What is the difference between a 590, A1 and special purpose

[JimFromFL]

Apparently there are a number of 590s available but have not been able to obtain a visual or descriptive enough information to clarify them all.
What is the difference between a
590
590A1
590 Special Purpose
590 Milspec

 

[jayhway]

From what I understand,
The 590 A1 or Milspec is the version that was tested by the military and meets military standards. The major differences btw a 590A1 or 590 special purpose is that the A1 has the following:

1) Heavier barrel
2) Metal safety
3) Metal trigger guard

To most people, the differences btw the 590 and 590A1 are miniscule and not noticable.