Ah...that oldie. Nope, its not a bug (well, ok...) but perhaps one of the many irritating "features" of how OFP loads missions and addons. No need to suffer through reinstalls. Help is near ;D
You get that error because the hunting rifle is defined in the Res\O_WP.pbo addon under the BIS_WeaponPack class and your mission isn't using anything from that addon while your description.ext lists it as a selectable weapon.
The solution is to open the mission.sqm and add "BIS_WeaponPack" to either (or both, can't remember atm) the addons[] and addonsAuto[] parts. An example of how the top of a mission.sqm could look:
version=11;
class Mission
{
addOns[]=
{
"bis_resistance",
"BIS_WeaponPack",
"ah64"
};
addOnsAuto[]=
{
"bis_resistance",
};
...
...
...rest of the mission sqm follows here
As you can see, the addons[] part has "BIS_WeaponPack" listed. That will sort out the error message.
To have that done automatically, place a Hunter in the mission somewhere and preview it in SP editor mode (i.e not MP) and save it.
In short: the error comes from the mission lacking the "BIS_WeaponPack" entry in the mission.sqm and sometimes one has to add that manually. Open the mission.sqm with notepad and add it.
And yes, in a way, its related to the error "cannot load mission, missing addon BIS_Resistance" that pops up sometimes. Do a search in here or in the official forums for "addon dependencies" and/or "requiredAddons" to (perhaps) learn more about the problems.
Good luck!