The depth of grooves vary from ·005 of an inch, to about ·125; and some rifles have been made with an alternate deep and shallow groove.
Form of grooves.
Grooves have been made round, circular, triangular, rectangular, and indefinite, alternate round and angular, elliptical, polygonal; and some cut deep only on one side.
Proportion of groove to land.
Some gun makers are of opinion that there should be a greater proportion of groove or furrow than of land or plain surface, because they say the ball is thus more firmly held, while others maintain that by diminishing the number of the grooves, the accuracy and range would be increased, and this has led to the opposite theory, that perhaps if anything, the plain surface of the bore should predominate over the grooved.
Form of early grooves straight.
The earliest rifles had two straight deep creases opposite to each other, the bullet being spherical, and furnished with small circular knots of lead, large enough to fill the creases.
Form &c., of ancient rifles.
The greater number of ancient rifles have a whole turn, with an odd number of deep and rounded grooves; hence we may infer these were considered the best forms.
Objects of rifling.