I have used one to give aspirin boluses. I have never, to my knowledge, placed an aspirin bolus in a horse's lungs. The gun is used to place the pill/bolus far enough back on the horse's tongue, and SAFELY for the handler, to prevent the animal from spitting it back out. Much like putting a pill on the back of a dog's tongue. You don't shove it down into their pharynx, just back near the molars. The horse will then have little choice but to swallow.
The aspirin boluses are large and not very slippery, so I'd always coat them with a little vegetable oil or cooking spray. Haven't used this method in ages, but it's perfectly do-able.
I used balling guns for years when I worked on the racetrack. If you do end up getting one, get the one with heavy rubber/plastic at the end, not the straight metal ones. They are more comfortable for the horses. I don't particularly care for balling guns myself. No matter how good you are with a balling gun, there's always some horses that gag and choke a bit before they get it down. I'd rather crush or soak the meds and using a dosing syringe. I don't think there's too many meds that you can't do with dosing. My favorite dosing syringe is a well washed out dura-vet pyrantel wormer syringe. They work forever as long as you keep them clean.
On a whim I just used my hand a couple weeks ago. Pulled the horse's tongue to the side and slid that tablet of bute to the back of his tongue before he knew what happened. Worked better than I thought it would!
For most anything other than bute I put it through a coffee grinder and mix it into their food. I find bute is too bitter for some to eat it that way.