In many ways the SST-590 is similar to the SST-870, but there were several design changes to make the concept more suitable for the Mossberg platform.
What is the same
1) T6 Aluminum Billet construction, Type III milspec hard coat anodizing
2) Designed to use optics effectively on your shotgun, existing iron sights cannot be used.
3) The docking port for the rail is the same as on the SST-870 so any future accessories will work with both units.
4) Allows the mounting of any AR15 buttstock and pistol grip.
What is different
1) The trigger reach is longer than the SST-870 (and longer than an AR15). This was unavoidable for 2 reasons;
a) cost of machining would have been greatly more expensive to position the pistol grip farther forward
b) for ambidextrous access to the slide release
We believe it was more important for the shooter to be able to put their finger behind the trigger guard to activate it. As the result of this longer trigger reach, grips such as the ergo grip and Fobus G27 grip may not work well for some shooters as the trigger reach will be too long
2) New Safety
The SST-590 comes with a new safety to replace the factory safety. This safety is completely ambidextrous, allowing the shooter to disengage it with either the trigger finger or thumb with relative comfort.
The safety is machined in such a way that no dirt or grit can get around it or behind it to impede function.
Installing the SST-590 requires complete disassembly of the gun to install the new safety. While this may be easy for many people who have experience and the correct tools, we have to officially advise that this is a gunsmith/armorer installed item. Due to the large number of Mossberg 590s/500s that have been produced over the years, we cannot guarantee that some minor fitting will not be required.