The Bullpup Unlimited kit can convert a Remington 870 or Mossberg 500 series to a bullpup without altering the gun, so it can be returned to standard configuration at will.
https://bullpupunlimited.com/
The up side is you get the reliability and strength of the standard gun without the complications of the odd-ball double magazine systems of many bullpup shotguns.
Accessories like magazine extensions work.
If you already own a compatible shotgun, you can get into a bullpup cheap.
The down sides.......
Like almost all bullpups other then the Kel-Tec's, the extended linkage trigger is wonky and the trigger is not very good. This has always been the failing of bullpup guns until Kel-Tec solved it with their KSG and the KS7.
The gun is not as smoothly operating as a Kel-Tec, or as smooth as the unmodified gun.
To deep clean the gun you have to totally remove it from the stock/cover by removing all the screws. Since the mounting nuts are brass and molded into the plastic, if you damage one it's going to be a problem.
Unlike guns like the Kel-Tec, it's right hand ONLY.
Unless you own a gun now that can be converted, I'd go with the far superior Kel-Tec KS7.
It appears to be the best bullpup pump shotgun on the market.