Depends on what you want out of it.
Ursus is a great assault rifle and really can hold its own against a lot of other weapons out there. It caters towards being used closer to its effective range than closer encounters, so you have to be careful of that. Other than that, it has decent overhead per shot, making it threatening and powerful.
The Volcano "AAEPD" JC (stuff Cannon) is essentially an alternative to the OSMAW and could arguably be claimed better than the OSMAW for a few reasons. The most notable difference is the longer it flies, the more damage it does. It's faster than the OSMAW in terms of flight speed, but has slight drop over distance, so you may need to angle your shot slightly upward to get those longer distances. It also flies in a very partial spiral motion, so be wary of that (but it really doesn't affect much). It's exceptionally good against all vehicles in the game, due to its flight speed, meaning you don't have to lead the target as much and it gives them less time to dodge the explosion. Furthermore, it's an AoE weapon, so it can hit multiple targets in one go.
Again, completely up to you. Ursus gives you a potent assault rifle, the Volcano is more for mucking about.