the banana shape comes from one side of the case being thicker than the other and having more elasticity, hence the shape, one side will stretch more than the other, especially under pressure, the really bad cases will never be straight even after full length sizing
you cant fix it like neck turning that uniforms the neck,
when the rifle fires there is expansion where there is no support from lugs (two lug bolt) the case can bend, as explained by Brett in his post above
if you index your cases with a concentricity gauge and notch/mark the case to place it in the same every time this should not happen,
or just get rid of the really bad ones all cases are effected to some degree
if you have multiple locking lugs that cover all the way around the bolt sharing the load it does not matter, as explained by David Tubb in his sierra reloading vid
this David Tubb video is IMHO very good for the reloader looking for precision, Utube has taken it down, copy right
so there you go, I'm happy to embarrass myself with stuff most reloaders would think, hmmm not for me
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