Low Tech cannons mortars and shells

[Dalillama]

Apparently there were explosive musket bullets in the mid-19th century, which saw some use in the ACW. The St. Petersburg Declaration of 1868 barred the Great Powers from using explosive rounds smaller that 400g/1 lb, a prohibition later ratified by the Hague Conventions, which drew more signatories. Presumably this is part of why there aren't many examples of explosive ammunition for small arms.

[Anthony]

Originally Posted by Dalillama

Presumably this is part of why there aren't many examples of explosive ammunition for small arms.

Bans on weapon types usually only actually work if the weapon type isn't very effective to start with.

[Pursuivant]

Originally Posted by Polydamas

I don't think that the average three- or four-pounder-crew or naval gun crew had explosive shells in the 18th century (which GURPS calls TL 5). Instead, a well-equipped European artillery park would have some weapons with explosive shells.

You're right. Shells were specialist weapons until the early 19th century, when Henry Shrapnel's invention was adopted by the British Army. Even then, they were tricky to use due to fuze timing problems and only really came into their own with improvements in fuzing and shell design in the 1850s.

Pre-19th century shells were mostly used as counterbattery and siege ordnance. I believe that they were mostly used by large guns, as you describe, since the ratio of powder to shell volume improves with shell diameter, making them a more useful weapon. Swivel gun or "galloper gun" light cannons probably wouldn't have used shells.

That said, the grenades of the era were spherical, cast iron and probably tough enough to survive being shot out of a cannon. I don't have any evidence for it, but I can't believe that artillerymen never experimented with the idea.* Shooting cast-iron grenades out of a Coehorn-style mortar would be a way to make them useful as a siege or countersiege weapon.

*

 In pre-20th century armies, artillery and engineering officers were the inventors and intellectuals, which is why top-ranked West Point graduates went into those arms. Infantry officers were a mixed lot, and cavalry officers had a reputation for aggression and lack of book smarts. (E.g., a joke from the British Raj, "Did you hear about the new cavalry lieutenant?" "No? He was so stupid the others noticed!")

[Polydamas]

Hand mortars were a thing (with no stats in GURPS Low Tech!) https://en.wikipedia.org/wiki/Hand_mortar but as long as the fuse had to be lit by hand there were practical problems shooting shell from a long-barrelled, small-bore gun.

Early gunpowder artillery is really complicated and could easily be a booklet by itself. Even GURPS High Tech has one gun (!) to represent all traditional muzzle-loading cannon of the 18th and 19th centuries.

[johndallman]

Originally Posted by Polydamas

Early gunpowder artillery is really complicated and could easily be a booklet by itself. Even GURPS High Tech has one gun (!) to represent all traditional muzzle-loading cannon of the 18th and 19th centuries.

High-Tech for 3e has a bit more detail about gunpowder weapons in general, including artillery. It includes TL4, whereas in 4e TL4 is in Low-Tech.

[Rupert]

Originally Posted by Polydamas

Hand mortars were a thing (with no stats in GURPS Low Tech!) https://en.wikipedia.org/wiki/Hand_mortar but as long as the fuse had to be lit by hand there were practical problems shooting shell from a long-barrelled, small-bore gun.

Early gunpowder artillery is really complicated and could easily be a booklet by itself. Even GURPS High Tech has one gun (!) to represent all traditional muzzle-loading cannon of the 18th and 19th centuries.

It also has one gun for 'colonial adventures', one gun for WWI artillery, one gun for WWII and Cold War artillery and it's not the gun that I, from a Commonwealth country would consider iconic of WWII (the 25-pounder), nor the most common Cold War gun (the Soviet 122mm D-30), so it's not as if HT is especially slighting any given period.

[Polydamas]

Originally Posted by Rupert

It also has one gun for 'colonial adventures', one gun for WWI artillery, one gun for WWII and Cold War artillery and it's not the gun that I, from a Commonwealth country would consider iconic of WWII (the 25-pounder), nor the most common Cold War gun (the Soviet 122mm D-30), so it's not as if HT is especially slighting any given period.

A good thing is that gunners really like charts and tables and statistics (especially before they had computers to do their arithmetic for them).

So once you understand the basics of how GURPS represents a type of weapon, you can often rough out stats for similar weapons with a bit of research (I think Bill Stoddard has sometimes had trouble with details like whether a listed weight includes the carriage).

