The dynamics behind this are very complex. The cue is part of a system that includes the shooter’s arm. Muscles act like springs, different people have different strengths and arms weights. Technique plays a big part, with some people much better at transferring arm/body momentum to the cue. For example, Nick Varner has a very powerful break, and he’s a tiny guy who weighs something like 115 lbs. The optimum cue weight will be slightly different for each person as a result of all this.
Given all the complexities and difficulty modeling the entire system, the best way to figure out optimum cue weight is through experiment and data collection. That’s exactly what Bob Byrne and Bob Jewett did - they removed the foot rail from a pool table set up in a garage, then started shooting balls off the table and marking where they hit the ground. From there they were able to work out the velocity of the cue ball as it left the table.
They tried multiple cue weights between 9.5 and 25.9 ounces, with multiple people, all experienced pool players. After 67 shots in total, they collected the data and graphed it.
What they found was that first, there was a lot of shot-to-shot variance. These guys were all college level players, and presumably had pretty good technique but their velocities varied by as much as 20%. So the sensitivity of the test was pretty low. However, they found that the optimum range of cue weights clustered around 18 to 21 ounces. That’s the range that produced the most consistently fast cue balls for the players as a group.
Within that range, the player’s own variance tended to dominate. However, one clear result is that there’s very little advantage to a heavy cue - not a single player was able to hit the ball as fast with a 25.9oz cue as they could with a cue between 18 and 21 ounces.
The speed difference in that range were really small, though. For example, one player’s 3 shots were measured at 23.72 mph with a 20-ounce cue, 23.29 mph with a 19.5 ounce cue, and 22.85 mph with a 17.9 ounce cue. Notice that the lightest and heaviest cues had better speeds than the middle one, meaning that the differences really just came down to the player’s technique from shot to shot.
The conclusion was that you might as well just use the cue you are comfortable with, so long as it’s in that range between 18 and 21 ounces. Personally, I have a 19oz playing cue, and I use it to break as well. That’s what Nick Varner does too. If you play with a really whippy cue, you might want a stiffer one for breaking, but don’t sweat the weight. Just use whatever feels right.