Swivel Guns (TL4-TL5)

[Icelander]

I want to play around with shipboard ordnance that can be used for boarding and counter-boarding. Because smokepowder in my campaign is extremely expensive, there is an economic incentive to make efficient and accurate high quality guns, to avoid wastage of powder that costs at least $700+ per pound.

The presence of flying monsters and dragons with armoured skins also means that powerful and accurate guns capable of swiveling to engage flying threats are a much higher priority than during the historical Age of Sail.

What I want are basically three sizes. Let's call them 1) Mounted rifle, 2) 1-pounder and 3) 3-pounder.

One would be a more powerful musket, mounted on a pintle, tripod or some other flexible-but-stable mount, with a rifled barrel. It should be light enough to be used from a fighting top in the rigging, but powerful enough to outrange any personal weapon.

The 1-lb cannon should be light enough to be mounted pretty much anywhere on the deck or quarterdecks. It would be used with a round ball at range, grapeshot for boarding actions. If it could be used with canister, that would be great, but for such a small bore, I doubt it's worth it.

The 3-lb cannon should be powerful enough to shred boarders with canister as well as grapeshot. It would be capable of using round shot against flying threats or the rigging of enemy ships during maneuevering.

GURPS Low-Tech features the .69 caliber Lantaka (TL3) on LT p. 88 and the 2.25 caliber Swivel-Gun (TL4) on LT p. 89. The smaller Lantaka weighs 30 lbs. and has a pintle that weighs 13.5 lbs. The larger Swivel-Gun is 140 lbs. and comes with a 65 lbs. pintle.

There is also an interesting weapon on LT p. 93-93, the 1.06 caliber Wall Gun (TL4). Weighs only 27 lbs., comes with a 7 lbs. tripod.

In GURPS Fantasy, we find another Swivel Gun (TL4) on p. 144. This one is a 3-pounder, weighs 315 lbs., but doesn't specify whether that includes a pintle. It also has some statistical oddities, such as the Weight Per Shot being less than the nominal three pounds of the round ball, which contrasts oddly with the similar WPS of the smaller 2.25 caliber Swivel-Gun.

Some adjustments might have to be made to the stats of the Fantasy Swivel Gun. At the very least, it would be nice to know the assumptions behind the weapon, whether it's supposed to include the weight of a pintle, why the WPS is so low, etc. The Fantasy Maximum Range seems low for pretty much all the TL4 cannon there, too, comparing the stats to LT.

In any case, I'm going to want to come up with plausible and balanced stats for these three sizes of cannon. The designers will be TL4+1 with TL2+1 and TL3+1 assistants and TL4+1 materials are available, if necessary, i.e. if the benefits of extremely light and strong steel, springs, clockwork or other high-tech doodads justifies the cost. If high quality bronze works, then it's greatly preferred, because there are a lot of unemployed people in that field.

Because of many factors, the PCs have access to quite a lot of good quality bronze and bronzesmiths without any business, so figure bronze costs about half normal. Because of the vast variety of uses for good quality steel and the limited production of it, figure that +1 CF is added to steel items that are better than rehammered slag, i.e. Good quality steel costs double and the Fine versions cost even more.

I want to keep weight down as much as possible, though not at the cost of performance. Flexible, but stable mounts are a high priority, as these make the weapons much more effective. I'd also like to make these small cannon more powerful and more accurate.

Smokepowder, as noted earlier, is extremely expensive. This means that historical tactics that relied on using a lot of it, because it was cheap, are not viable. Any weapons that use smokepowder are potentially competing with heroic archers using enchanted arrows, not just regular archers and slingers. There exists some REF 0.4-0.5 smokepowder, but the variety that the PCs use is REF 0.8, comparable in power to early smokeless powder.

a) How can swivel-guns be made more accurate?

Whatever it is that allows the Wall Gun to have Acc 2 as compared to Swivel-Gun/Swivel Gun Acc 1, I'd want to replicate that in the larger calibers.

Would making a better pintle be enough? If so, how much more expensive than a regular pintle would that be? Would it weight more?

Would mounting the swivel-gun on a tripod work better than a pintle?

b) How much more power can the swivel-guns listed take without risk of bursting or other accident?

Using REF 0.8 smokepowder* instead of REF 0.4-0.5 black powder allows for either a smaller powder charge and the same power or more velocity from the same load. This does create more pressure, but a lot of historical cannon were significantly over-engineered, because poor quality control meant that the occasional barrel was flawed.

Assuming better quality control (to the tune of +2 CF or so), would the swivel-guns listed in Low-Tech or Fantasy be robust enough to fire those ca 1.5-lb and 3-lb iron balls at significantly greater velocities than listed?

c) If you load these smaller cannon with lead balls, instead of iron, how do the stats change?

*Which has gasses going at almost 4,000 fps, as contrasted with the less energetic black powder at around 2,000 fps.