Don't think you will get 40ftlb out box I can get about 36ftlb with 18grain jsb but mines a 500mm barrel you put a 6 or 700mm barrel and you will get the speeds you want iv got a 700mm impact 22 barrel and if I put it on with same settings as the JSB it goes to 1040fps you can't put a power plenum like impacts on one but you can get a bigger hole in barrel port and heavier hammer Weight what people use to get speeds up to shoot slugs hope this helps iv probably missed something out but I'm sure mitch will be along to give you his thoughts as he has crown aswell