It would be nice to have stats for a mortar in GURPS Low Tech, but not sure I can name a page of High Tech or Low Tech which I would happily give up for more artillery.

[Varyon]

Originally Posted by Pursuivant

There is general information about black powder artillery shells in GURPS 3E High Tech 3rd Edition. It says that BP shells do explosive damage based on filler weight, which is usually about 10% of total shell weight. Fragmentation damage is 2d to 12d+ cutting damage per hex within the fragmentation radius. Material picked up from a ground burst material might add 1d-4 to 1d-2 HP cutting.

LE also shows up in 4e HT, on page 169. It notes they first showed up in the late 15th century, but didn't become common until the early 19th. It gives a value of 15% shell weight being explosive filler (black powder); it notes there is fragmentation involved, but as is the general rule for generic explosive shells in HT, it doesn't give any inkling of how much fragmentation is appropriate. I use a general rule of thumb that you can give up 1d of cr ex for 2d of fragmentation (to a maximum of fragmentation equal to the explosive damage), with the exact amount basically determined by whoever designed the shell. At low TL's, I don't think fragmentation is properly understood, so it might have a maximum of 1/2 the explosive damage, and I'd probably treat shells that aren't designed with any specific amount of fragmentation in mind as having 10% to 20% of the explosive damage in fragmentation (to be clear, this means a nominally 10d cr ex shell would typically max out at 6d+2 [6d+2] cr ex, at low TL's would max out at 8d [4d] cr ex, and if not specifically designed with fragmentation in mind would probably be around 9d+1 [1d+2] cr ex. This isn't necessarily realistic (it's based on the performance of the Diehl fragmentation sleeve, which is rather different from typical fragmentation), but at least it allows one to make explosive rounds that didn't exist historically (or at least aren't statted up anywhere)...

[Rupert]

Originally Posted by Varyon

LE also shows up in 4e HT, on page 169. It notes they first showed up in the late 15th century, but didn't become common until the early 19th. It gives a value of 15% shell weight being explosive filler (black powder); it notes there is fragmentation involved, but as is the general rule for generic explosive shells in HT, it doesn't give any inkling of how much fragmentation is appropriate.

Fragmentation damage is 1d per 20mm of warhead diameter if there's enough fragmentation to give a fragmentation effect as GURPS understands it.

I use a general rule of thumb that you can give up 1d of cr ex for 2d of fragmentation (to a maximum of fragmentation equal to the explosive damage), with the exact amount basically determined by whoever designed the shell.

UT doesn't seem to assume any loss of blast damage to get fragmentation. Nor do the stats in HT for the 105mm M2A1's HE shell and the 3" Stokes mortar.

Of course the DM51 hand grenade does, going from 5d to 3d+2 exp when the fragmentation sleeve is used (3d frag). This suggests half the explosive energy goes into the fragments. However, the way GURPS does fragments implies that as the warhead gets bigger the energy put into the fragments drops, as a fraction of the total (which is questionable), and for artillery shells it could well be negligible.

At low TL's, I don't think fragmentation is properly understood, so it might have a maximum of 1/2 the explosive damage, and I'd probably treat shells that aren't designed with any specific amount of fragmentation in mind as having 10% to 20% of the explosive damage in fragmentation (to be clear, this means a nominally 10d cr ex shell would typically max out at 6d+2 [6d+2] cr ex, at low TL's would max out at 8d [4d] cr ex, and if not specifically designed with fragmentation in mind would probably be around 9d+1 [1d+2] cr ex. This isn't necessarily realistic (it's based on the performance of the Diehl fragmentation sleeve, which is rather different from typical fragmentation), but at least it allows one to make explosive rounds that didn't exist historically (or at least aren't statted up anywhere)...

The big change in fragmentation of shells hasn't been an increase in the damage they'll do, but in the amount of fragments and the consistency of fragmemtation. In fact, a TL3-4 shell would most likely create only a few fragments, but they'd be very dangerous for a great distance assuming you were unlucky enough to be hit by one. The primary damage from those old shells was the blast and incendiary effect.

[Fred Brackin]

Originally Posted by Rupert

However, the way GURPS does fragments implies that as the warhead gets bigger the energy put into the fragments drops, as a fraction of the total (which is questionable),t.

With a fragmentation jacket of constant thickness (resulting in fragments of unchanged size), fragmentation will go up as the square while explosive charge will go up as the cube.

Even if fragmentation jackets got thicker the interface between the jacket and the charge only goes up as the square.