Hi Bill, welcome to this part of the world
.
It is only natural that we tend to think that the sense of rifling is that of causing a determined rotation in the
bullet, to avoid air resistence and therefore promote projectile direction 'straightness', which is an undeniable fact. Such rifling is grooved in the different barrels with a diverse rotary 'pass', depending on the distinct models and respective patents.
But it seems as there is another version/option called 'straight rifling', which is a rather different thing and serves a somehow opposite purpose, mainly uded in
shot guns, i think relative to avoid the wad (and the lead) to freely (randomly?) spin, causing the dispersion of the shot pellets charge, when wanted otherwise.
As rifling is a term intrinsecal to the spinning efect, one (i) thend to think it would be more correct to call this version straight grooving ... but i am wandering
.
Just try and draw what is usefull, from all i said ... and with some reserve
. Others can put this more technicaly, of course.
Fernando