There is no need to reinvent the weel.. 1-14 the correct twist for up to 80gr .. no matter if it is for hunting or longer shoot.. 1-14 twist is useless for anything heavier than 85gr .. bullet lead in should be designed around the 257-420.. at 74gr is probably the most cast bullet for the 257 .. and the easier mold to find .. please don't come with a lead in for 100gr+ bullets that don't even work on that twist.. all this was already tested in the last 12 years + of the 257 being used on airguns .. if you do this rifle around the 257-420 .. it will be one of your most selled rifles.. also please make it aviable with the longer tank that you already have aviable.. so the beach guys could have some extra shoots to ..I haven't gotten to play with the 257 all that much. It was tested with 6x, 7x, 9x and 10xg slugs. All with good results at 50 and 100 yards. Of the 5 rifles that went for testing,only 3 were returned to engineering. The boys in QA liked them so much they have "held on to them for further evaluation ". 1 is set up for coyote and the other for woodchucks.
I cannot remember the barrel twist off the top of my head.
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