From Halopedia, the Halo wiki
It is a remodeled version of the Gravity Hammer with no change in attack power or speed. It is the same size, but has the shape of a standard, real-life, golf club.
In Halo 3, it has the icon of a Grunt on its head. Its reticule is circular and perforated like a golf ball. Bungie likely remodeled the Gravity Hammer design for the 7 wood for novelty purposes, as no other use for the 7 wood (or the other golf items, such as the Golf Ball and the Cup) has been found. The 7 wood was a new addition to the Forge scenery selection, with the arrival of the Mythic Map Pack. It only appears on Sandbox, a large open terrain map with three levels. Sandbox is considered a very Forge-intensive map. Due to the expansive forge abilities of Sandbox, this is likely the reason the 7 wood is only available on that map. The 7 wood is also found in Forge mode in Halo: Reach and also in Firefight.
In Halo 5: Guardians, the 7 wood was added as part of the Warzone Firefight update to Forge. It lacks the icon of the grunt on its head, instead replacing it with a UNSC icon; on the bar of the club is the word Misriah.
In Halo 3, Halo Reach, and Halo 5: Guardians, the 7 wood is available in Forge; more specifically, in the Mythic Map Sandbox. It can be found under the "Scenery" section, as opposed to the "Weapons" section. It can be used as a melee weapon in a similar way to the Gravity Hammer.
Bungie added two other golf objects to go with the 7 wood - a "cup" and a golf ball, most likely as a novelty object for Forgers and custom gamers to mess around with while playing Halo 3 with friends. Players who use the 7 wood for combat would use Gravity Hammer tactics, as they have the same effects, but as a novelty item, it was likely that players using the 7 wood will only be messing around in Forge.
The 7 woods have also appeared in matchmaking in the Grifball court Chichén Itzá, where teams starting in the lower court can grab a 7 wood right after spawning. Halo: Reach's Grifball variant, Hand Egg, also features the 7 wood as its primary weapon. They also appear in Halo: Reach in the Action Sack playlist, and in the HaloBall and HaloBall Xtreme gametypes.