Nearly every cannon round fuse is built in and goes off when it hits something hard.
Howitzers and mortars call this a PD fuse for Point Detonation. It's an impact fuse.
This is the default fuse for GURPS: Gun Stats.
But there are others!
Most common is the Mechanical Timer Super Quick or MTSQ. This is a fuse that has a clockwork timer in it the gunner can set to go of before impact, if they time it right, it creates an airburst over the target and more things get attacked by fragments.
Proximity fuses do the airburst thing too, but at a fixed altitude rather than selected time from firing.
MOFA are Multi-Option Fuse, Artillery. This is everything all in one. Impact, timer, and proximity.
Just to be confusing, there's also an anti-aircraft proximity fuse, but the sensors look sideways instead of forward.
All of the timed and proximity fuses I've found have an impact mode too.
Where this gets sticky for GURPS terms is your basic M107 HE round can take any type of fuse changing it from a PD to an MTSQ to a Proximity round with the spin.
The entry for all of the timed and proximity rounds is "Airburst". +2 cost factor to any round. Which makes it a little awkward because the various base rounds will have different prices. One would think that the price increase would be the same for all rounds of the same chambering.
Worse, some rounds have to be timed fuses or they just don't work right. DPICM, for example.









