...If you have an actual forcing cone cut in your barrel, the ball will be in "free flight " with no stability. Accuracy will suffer...
I disagree & here's why: the projectile is "dumb" and only "knows" diameter until it engages the rifling. If the FC end (technically called the leade) is the same diameter as the groove diameter then all is well. Most importantly the projectile needs to transition into the rifling as unmolested as possible. That's why it's best to have chamber diameters as close as possible to actual groove diameter and not undersized chambers as then you get gas cutting around the projectile which can cause leading and degraded accuracy. There is no "free flight" as the projectiles is trapped within the chamber & then FC area. There is however a short stint of unrifled passage into the FC but that is unimportant per my experiences
IF the chambers are properly reamed to match the groove diameter of the barrel or 0.001" greater.
Do a Google search on Taylor throating for revolvers with grossly oversized chambers to better understand this principle. Just my 2 cents.