I have a quite nice old Western Field 16ga pump action that I paid $100 for some years ago.  It is in good shape for its age and after I bought it I realized that it was actually a JMB designed (design then sold to Stevens) 520 from probably around 1927 to 28?  It works ok but the barrel is worn out and not really trustworthy IMO.  

The rest seems fine but it almost seems impossible to find barrels for these old things.  I did finally find a 24" 620 barrel in 12ga (which is fine if it works) for $75 and so I ordered it.  Based on everything I've read the early 520's like mine all used the same 12ga frame (whether 12, 16 or 20) and it was only later that they started to build lighter frames for the sub guages.  The 620 used the same platform but had an improved receiver design. Mine is a double hump with a suicide safety. The 620 used a smooth back with a tang safety but I can find no indication that barrel design changed.

They have quite good reputation and looong history but does anyone have any experience with these old warhorses?

The newer 520's/620's used a slightly different receiver height, just enough to where the older ones can't use the newer barrels. Found this out when I bought a 20" barrel for mine and it was about 1/8inch too tall

When you say newer 520 do you mean the 520A? I think that was around the 30's that Stevens started to modify the receiver. I think they stopped and switched to the 620 around WWII but from what I understand even the 520's were still in use clear up until vietnam in military versions.

Either way it is good to know because I am not completely certain of the age of mine.  I'm hoping though that what you say is true...that an older 520 can take a newer(ish) 620 barrel.

Apologies as I was typing that on my way out the door and made a typo (corrected in my OP), the older shotguns cannot use the newer shotgun barrels. My bad.

I've had three of these, one I tried to just replace the barrel (and keep the old 24" inch because why not) but as stated they don't fit and I couldn't find an older barrel so I just cut the original down and sold the extra barrel on the EE, had it cerakoted and added a fiber optic front sight, fun gun! One of the others had already been cut down to 18.5" and fired but the lifter pawl was MIA so you had to manually load it, also stenciled "stolen Seattle WA 1973" on the side of the receiver , minus the stencil its the nicer of the 3 (and has been claimed by the wife). Stole the lifter assembly from the 3rd which was a westernfield knockoff in pretty rough shape and gave that one to a buddy for a "man cave wall gun" with the understanding that even though it fired it would never cycle. He was pretty stoked.

All of mine have the hump and suicide safety, so they are very early guns.

I just took a 620 barrel and installed it on a 520 humpback and it fit and cycled fine.

I've never had a double hump one, but I can say the 520a and 620a can use the same front ends.  Some minor fitting may be required though.

Thanks and good to know. Beautiful old shotguns you've got there btw.  I should have the barrel tomorrow so my fingers are crossed.