Spot Shooter,
I found something similar when I switched the forearm off a Survivor, and added a forearm from a Ultra Rifle. The action would open very stiffly. It first appeared that the heavy barrel was too big for the barrel channel in the forearm, but after looking more closely, I found the barrel stud was located on the barrel just a little closer to the reciever on the Survivor, than it was on the Ultra. I just removed the spacer and took a bit off the reciever end of the forearm. It didn't take much.
Don