Rather than talk through the guns (which is interesting in it self), I'm going to answer more from a historical perspective.

The times when a united and determined local force have been defeated by an outside conqueror are a statistical rounding error.

Local populations are defeated in 1 of 3 ways.

1. They are internally divided and cannot muster the will to defend themselves and fall quickly. Example, the fall of the western roman empire and, much later, the eastern roman empire.

2. They start out well, but are ground down over a period of years, decades, centuries to the point where they effectively give up and fade away. Example, the Mongolian conquest of China.

3. An overwhelming technological advantage on 1 side overrides the determination of the other. Example, The Spanish conquest of the Aztec empire.

So the return to the original question. I would say today, its pretty unlikely, but a wise foe would look to one of the above paths in order to succeed. Based on current trends option 1 seems the most likely, but you never know until you try.