I originally posted this on the collectables thread, but I am kindly advised that the knowledge is more likely to be found here

I've recently acquired an FWB 300 (what an absolutely beautiful thing, what a shame they don't make airguns like that any more) which has been fitted with a new breech seal. The problem is that it is a very tight fit against the loading cone, which means it is very hard to close the sidelever (far more force needed than I like to use) Could someone who knows this model please tell me if this always happens with a new seal, and it will bed in eventually, or should I be investigating why it is happening as a one off on this one?
Thanks for any help you can offer.