This is very common down here. Both our daily papers from regional centers and our weekly papers are very poor about printing pro-gun letters. They will print both sides of the abortion debate, they will print taxation issues, and they will print both sides of religious debates. However, they will not print gun issues.
The weekly papers publish editorial views written by senior class members of the high schools. It is a requirement for the students of a class to write these, and the paper runs them. This is the only place I have ever seen a pro-gun stance published.
The sad thing is that it is easy for a newspaper to not print a letter to the editor. If you read their policies you will find they leave themselves a lot of exclusion rights that you can't attack.