I replaced the stock barrel with the Madbull Black Python V2 509mm TBB, along with replacing the stock hop-up rubber with the PDI W-Hold rubber.
The first approx. 150 shots (with BB Bastard 0.25g) were very accurate out to 33 feet with nice tight groupings on the target (better than stock), however after that the shots became less and less accurate until they went well off target, despite me attempting to re-zero the sights multiple times to compensate. I pulled the new TBB and plastic hop-up unit out of my AEG and saw that the barrel had moved slightly, as the visible bulge from the W-Hold hop-up rubber (looking down into the barrel from the hop-up unit end) was no longer perfectly centered. I then discovered that the plastic c-clip that was supposed to secure the barrel wasn't on all the way, and for some unknown reason, I couldn't make it clip in all the way. Later, I even tried to refit the stock barrel back on, but the c-clip again didn't want to clip in all the way.
I was a bit disappointed, but hopefully the Lonex metal hop-up unit I ordered today from Airsoft Store Canada will be better than the plastic one-piece Sportline hop-up unit.
My airsoft guns:
Classic Army M15A4 Sportline Rifle (SP003P-1)
Umarex Beretta 92FS Walmart Clearsoft Spring Pistol