The latest count of the number of guns in Borderlands is 17,750,000. 1 Each has a unique appearance and effect. Gearbox is using a process called procedural generation to create all the guns. In this practice, developers write procedures into code which dictate the generation of content.

Attributes

Color

According to Gearbox producer Simon Hurley in an interview silver weapons have greater chance of achieving an extreme critical hit, while black weapons will fire quickly, but lack accuracy. 2According to a poster on the Gearbox forums, the color of a weapon signifies its type. Yellow is high explosive, green is acid, and blue is electrical. There are also red weapons.3 Electrical weapons are significantly more effective against warp shields than other additional damage types.4

Length

According to Randy Pitchford in an interview, the length of the stock affects stability.

Weight

According to Pitchford, the weight affects how much the gun moves when a player is running. (Also see "Three Ways Borderlands Differentiates 1 Million Guns")

Ammunition

According to Randy Pitchford from Gearbox,"in this science fiction world, the fabrication of matter is not a problem. You can jam particles together and make something......you still have a magazine, you just need to create the matter somewhere and it goes into the magazine, which loads into the gun." (Source)

Manufacturers

Each gun in Borderlands has a manufacturer. Varieties include Jakobs, Hyperion, Vladof, Dahl, S&S and Maliwan. Each manufacturer's weapons have certain strengths and weaknesses.

Jakobs

The Borderlands Game Informer preview discussed the manufacturer Jakobs. This family-owned manufacturer makes low-tech but high-quality guns. Jakobs weapons typically do above-average amounts of damage and have a lower rate of fire.

In a recent interview, Randy Pitchford commented that Jakobs guns tend to use wood as a construction material. this makes their guns sell for larger amounts of money, since there is no wood to be found on Pandora, and therefore must be imported from offworld. He also commented how jakobs guns tend to have an 'old west' look, which is due partly from the use of wood,  (ie wood stocks, revolvers, lever action, etc) 

Compared to Jakobs products, Maliwan guns tend to be made of cheap, highly available materials, such as plastics. this tends to lower the resale value.

Statistics

When examining a gun, a box appears at the bottom of the screen with a list of numbers. These represent ratings on the gun's power, accuracy, rate of fire, reload time, ammo regeneration, and secondary effect.

Trading

A player can trade weapons with another player during a co-op game. The new equipment remains with the character when he returns to a single player game. 5

1. 1. http://www.computerandvideogames.com/article.php?id=220328
2. 2. http://www.computerandvideogames.com/article.php?id=171982
3. 3. http://gbxforums.gearboxsoftware.com/showthread.php?t=68730&page=4&highlight=high+explosive
4. 4. http://www.gearboxity.com/component/option,com_transmissions/task,/Itemid,33/p,5/
5. 5. http://borderlandsguide.com/forum/ill-swap-you-green-shotgun-two-purple-rifles