why would a normal person need this loophole thing ?
because we normal people also shoot rifles around objects and we don't want to accidentally hit something in the path, like a tree branch or wall or rock etc.
granted, MOST people just move well clear of those objects so you don't have to calculate anything. but sometimes you don't have a choice.
sentry and mike, there is more than one way to compute this, but i just use the old junior high algebra "slope intercept form" y = mx + b and built a quick excel spreadsheet to calc it for me.
it's not hard to figure out, and it only works to your zero distance, so it's not perfect. basically, i find the slope of my trajectory between the muzzle and zero distance by first figuring out how much the dope i have on my rifle subtends at the zero distance, and then adding that to my height over bore (rise) and dividing that by the zero distance (run). then if i want to find the distance to the optimally small loophole, I solve for x and if i want to find how tall the loophole needs to be at a given distance i solve for y.
pretty simple, but it doesn't exactly take into account the small drop from gravity between the muzzle and your zero distance, so it's not perfect. i mean, it assumes a straight line, where the line is curved up usually an inch or so