I am trying to decide which lever 44 to get for deer season now that my state allows straight wall cartridges in slug counties. I like the Marlin 1894, but see that the rifling is only a 1-38" twist. The Henry Big Boy has a 1-20", as does the Ruger 77/44 if I wanted a bolt. My question is, in the event I ever wanted to shoot 300gr bullets, would the 1-38" twist in the Marlin stabilize them, or is it too slow and limited to the 240gr bullets. I do plan to shoot 240gr XTPs, but wanted the rifle to be able to shoot the heavier bullet.

If the 300s really matter that much to you, you already know what to do. There are a few in the middle ground like 270gr GDs and IIRC 280gr Swifts that may offer more weight without compromising your preferred choice in rifles.

FWIW, Barnes 225gr XPBs, which require a 20" twist, scattered like buckshot from my MGM Contender, a 20" twist. Very disappointing. I also have some FTXs to try, but will have to trim cases if I want to run them in my levergun.

I personally think that a 300 grain bullet is too heavy for the cartridge. I you want to shoot a 300 gain bullet, buy the 45-70.
The Hornady XTP in 200 or 240 grain will be fine for deer. I prefer the extra speed of the 200 grain. I tried the 180 grain but fear that it is too fast and will come apart too quickly.
In a 240 JFP, Roze bullets offers one in a .431" diameter. Marlins have a larger bore in 44 mag. Usually reported at .431" or even .432"

I haven't had any problems with the right 240 grain
out of my 44 mag rifles. One shot one kill.
Been using the deep curl speer. They were getting
kinda scarce back a year ago or so, so I picked up
a box of xtp's. Never had a chance to load or try the
hornady yet. Picked up a box of the speer yesterday
off a nearly bare shelf at the store.

speer # 4455
2400
regular primer
remington or starline empties

the bigger the hole- the faster the boat sinks

The old 265gr Hornady will kill any deer that's ever lived, and do it at any angle to the vitals presented, get some H-110 WW-296 and lean on it hard.

Well dang, went and had a look, seems the geniuses at hornady have decided to drop the grand 265gr 44 cal flat point bullet.

Oh you can still buy them, you just have to buy
them stuffed in powder filled brass cases in a
Superformance box.
I bought the last #4300's the other day that'll
probably be the last ones we see in my area.
After those are gone, I don't know what to do
from there.
JMHO- I think hornady is throwing all their
efforts toward ammo and components for
the panic buyers, and for the long range folks
buying the elo or plo or whatever the popular
hornady long range bullets are

I should also add that it sucks badly to me to
spend all the time, effort, and money to work
up an exemplary load for a specific firearm
only to find that one or more components are
now discontinued,and you pretty much find
yourself back at square one.

But what can you do? . .

^^^^^^^^^^^^^This^^^^^^^^^^^^

Substitute 270 Speer if you can't find the Hornady.

Yep, that does suck Ranger99, Vic had a nice suggestion, any bonded [plated] Speer bullet upwards of 270+ grains should do a damn fine job, and, if available, I'd grab a thousand to be set for life.

Crap. I've still got a box of them for my 444 but that's not good news. Wonder if Hornady will make the 265 FP available again. This could be temporary while they focus on ammo due to high demand for the time being.

Might also try Beartooth Bullets. Been a long time since I ordered from them, so not sure if they are still in business or what the order backlog may be, but they have quite a selection. Use the vertical scroll bar on the left side of the screen to see all of their caliber selection.

Beartooth Bullets

Stock up when it's made.

This has been 15 or so years ago so Iam fuzzy. I have 3 Marlin 1894 levers. A 24" octagan ballard 1-38 cowboy, a 20 " round 1-38 stainless and a 20" microgroove round 1-20 blued. IFIRC I saw very little difference between them. I shot a a range of bullets and loads including the 300 grain the OP mentioned as well as lots of 240 grain stuff through all three guns including my 7.5 redhawk. Range was 50 yards for a bunch of shooting then a few at 100 yards through the one rifle I was going to hunt with. I do not remember powder or amounts but all was fairly stout but not hot rod. I too feel the 300 is not worth it. 240s hit hard and penetrate plenty. Also 2 of the three 300 loads did not cycle well on my stainless gun. They were RNFP hard cast and OAL was just a hair longer than most my other stuff. One of the problem loads was commercial. The first handload was developed using the commercial OAL. The third load I modified and shrunk the OAL and problem solved. The only reason I bothered was the stainless gun I had put the XS sights and rail and a scout scope and wanted to take it and my revolver all with the 300s on a hog hunt. I had just finished getting the sights/scope mods done and I do remember being pissed that the stainless gun gave me this greif. So there could be older 1-20 microgroove Marlins out there. BTW have shot plenty of non jacketed through microgrooves with no issues.

When I was getting ready for this hunt I thought the 300s were going to be the DRT end all but truth be told the 240s are plenty. These levers are handy up close guns and I just don't see much value in pushing the 44 too fast or too big out of this type of action.

If you are a reloader/caster het a ranch dog version of the 265 gr .432 mold from NOE. Load it on top of a stiff charge of 110, or Win296 It will put big holes in things causing them to die quickly. Works well in both my Henry, and 1894 microgroove marlin. I tumble lubes with Lee liquid Alex, unsized, plain base, no gas check.

Beartooth's are great cast bullets, had to give up on them severall years ago because I couldn't make contact by phone or e-mail, must be severely back logged, I'd certainly be happy with any Speer bonded bullet North of 265 grains.