The suitcase itself doesn't need to be hardsided, but the gun case that goes inside the suitcase does need to be hardsided. I use a softsided carry-on bag*, though obviously I check it when traveling with a gun. I lock the suitcase with a TSA-approved lock, but not the gun case. Also, I use a heavy duty cable lock to secure the gun case to the suitcase's frame (accessible via a zipper in the lining). A potential thief may be able to get into the suitcase, but likely won't bother with taking the time and effort to try and get the gun or gun case out.

I also recommend printing out the airline and TSA regulations and bringing them with you whenever you travel in case an issue comes up.

* ETA: Double-check with your specific airline's regulations, but I didn't have any issues with my softsided suitcase.