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the bit where your rifle still grouping as such....but group is moving...suggests to me its NOT the load,but something changing. weather thats your technique (probably not from what youve just said)...

goodho ol chap......no one is having a go at you.....some of us have seen it turn to shit in past so passing on the hurt so to speak....
check them screws etc and let us know how you get on.

do you shoot freeroiling/hug yourself/off bipod with off hand under rear of stock??????
if so....do you sight in off hard surface and hunt with soft surface under legs of bipod???????or vice...