Reddit mentions of Chemglass CG-1712-04 Glass Cylindrical Style Graduated Addition Funnel, with 4mm PTFE Stopcock, 500mL Capacity, 24/40 Joint

Hey, I'm a chemist. The reason for the conical shape is because the point of a separation funnel is to separate two layers of material. So you're interested in stopping flow right at the interface between the layers, so you want the bottom to be narrow so you can to minimize error.

Another type of funnel with a stopcock is an addition funnel, which lacks these separation-optimized features but frequently provides a means to cycle atmosphere between the vessels above and below to ensure a perfectly continuous flow. This is particularly helpful if you wish to control the atmosphere (for example, to brew under nitrogen or argon).

That said, chemistry glassware is generally far more expensive than its value to anyone who is not deadly serious about certain outcomes. If your drip can stand to sputter for a second, you don't need a $200 addition funnel. If you've got the $200 and want to prove that sputter-free coffee brewed under xenon is the wave of the future, be my guest.