TLOPO copperhead (pre-patch) was indeed firing at 15%. I don't remember POTCO well but I think I read somewhere earlier that TLOPO explosive does more damage than in POTCO, if true, it explains why the copperhead damage was higher as well, and thus this nerf was needed. As of this recent patch, copper is back in 1st place (but only just barely), followed by storm chaser and then S&B.
On the copperheads I own on different alts, sometimes, be it random accident or whatever else, there would often be 50 to 100% explosives fired, repeatedly.
That's not how it was in POTCO.
Copperhead was slow, cumbersome and safe. It had powerful shots, which were not as potent as they are in TLOPO. I've seen damage from my explosives in the high 5 digit range, which is absurd, tbh.
I've been able to singleshot an En-Garde many times with a copperhead, and that's just not possible. It used to require 5 solid broadsides on a tally-ho to sink it with a copperhead, and now 3 is enough if all rounds hit the target.
Also, the armor on a copperhead was meant to sustain one explosives broadside from a ship of the line or a Corsair. Now it can take up to 3, depending.
15% chance means 15% chance.
In Potco, that meant 15 out of every 100 shots fired, on average.
In Tlopo, there's never a broadsides that does not have at least 1 explosive round in a single broadside, which mathematically speaking, is well over 15% per 100 shots fired. In fact, the number of explosives is well near 50% on any random broadside.
Potco was more challenging and balanced the "pros" and "cons" of each ship type.
The concept in Potco was to not have a single class or type of ship that overwhelmed all other ships, which seems to be the case in Tlopo.