122mm rockets

I have just finished reading another book on SOG and I am somewhat confused about just how accurate these rockets were/are. I also watched a video I have in my collection where Chechnian Rebels are firing them from a mountain top towards a valley that is miles away and this got me to thinking.

I understand that they have a "maximum" range and that Trigonometry has to come into play here since they technically are not considered "guided". So, if one were to set these up to bombard say the camp at Dak To they would need to know the range from where they are to Dak To. Then, they would have to figure out the correct angle to set the rockets up at so that the rocket(s) would reach "burnout" at the right moment and fall to their target(s).

Is there more to this or do I have the basics right?

It seems to me that during Vietnam the enemy became really accurate with them or was it really just pot luck?