An easy way to look at the barrel roll:
If you take a flat gift-wrap string and make a loop with it (looks lika loop) and then pull the ends apart until the string is straight, there will be a 360deg twist on it (roll). Everything in between is a barrel roll.
Too easy!
If you now take the ends and tie a knot instead, it's a snap-roll or lomcevak, depending on the knot-tieing skills.
As a final show: Take a pair of scissors and cut the string (and maybe your fingers) in multiple small pieces and suddenly you have a R44 doing a barrel-roll
/2beers