The cross drilling is to vent the blowback gasses to the side to prevent rearward blowback of the hammer which keeps the cap on the nipple so it doesn't get dislodged until the cylinder is rotated when the gun is cocked. Ideally, the cap stays on the nipple and then falls off with rotation or recoil when the next chamber is fired.
With over 15 perCUSSIN' revolvers, I've been called the Imelda Marcos of cap & ball.
SASS#3302 (Life), SASS Regulator, NRA (Life), DGB#129
Wolverton Mtn. Peacekeepers (WA), former Orygun Cowboy (Ranger, Posse from Hell)