The pulse rifles and the smart guns were designed by James Cameron, along with a lot of other equipment in the movie.
Excerpt from the Aliens Special Widescreen Collector's Edition (laserdisc)
"Chapter 15
MARINE WEAPONRY
Looking for the biggest, brightest muzzle flash the crew could find, they selected various different contemporary weapons and converted them, with the assistance of the art department, into the Marine weaponry. It was very important to the filmmakers that the weapons were believable, and not so unusual that they distracted the audience. For each weapon, a lightweight fiberglas version was made as well, so that it would be easier for the actors to carry around.
M-41A PULSE RIFLE
The pulse rifle is a complicated hybrid between a Thompson submachine gun, and a Franchi SPAS 12 pump-action shotgun mechanism, placed in a futuristic shell created by the art department. The pulse rifle is, in every practical sense, a working firearm.
SMART GUNS
The smart-gun was also designed by James Cameron, and is a hybrid of a Spandau-type machine gun, the MG-42, and a Steadicam harness, which enables the operator to move the heavy gun with ease and grace.
The smart gun has a place for an ammunition magazine on the side, and is a fully operational weapon. However, the smart-gun had a baffle in the barrel, so that the wooden blanks being fired would simply disintigrate, and disappear, rather than actually firing."
The flame throwers and sentry guns were also real, although there isn't as much detail about them. The flame thrower was made with different versions that could fire 5 feet and 45 feet. The sentry guns fired 600 rds/min, and the tripods were radio controlled.