My first marker was a good old Splatmaster, which looked a lot like a .45. That is probably the one you remember your friends having. I do not think that they will shoot as high a velocity as the new markers are allowed (velocity is normally capped at 280 - 300 fps and you must fire the marker over a chronograph prior to play). I think that the Splatmaster probably fired around 250 fps.
Until the late 80's, the Splatmaster was about it. Then you started getting stuff like the PGP, which looked like a thick pistol with a pump on it. Pumps were the main marker from about 1988 to 1993 or so, if memory serves. After that, the semiautomatic market took off. Electric markers, which you pull the trigger and a solenoid actually fires it for you, started a while ago but I noticed them getting big sometime around 2002-2003. Nowdays, pretty much anyone who plays competitive uses an electro (as do legions of annoying 16 year-olds).
However, you are right that the paintballs might have had a thicker shell back then. Since a paintball must break for the hit to count, most paint these days is manufactured to be fragile as possible without leading to breaks during loading or firing. I still get bruises most of the time through the old set of cammies I wear to play. Tournament players buy special paintball jerseys and pants that are loose and billowy (to reduce the chance a ball will break).
My newest marker must run on compressed air. I could use CO2 in it, but the valve was made to run on compressed air. The tank you see on it is a 68 cubic inch, 4500 psi fiber-wrapped tank with a 800 PSI regulator. I get about 1000-1100 shots from a full tank. Any marker will run better on compressed air, because you get better consistency. My old Automag did use CO2 for a while, but it used the classic valve that was originally designed when CO2 was the only thing available.
Very few people use 12-gram anymore, except those who play in pump leagues. Pump is a lot of fun, so I have been thinking about picking up a good pump marker like a Phantom:http://www.phantomonline.com/paintballguns/paintballguns.htm
I have never read Tom Kaye (the daddy of Automags) say that the design was based on a real firearm. I would say it looks that way because it is designed around the valve. Maybe you think it looks like a M3 submachine gun?http://world.guns.ru/smg/smg32-e.htm
This is a thread on the Automags forum with lots of pictures. People tend to customize their markers, because the AGD products are very modular:http://www.automags.org/forums/showthread.php?t=5895&page=72&pp=30
The valves, both the old and the new (this is the new, same as I have), all look roughly like this:http://www.airgun.com/Products/xvalve.shtml