IMO You have a most desirable NIB Factory fired semi-auto SIG AMT .308 rifle.
I admire your restraint because I would have fired it many many times by now.
By having restraint you have in your collection one of the rarest guns in the world.
If you sell it - it would leave a hole in your heart UNLESS it accomplished your goal of obtaining an MP40 but the MP40 would not be in the same condition of the gun you have now ... ME I would keep the SIG and wait for the MP40 to come along ... I am a firm believer that if you're meant to have one it will come to you.
https://www.youtube.com/watch?v=m3S8YvIVK2g