That is the copper and there is no give in copper. You might try the 350 gr lead one. There should be more give to that bullet
I had the opposite problem with them. Tried both the lead and copper ones and both slid up and down the barrel after seating them. I've measured both with calipers and they're right at .498". My gun uses .500" Thors which go down with what I would consider the right amount of force, so I think they're just a bit too small for my barrel.