Understand that I am not a smith so please excuse my terminologies. I have repaired this problem before and have more Contenders than I care to admit. Problem seems to vary from barrel to barrel. When you pull down on the trigger guard, there are two fingers inside that push on two lugs of what I call the locking wedge. You will find these on the lower rear of the barrel lug. If you hold the muzzle up on the barrel and push down on a soft surface, you will see this lacking wedge retract against a compression spring.
The problem is that this locking wedge, is not clearing the engaging area of the frame. You will notice that the locking wedge, has a taper on the top side. This tapered area is where it locks on the frame. The taper has a range to accomodate the difference in frames. You are on one end of it's range and thats why it's so darn hard to disengage. I would leave the frame as is and sure you will see a difference if you try a different barrel. So what was my fix??? Hope I say this right!!
There is a flat face on the end of the locking wedge, that ransitions to the taper. Carefully file and hone this flat face and it's important to keep it square and true. Perform this a little at a time
just until you see some results. Listen to the project and it will tell you when you are there. Be very conservative.
Or, you can send it to these guys and they will take care of the problem and more. .....
http://www.bellmtcs.com/store/index.php
Be Safe !!!