I have two 390's for trap - HOWEVER, they are Super Traps and designated accordingly. There were two versions of the 390 Trap, a "TRAP" and a "SUPER TRAP," and a 28" and 30" barrel. The ST was ported, both had screw-in tubes and both are designated on the trigger guard. The ST was way overpriced at $1200 retail. I didn't figure it was worth more that a Remington 1100 Trap at about $800 then.
The field version (square back) operates a bit differently than that trap version too, in terms of unloading, if I remember correctly. I bought a square back, built it up for birds, but got rid of it due to its inability to hold the action open without ejecting a shell from the magazine tube.
If you want to shoot trap with a 390, I would NOT shoot the field version, unless you can get a parallel stock or an adjustable trap stock.
I don't shoot mine for trap, but installed synthetic field stocks and then fiberglassed them to be Monte Carlos as in a trap stock. In fact, many of our pheasant hunting gang did the same thing. We can't shoot a field gun for birds, because we shot trap for so long. Additionally, I normally shoot Remington Nitro Mags in the gun and even with its aluminum receiver, I have NEVER, EVER had one single problem.
A field gun's butt-stock's dimensions are absolutely counter to what a trap stock is.
IMHO.
Whiz