I beg pardon for the thread necromancy, however, there are a few factors that were not considered in this discussion, along with one major error.
The major error is equating the radius of a gas cloud with the effective area of burst of a High Explosive shell. A U.S 155mm artillery round weighs 95 pounds and carries about 15 pounds of high explosive. The effective area of burst for one round is approximately an oval of 180 feet perpendicular to the trajectory of the shell and 54 feet along the path of the shell. This is taken from FM 101-10, Staff Officer's Guide for 1956. It might be a bit optimistic, as the 1959 edition gives a more limited area. That may be based on a different angle of burst, as the area covered by a shell burst varies greatly depending on the angle of impact with the ground. It should also be noted that the effective burst area is defined as the area where a standing, motionless man on level ground has a 50% chance of being hit by an effective fragment.
A 155mm chemical round has a weaker shell body so as to be shattered by only a burster, basically a tube of explosive running down the center of the shiell body, the chemical filling, and the burster tube. Fragmentation is minimal. The purpose of any chemical shell is to place a lethal concentration of gas in a given area. You do not fire just one chemical shell. The most effective weapon for delivery in World War One of phosgene, chlorine, or a mixture of the two, was the Livens projector, an 8 inch smoothbore mortar that fired a gas canister to an effective range of 1450 yards. The British used them in massive batteries to place a lethal concentration of gas on specific areas, such that a single breath without a mask was possibly lethal. You can find the 1942 manual for the Livens projector online if you look for it.
The next thing that was not considered is exactly what the chemical round was loaded with. Generally, you were looking at phosgene, some form of a tear gas agent, hydrogen cyanide (which did not work that well), or liquid mustard agent. Mustard was not a gas, but a persistent liquid that was best as a blister agent, although if you had bad luck, it might kill you. The gases dispersed fairly quickly, hence the use of massive numbers of shells, and had this distressing tendency to drift downwind. That means that the user needs to take weather conditions into account before cutting loose, otherwise you might just gas your own men. That did happen. Mustard, being a liquid, did release some vapor over time, which did the eyes no good at all, and could stick around for anywhere from several days to several weeks, again depending on the weather. During World War One, where for a while the Germans had a monopoly on mustard, the Allies figured out that the Germans did not shell areas they were planning to attack with mustard, because of its persistence. So, if you were shelled by mustard, you probably were not going to the target of an attack in the next couple of weeks. Mustard also makes for a great land mine filler.
Smoke shells are typically base-ejection rounds, using a time fuze to blow the base of the shell of, and then eject 3 to 5 smoke candles along the trajectory of the round. That way the smoke is not all concentrated in one spot. Now, if you decide to use Adamsite smoke candles, you get both a smoke screen, and a nice cloud of vomiting agent. There are other chemicals that are used for smoke rounds without the vomiting bonus. White phosphorus is simply a shell filled with white phosphorus and a burster to spread the white phosphorus around, making for a nice smoke cloud, and also burning the daylights out of anyone hit by a piece.
The current crop of nerve agents are similar to the World War One gases in how they are delivered, with a lot lower lethality limit. VX is the equivalent of mustard for nerve agents, as it is also a persistent liquid, and not a gas at all.
Using chemical rounds is not a simply or easy matter, but it is an efficient means of causing casualties. Then you also have chemical bombs, the mustard land mines mentioned, cloud gas attacks, and the aerial spray tanks.
Took a while though since I focused on the smoke round parameters.
