Quoted from dmarston:You can find a list of gun games and their features in the Mr. Pinball Price Guide. IMHO, the principal differences are in the type of gun (most are rifles, but there are pistols, machine guns, and space blasters) and whether you play based on time or number of shots. Interesting variations: free-handling gun (tethered instead of mounted), shooting physical projectiles, and 2-player games.
.................David Marston
I think one of the key features that makes gun games unique or more desirable are their sound properties. Early games have more simplistic Bells, by the late 60s and early 70s, many games had switched over to primitive solid-state sounds. More interesting also additional background music. If you look on these threads, you will see most of the popular games incorporate those later more sophisticated sounds.