I should probably mention more on the scope. Smaller engines could carry mortars for residential fields near streets. Larger guns would be used on more catastrophic fires. I see firefighting artillery used more for open fires rather than on buildings, though I wouldn't limit the idea. Guns could be mounted on helicopters to strike at high-rise fires.

The MetalStorm company is already working on this. Their specific application was high rise fires, though. Current fire engines cannot pump much above around the 6th storey, so higher level fires cannot be attacked by appliances. MetalStorm was talking about using their ultra-high rate-of-fire automatic grenade launcher (which is still a prototype at this point) to launch a barrage of thousands of tiny fire extinguishing grenades into high rise fires.

I believe, very strongly, that use of artillery for the delivery is necessary because of its accuracy, mobility, potential for use in daylight or dark and because the concussion could effectively blow out some parts of fires. It could be coupled with GPS tracking systems on firefighters and monitoring devices that could ascertain if personnel were in the affected areas. Development of this delivery system could cause a monumental change in the future of firefighting. Mike E.

This might work, but you would have to shoot hundreds of shells to equal the amount of retardant delivered by one pass of an airplane. Shooting into buildings doesn't sound like such a good idea. I sure wouldn't want to be in the building when the FR howitzer shells start flying in! You might be able to ensure that no firefighters are in the building, but what about civilians?

If we're going to talk about using military weapons to deliver FR, how about using Unmanned Aerial Vehicles? Or guided bombs? Or even cruise missles? They're all very expensive, but at least they could deliver much larger payloads than cannon shells. The UAV could be used over and over, making it possibly more cost effective. Guided bombs would allow the airplanes to stay a safer distance from the fire. Cruise missles are way too expensive of course, but they would have the advantage of being able to be kept at the ready for use at a moments notice. (Well, not that fast really; they have to be fueled up right before use. But they could be ready in minutes.)

For use on tall buildings, how about using a helicopter with a water cannon? A CH47 with a side shooting water cannon would be awesome! A helicopter water-gunship.

You never know... Cruise missiles would be awesome for centralizing fire control. Imagine fighting a fire on the other coast.

But seriously, it seems this idea might work better in some sort of grand scheme where careful placement of retardant, accelerants, and ignition can help control a large fire, if not extinguish it.

Use in buildings would indeed be tricky, but need not be harmful to people. The shells would only have to penetrate into the building (obvious broken glass and projectile hazard), but not necessarily explode like standard weaponized shells.

The only problem I see with water cannons on helicopters would be balancing the force of the projected water with all the other forces at play. Might be better to keep a rooftop water supply for emergency use, and only rely on external stores in dire situations.

Any shell fired at cannon velocities will be potentially lethal due to its kinetic energy even if it doesn't explode. Even the FR itself would likely kill people if it hit them at supersonic velocity. OTOH, if you only use the cannons on the worst parts of the fire, it probably wouldn't matter. Anyone there would be dead anyway.

There is also the issue of impact damage. A shell hitting the wrong spot might cause a building to collapse. Can you imagine what the conspiracy theorists would have said if the NYFD had hit the WTC with a few volleys of artillery before they collapsed?

The recoil of a water cannon would affect the helicopter, but with practice, the pilots could learn to handle it. Those guys do practice a lot. The water cannon would be a bit small as water cannons go, being that the helo can't carry all that much water. A big cannon would also be heavy, which would further reduce the water capacity. So the recoil might not be all that severe.

The biggest disadvantage of the helo water cannon would be that it would be hard to get the water well inside the building where the fire is. The artillery would certainly be more effective in that regard. I suppose you could use both, but then you have to avoid shooting down the helo with the arty!