Here is what the Remington site says (just copied/pasted)
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We do not recommend the use of steel shot through any barrel manufactured before 1963 or through any barrel having a fixed Full choke. Anything larger would not perform well out of a fixed full choke and could open up your muzzle over time.
If you have barrels manufactured after 1963, with fixed Modified or Improved Cylinder chokes, you may shoot up to size #2 steel shot. The use of steel shot larger than size #2 is only recommended in modern barrels with the Rem Choke system.
If you have the Rem Choke system, you may shoot any size steel through the Improved Cylinder and Modified choke tubes. The Full choke tube must state "For Steel or Lead" to be capable of handling steel shot.
