Sorry OP, after 5 months of you not noticing the barrel being shorter than what you wanted I see no reason for the store to take it back. If it had only been a few days then yes you could argue it, but not 5 months. Stevebot brings up an excellent suggestion though, just look for a new upper and swap it out. Do your homework for any future purchases and you'll be less disappointed.
Seems a bit harsh to ban you from the store, but then I'm wondering if you perhaps left out some of the info regarding how you handled it. If you lost your #### and flipped out in the store then I'd have banned you as well. However if you were calm and polite to the employees then I'd say a ban is harsh.
From the very beginning of my raised concern, i was polite and civil talking to store reps. It went to the point where they told me that they are closing the door for any discussion and back-off. I asked them for the replacement of the upper but was declined. To be honest, i was surpised with the attitude and the way they treated me, they leave me no choice and chose to elevate the issue legally thru Sales of Goods Act. I refunded everything including tax but wasn't happy at all because i lost some good friends in there. I would like to just keep the rifle but they keep on ignoring all my calls so bE it.