If you're doing an Internet search for "mortar gun," it will be more productive to try "grout gun" instead. Otherwise, you'll be confronted with products that help you clean out an enemy machine gun nest. The type of mortar gun used for home renovation and repair has a much more modest aim -- to ease the pain of applying grout between wall and floor tiles or bricks.
Most of us who own homes have been there. The problem is figuring out a way to get a material the consistency of toothpaste to remain inside a narrow path without slopping over onto the tiles themselves. It's difficult to find a trowel-type tool that can operate effectively in such close confines, and the result is often doing the same job twice -- once to apply the grout, a second time to clean off the spatter.
Among the devices that have been developed to deal with this sticky -- literally -- dilemma are the grout float and the grout bag. The former is a flat hand tool that aims to funnel grout into crevices more efficiently than a standard trowel. The latter operates much like a tube of toothpaste, squeezing the contents out from the bottom. As with toothpaste, however, dispensing the grout becomes more and more difficult as the level diminishes.
The mortar gun, meanwhile, features a long, anteater-type nose and a larger capacity for grout than most bags. It actually won't reduce the need to stretch or stoop, as one mortar gun Website promises, but it is easier on the body when the job requires applying grout to a wall or ceiling. Some guns use a plunger device to feed the grout, others a trigger. Aimed properly with both hands, a mortar gun can facilitate inserting mortar more neatly between tiles or bricks, with less of the inevitable overflow that makes grouting a multiple-step process.
The mortar is loaded into a cylinder that fits into the rear of the gun. One advantage over a grout bag is that the mortar gun can be taken apart from the side and cleaned more easily. Another is that the device feeds the grout toward the nozzle more efficiently.
Try to estimate the amount of grout to be used as closely as possible, so it doesn't remain inside and harden. Clean the device after each use. Also, adding WD40® or some other lubricant to the inside of the cylinder helps keep the grout from sticking to the sides.