Been there done that lol!
.457 is the size of roundball you want to use. For conicals you want them to be at least .454 diameter, and preferably .455-.456. It can be tough to find boolits that are .455+ that aren't .45-70 rifle designs (read very heavy). Though some designs shoot great out of a 909 at up to .458 diameter if the bands are narrow.
Keep the BHN as soft as possible.....if you go much harder than BHN10 velocity suffers.
For a lightweight boolit, the EPP/UG's beat ALL! Very accurate, and more efficient than RB due to a better seal (I get an extra shot or two over roundball from the same fill pressure). Designed to exceed the performance of roundball in cap and ball revolvers, EPP/UG's duplicate the RB's speed while also being a little bit heavier (read more fpe). My go-to small game boolit. Get the mold at biglube.com
These are the only two groups I shot with the EPP's that day, off the steady sticks
The 180grain Buffalo Bullet Company 180grain Black Powder Pistol Bullet (advertised .452, but actually .454+) is a real nice shooter. I haven't tried the 190grain version yet (advertised .457) but want to soon.
Medium weight 200-220grains-the 200grain REAL, and Lee 220grain ROA slugs are long proven performers in this weight range. I think the 210grain BigLube version of the ROA slug may be just as good. They shoot very hard out of my power-tuned 909, but I have yet to do any real accuracy testing with that version of ROA slug. 205grain Black Widows work excellent (very EXPENSIVE), though they are designed for muzzleloaders, and do not expand as designed at airgun velocities.
Heavyweights 230+grains-Maxi-Balls (EXCELLENT accuracy) 250grain REALs, Lyman 452374 (I think that is what the RN's I have are), the old Lyman #45266 (I hear the old Lyman 452 molds dropped them a bit fatter way back in the day) , #454424, and #454424 hollowpoints, #454190........ I would try every old Lyman #454*** design I could get my hands on, as they should drop a fat enough boolit.
I see you live in Missouri, where hunting Whitetail Deer is legal with bigbor eair rifles of .40cal and over (you lucky dog!).
The ultimate big game hunting heavyweight for the 909 is the "J.R." WFN series cast by BlackHogDown. They were designed specifically for the 909 barrel profile.
Here's the 240grain version (my favorite) of the "J.R." WFN (available in four different weights), and a typical 50 yard group I shot with them off of my steady stix (the small squares on the target are .5 inch). I think they'd make one hole using a solid rest. I had a limited supply, so this is the only group I have shot with them after doing the fill pressure/chrony testing, and getting them sighted-in.
These hit harder than Mike Tyson!
If you want to boost the power of your 909S to 350+fpe, I know a couple of guys that tune SamYangs. You'll get two power shots, and a finisher or two. My 2tube 909 is tuned for four 250-260fpe shots, with a few finisher shots. The 2tube can also be tuned to 270-280+fpe for three power shots, and a couple of finisher shots.