The outcome of an untreated fracture does largely depend on the nature of the fracture. There are many factors at play, including whether the fracture is open or closed, the stability on the fracture, and whether the leg is rested.
A simple, stable fracture can often heal uneventfully without surgery. Splinting can be helpful in some cases. Pain management and strict rest may be all that is necessary.
A more complex fracture, where there is an open wound, severe instability, multiple bone fragments, or (as in your cat's case) foreign material such as bullet fragments in the fracture site - these are not likely to heal on their own. A fracture with an open wound or bone exposed to the outside will be very susceptible to infection, which could be life-threatening. In the event the fracture does heal on its own over time, the leg will likely have chronic pain and be susceptible to arthritis. Muscle atrophy is a possible outcome also.
I did see a recent case which I think is quite relevant here. This is a young cat who fell off a third floor balcony, not once but twice over the course of a couple months. After the initial fall, he was limping but this seemed to resolve fairly quickly so the owner never brought him into a vet. A few months later, he had another fall, but did not seem to be getting better as quickly, and this is when I saw him. It was over a week after the second fall that he was brought in to me. These were his x-rays:
Similarly to your cat, this cat had fractures of his tibia and fibula. There are two primary fractures on the tibia. The more distal (lower) fracture shows some periosteal proliferation and callus formation, which is what happens when the body tries to heal a fracture. This is the hazy area between the fracture segments. I suspect that this is the original fracture from a couple months ago. The more proximal fracture appears more recent with no callus formation.
On his exam, the fracture did appear very stable, meaning I was unable to move the individual fracture fragments much. The cat was partially weight-bearing by this point.
I sent this cat to an orthopaedic surgeon for a second opinion and they advised to hold off on surgery for this cat, as the fractures were stable and minimally displaced. Although the x-ray looks terrible, with rest and pain management we are hopeful that this one will heal on its own. Time will tell, hoping to have a progress check on this cat in a few weeks.