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, only on 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 away as soon as they spawn. Halo: Reach Grifball variant Hand Egg also features 7 wood as the primary weapon. They also appear in Halo: Reach in the Action Sack playlist, in the HaloBall and HaloBall Xtreme gametypes.