Question

What was Billy Dixon's lucky shot that ended an attack?

Answer 1

I think this is about the Second Battle of Adobe Walls June 27, 1874, isolated group of buildings in the Texas Pan Handle near the Canadian River.

Explanation:

The Native American tribes in the area were attempting to stop the commercial buffalo hunting that was occurring. The buildings housed services for the 200 to 300 buffalo hunters in the area.
A small group of 28 hunters hold up in the buildings held off repeated attacks by about 700 Native Americans on that day.
Later in the day some legendary long range shooting drove off the tribes.

The engagement of led to the Red River War of 1874-75 in which the US Army cleared out most of the hostile tribes from the panhandle by mid 1875.