Purpose of "rifling" is because in many applications the wrist pin does not even have oil supplied to it, meaning assembly lube is the last "oil" actually applied and supplied to the wrist pin... once assembled, on a motor with rods that have NO rifling, the wrist pin only gets oiled by "splash" which doesn't obviously supply much to the inside, meaning lubing the entire wrist pin and rod around it.

So anyways, the rifling of the hole or passage thru the rod helped the oil make it to the wrist pin. A straight thru hole as the motor turned and with the extreme lunges up n down as cylinders fired, along with rotation and enertia, the straight thru hole did not work as well as one rifled. The spiral allowed the oil to have a lil resistance to NOT just flow back down the passage, therefor making it to the desired location... the wrist pin.
*Some people believe the rifling also created better cooling, by way of more surface contact with the oil so it could withdraw more heat as the oil passed thru, but I do not believe this has ever been proven fact.

Yeah I am dumba$$... Get over the other thread and read the answer to your question! Sheesh!
Rifle drilling was first used in Diesels I believe, then later used on motors with alot of wrist pin "slap" to help supply more oil to the bearing journal to eventually make its way out of the journal and fill the space to prevent or reduce "slap". This "hole" through the rod has a beveled entrance to assist with creating pressure to help the oil push thru the tiny hole, the hole is just the "A HOLE" with out rifle drilling they would likely just call the rods "drilled". When testing was done it was found that using "rifle drilling" that this assisted in the oil not being "slung" out by the rotation. So now there is such thing as "rifle drilled con rods"

Which our motors don't really require because our's have oil squiters (I just can't recall if they have rod or block squiters of the top of my head)

"you're welcome"

CON: is the rod can be weaker. (typically this is the case when a rod NOT originally designed with the rifled hole). If you buy rods designed and tested with the rifled hole, you'll be totally fine.

PRO: is that oil will be "supplied" to the wrist pin, not simply lubed by splash/squirt.