I have a Mossberg 500A 12 guage. It has a 26" barrel, and holds six shells in the mag tube. I'm wanting a Mosssberg 18.5" replacement barrel for home defense. I want to make sure it will line up, with the threads for the barrel nut. Any help is much appreciated.
Speaking from experience, take the shotgun with you when you go barrel shopping.
I purchased a 19" for my 870. Got it home, took the old one off, did a quick comparison, then installed the new barrel. The next day, I discovered that the action would not completely close with my new barrel. So, I called the store of purchase and they told me to bring the gun and the barrel. They could not get it to work either. So, they happened to have another 19" barrel in stock and swapped them out. No problems since.
The lesson I learned: Save a trip and bring it with you.
I do not know what to suggest at this point. I will say that I looked around the internet prior to purchasing locally. I opted to go local for the very reason of your post. My two cents.