The proof house is certainly a very good visit especially the museum, it will certainly make a few reloaders slightly more cautious. Sweep you're right about numbers being limited.
The Royal Armouries museum is also another great place to visit if you have an interest in firearms though I found it a little upsetting, I kept saying "I used to own one of those" over and over again. It makes you realise what has been taken off us over the last 25 years.