I got an email from the Mossberg store. Listed barrels are 30% off. For the first time I have seen 590a1 barrels. Also there are model 835 barrels which do fit the 590 and 590a1 frames--as long as they are the 6 shot models with 18 inch barrel. This requirement is determined by placement of lug fitting over mag tube.
Note: the company's specs given don't show the compatibility described above. I have an 835 barrel that I use on my 590a1. Fits perfectly. Works perfectly. Chamber is 3.5. That matters not.