as projectiles. A round the equivalent to the modern .410 shotshell could be used in pistol-sized weapons.