For many years, my shooting partner used a skeet grade 870 .410 bore with 2 1/2" #9 shot loads. Bob was semi-retired from truck driving and was an all around, handy-man type. He kept quail dogs, an English Setter (Sam) and a Brittany (Lucky). In season, Bob hunted with Sam and Lucky almost every day. I was fortunate to be able to hunt with Bob several times.
Bob was a Kentucky handicap champion and worked for about 10 years as an Olin "Learn to Shoot" instructor. Bob and I also shot "games" together on a weekly basis in the Fall.
I watched Bob kill "three on the rise" with his .410 bore, several times. He was quick as a snake. While quail hunting with Bob, I used an 870 with a 26" improved cylinder bore and 1 1/8 oz. of #8 shot and concentrated on one bird. With Bob's dogs and his skill, we would almost be standing on a covey before it flushed. Sam and Lucky would then work together, Sam at a distance and Lucky up close, as we followed up on single birds.
IMO, #9 shot is very effective on quail if you are a dead shot and quick. I prefer #8 for quail and # 7 1/2 for most very thing else. For late season, high flying doves, I use # 6's.