As regards handloading:
What is the difference between reloading recipes for a handgun and a rifle?
A handgun and a rifle firing the same ammunition?? Generally speaking no differences.
And don't assume that just because the data is in the rifle section it was developed for and in the rife to maximize the rifle's advantages.
I have loading manuals where the data (.357 & .44 Magnum) for rifles and pistols is EXACTLY the same. Pretty sure all they did was take the pistol loads, firing them in a rifle, record the velocity increase from the longer barrel, and print that in the rifle section of the manual.
Not saying everyone does this, but it seems to be the usual pattern.
Is there a formula to convert handgun recipes to rifle recipes?
No, or rather, only in the most general terms, a rough XXfps per inch of barrel, and that varies widely with the powder used and other factors specific to the individual gun and ammo. You get more velocity from a longer barrel, but how much is not a straight constant that is accurately predictable due to may factors.
Anyone have a pet rifle load any where from 240gr to 300gr?
I don't load heavier than 240/250gr in .44 Mag. That weight, with a properly constructed bullet will do anything that needs doing and some things that don't...
I never fell into the popular hype that 300gr bullets were "needed", something that became popular a few years ago thanks to some gunwriters who thought it was a neat idea. Another factor might be that I simply cannot use, and do not wish to use them in some of my guns.
Lever guns are a bit picky about length, and especially the Marlins. Its entirely possible that 300gr bullets may be too long to cycle in your lever gun. Some loads are too long for S&W cylinders but work ok in guns with longer cylinders like the Ruger Redhawk.
Point here is you need to TEST before loading up a bunch to ensure proper fit and function. Load 2 rounds in the lever gun tube, (recommend dummies,) and cycle them through the action. If the rounds are too long, you'll know right away..
Saw a friend do this with a Marlin .357. He had some long 210gr RN slugs loaded in .357 brass and they were too long. Rifle locked up solid jam and required me taking it apart to get the ammo out.
Also, there may be accuracy issues. Different lever gun .44s just seem to prefer a certain weight range and shoot like crap when you go outside it. Again, something you need to test with your gun to see if it is like that.
Personally, I'd look at 240gr JSP slugs. Not hollowpoints. The extra speed from a rifle barrel may have an adverse effect on bullets intended to open up at pistol barrel speeds. I know this for a fact in .357, and I expect it would also apply to the .44Mag.