Gun Nac is a video game made for the Nintendo Entertainment System. The game was released, September 1, 1991, developed by the Compile company, and published by the Nexoft Corporation.
The player assumes the role of commander Gun-Nac, and throughout eight different levels, fights a host of enemies from a space ship. The ship can be upgraded to a larger ship by acquiring a "wing", which allows the player to sustain two enemy hits rather than one, and allows the player to upgrade his weapons further than the standard ship would allow.
The game contains five primary weapons, as well as four types of secondary bombs. Examples of primary weapons are a standard gun, a flamethrower and a type of guided boomerang. All weapons have unlimited ammunition, and they are obtained by collecting circles with numbers that correspond to one of the five different weapons. The primary weapons can be upgraded by collecting multiple "Power P's" that float throughout the stage. The secondary bombs work on a similar principle; they are obtained by collecting circles with corresponding letters in them. The bombs, however, are limited in supply to however many the player can collect.
Both the weapons and bombs can be progressively upgraded throughout the game. When a player has acquired a particular type of weapon, she can upgrade her weapon level by collecting several of the same circles in a row. Each time another matching circle is collected, the weapon is upgraded. This is limited to five upgrades.
Using the same method, bombs can also be upgraded. When a bomb is upgraded, it becomes twice as powerful, but also consumes twice as much ammunition.
Money can be found randomly floating through the levels, or being released by destroyed objects or enemies; it can be used to purchase upgrades in a shop in between levels.
Source: Wikipedia, "Gun Nac", available under the CC-BY-SA License.