JOHN BIDWELL says: It’s not that long ago I had to answer a similar query to this and what I said then still applies.
Any large weight gain (or loss) can impinge on the fit of a gun, in particular to the amount of cast and drop needed to place your master eye in alignment with the centre of the rib when the gun is mounted.
Overly fleshy cheeks, caused by putting on too much weight, can alter this sight picture, as too can a noticeable weight loss. The differences might only amount to fractions of an inch but these can be sufficient to alter pattern placement quite noticeably at longer distances.
Bulking up through weight training is unlikely to affect gun fit except in relation to the amount of cast at toe. A larger, broader, chest might require the toe of a previously adequate stock to be cast out a little more so that the butt again sits properly in the shoulder.
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