Advocate,
Glad to see that you purchased your first(?) lever gun. I see that you chose the .44 Magnum over the .357. It's a powerhouse with the right loads in a rifle.
I see Winchester finally put a faster 1/26" twist in their .44 Magnum. For years 1/38" was the industry standard, for whatever reason(?), and did not perform very well at all with low velocity loads and heavy bullets. Marlin still uses the 1/38. I had a Marlin 1894 in .44 Magnum a number of years back and the 1/38" twist would just about stabilize a 300 gr. bullet with a max. dose of 296. It shot 250 gr. bullets fine......as long as velocity was above 1,300 f.p.s.
The .44 W.C.F. Model 1873 Winchester, fired a 200 gr. bullet at 1,200-1,300 f.p.s. and the twist Winchester used was 1/36". My 1873 will shoot 200 gr. bullets just fine at 1,000 f.p.s. as well.
For a load that would be in the 900 - 1000 f.p.s. range in your 16" barrel with a 240 gr. bullet, I would suggest you might try 6.5-7.5 grs. of Unique, or 6-7 of 231. For Red Dot, Bullseye or American Select try 5-6 grs. to start.
With regards to a 300 gr. bullet, I would try .5 grs. more of any of the above powders to begin with. It would be interesting to see if a 300 gr. bullet would stabilize in a 1/26" twist at that velocity. In a 1/38, they would definitely keyhole.
Revolvers have faster twists and a 300 gr. bullet @ 1,000 f.p.s. works fine in them.
Have fun with your new levergun!
w30wcf