ONLY for the very rare gun with tight barrel, correct chambers and throats if you happen to get lucky. Having bullets smaller than .429 shoot at all well has Not been my experience at all. The SAAMI drawing dimensions have little relationship to reality compared to guns out there in the real world.
I have lots of experience with the .44-40 in both old guns and new. A few examples:
The cylinder throats on my 1908 Colt and both .44-40 New Services of 1920 and 1928 manufacture are .430, and while their barrels slug .425-.429 bullets MUST fit cylinder throats to shoot accurately. Post-WW2 Factory jacketed ammo is mediocre at best.
Pre-WW2 WRA assembled with thinly jacketed bullets, having dead soft lead cores, loaded with Sharpshooter powder was and still is VERY much better. I still have a few boxes which were my Grandfather's I keep as reference ammo.
Closest matches you can produce in modern components to approximate payload, velocity and pressure to Pre-WW2 loads which shoot quite well in the great majority of guns is to assemble loads with the 200- grain .430" diameter Hornady XTP in Winchester factory-primed case with 24.5 grains of RL7, 21 grains of IMR4198 or 18.5 grains of IMR4227. These all have been pressure tested, safe, accurate and effective on game in the 1873 Winchester, Colt SA, the New Service revolvers and modern Italian clones.
My 1920 Winchester 1892 barrel measures .433" and 1889 Winchester 1873 is .435". Post-WW2 Factory jacketed ammo keyholes from rifle barrels larger than .428 and will not stay on a B8 repair center at 25 yards from the original 1873 Winchester, which with correct loads is otherwise our most accurate rifle. Revolver groups from Colt, Pietta, Uberti clones, Ruger Vaquero and S&W 544 all exceed 8 inches at 50 yards with visible yaw on targets!
With Accurate 43-215C cast 10 Bbn, lubed with 50-50 olive oil-beeswax, sized .430 and loaded with 6 grains of Bullseye or TiteGroup, 6.5 grains of 231, 452AA, WST or Red Dot any of my revolvers will group 4-5" at 50 yards for 12 shots from Ransom Rest. My best 1920 Colt New Service tuned by Sandy Garrett at Nova Gun Works consistently under 4 inches.The same loads produce 3-4" ten-shot, iron-sight group at 100 yards off sandbags from the various .44-40 rifles and remain subsonic with low noise.
Accurate 43-200QL is a heeled bullet of my design intended for .44-40 rifles and revolvers having tight chamber necks and large barrel groove diameter without having to rechamber revolver cylinders or rifle barrels and is suited for loading with either black or smokeless powder, having sufficient lube capacity to mitigate foulout in carbines using Goex powder and SPG lube. It will stand "full charge" loads to 1350 fps with suitable powders. RL7 is the powder of choice, but 4198 and 4227 are also satisfactory. While Alliant #2400 is frequently recommended I did not finds its ballistic uniformity as good in standard-pressure loads suitable for the older guns.
My Pietta and Uberti Colt clones have .430 cylinder throats and .429 barrels. My 1894S Marlin .44-40 barrel is .432, same as my .44 Magnum. MANY modern .44-40 rifles have chamber necks which are too tight to enable loading bullets to fit cylinder throats and barrel groove. Neck diameter of .445 is fairly typical per SAAMI, but is too tight with modern barrels of .44 Magnum dimensions.
My older Colts and Winchesters have chamber necks .447-.448 and freely chamber .430 bullets loaded in Starline brass. My S&W Model 544 and Ruger Vaquero both had .427 cylinder throats and .429 barrels and wouldn't shoot for sour apples until the cylinders were rechambered to enlarge necks to .447 and throats to .4305, and shooting soft bullets of 1 to 30 tin-lead at .430 diameter they group almost as well as the old Colts which have provided reliable service in my West Virginia family for over 100 years.