Quoted: Watch gunbroker under "Mossberg car"! He has the adapter alone with no rail system and it,s much cheaper, around $50!

And it does about a 1/4 of the things that the SST-590 does so the price is right for that part.

Quoted: The Gunbroker adapter also does one thing the SST unit does not: it allows use of the stock sights.

Mine was never intended or designed to do so.  The SST should be looked at as a total conversion, not merely a stock kit.

The SST adapters do the following the "gunbroker" adatper does not

1) Allows the use of Any AR15 stock
2) Allows the use of Any AR15 pistol grip
3) Includes docking ports for single sling attachments via uncle mike's QD sling swivels
4) Includes and optics mounting platform for propper check weld to use optics, and increased modularity for use with future accessories.
5) Mirrors the geometry of an AR15 identically in terms of  cheek weld, rail height, sight picture, and grip for easier cross training.*
6) Includes a new ambidextrous extended safety for improved ergonomics and easier manual of arms.**


* (the Mossberg's trigger reach is slightly longer)
**(Mossberg only)


Comparing the two products like comparing an Aluminum rail that bolts onto a plastic handguard to a SIR or RAS.  They both have their places, but they don't do the same thing.