Did a bit of experimentation with shotguns as a practical fortress defence. In particular, I was tring to find out which ammo worked well, when the intention was to reload and fire repeatedly. Some findings:
1) Boulders: quick to load, taking only five per cart, but despite their massive size they seem to be decidedly non-lethal. Most impacts involve only bruises, and any more serious injuries are more likely from secondary damage due to the target. Also, with only five submunitions per shot, the hit probability is quite low.
2) General observation: if your ammo can be stored in a bin, and you have the source stockpile for the cannon set to have bins, then dwarves will load up whole bins....including the bin, which defeats the point. Solution is of course two stockpiles, a binned pile set to give to an unbinned pile, set to give to the cannon.
3) General observation 2: If you have a cart set to push at 100% capacity, and you change the desired items mid-fill, then it will never reach capacity and never launch. Issue a temporary timed always launch order to despatch the mixed-ammo cart.
4) Blocks: probably the best cheap ammo - you can create hundreds of them early game once you hit stone, and if you don't need to weaponize them, they can be used for prettier buildings! 100 per shot can take some time to load though so you might want to fire at 25/50/75%, and you need to set up a two-stage stockpile (see above)because they're binnable. Also, 100 blocks takes up quite a lot of space, so your ammo store needs to be pretty large and well planned in advance if you want enough reserve for several shots. Damage wise they are superior to boulders, typically shattering bones or dislocating joints. A hit on an unarmoured chest is nearly always a fractured rib through the lung or heart, so without access to medical help they're done for.
5) Spikes/Corkscrews: upping the lethality on highly wooded maps. You will need to create a LOT - about 75 spikes/corkscrews are required for a 25% full shot. Take some cheap wood with you, and spend some points on a skilled carpenter, because quality makes a difference - a *menacing wooden spike* will cut and main, where the standard quality equivalent will scratch and bruise. Devastating against unarmoured opponents - an 80 spike shot will cause targets at 7-10 square range to be hit by between five and fifteen projectiles. The high number makes reloading very slow though. Verdict, more effort than blocks for early ammo, but much more lethal and with the advantage of a skilled carpenter to make the beds for the hospital you'll need after testing. Warning: dwarves caught in the blast have a high chance of being killed before they can be given medical attention. In testing, corkscrews were slightly more effective than spikes, causing hand and finger severing, and dealing two internal hits compared to only one for the spikes. Targets that aren't killed by blood loss inevitably give in to pain from multiple lacerations, so could be safely mopped up by melee infantry.
6) General Observation 3: Once you have defined a stop, you can remove the launch condition, meaning you can have it loaded ready with ammo to fire. Just add a suitable launch condition when the enemy is in range and so long as a hauler is close by, you should have quite fine control over when the shot is taken.
That's it for now, next tests are to try some metal trap components and spears, and also to see the effects on armoured targets. Any requests welcome, once I've mopped up the blood....again!