As long as no one tries to convince me to drop the .45-70 and go to the .444 instead, we're good.
I agree. That's why I left things where I did.
My religion is my religion. I try not to preach, nor force it down other people's throats.
I may not believe in yours, but I'm not going to condemn
you for believing - or disbelieving.
Just don't try to force yours down my throat, either.
(Minor insight into the above statement: I grew up in Utah...)
444 bullets heavier than 300 gr? Yes, many. Beartooth Bullets is a notable source. But only a few can be found in loaded ammo from 'boutique' manufacturers. The rest are hand-load-only propositions. ...And many are cast from custom molds, usually designed (and sometimes machined) by the guy pouring the lead and pulling the trigger. I have a huge variety of .44 caliber molds. Some are "handgun only". Some work in handguns and rifles. Some are "rifle only" - such as my 437 gr "One Ounce Suppository" and 420 gr WFN (with a .390" meplat, it's almost a 'full' wadcutter).
There are a group of us over on the MarlinOwners forums that have been experimenting and sharing experience with bullets that push Marlin 444 and Winchester Big Bore chamber and action dimensions to the limit. For the most part, we've settled on 425 gr WFN as a realistic maximum bullet weight and style (with bullet design trying to keep it as compact as possible). But, of course, single-shot and double rifles don't care. In Marlins modified for a longer COAL, custom Mausers, single-shots, and double rifles, we've gone as far as 620 gr (and stupid velocities that will not be repeated).
Most 444 owners won't touch 240 gr or lighter bullets, once they use them on game or try the Hornady 265 gr FP. Plinkers and casual paper punchers love the Remington 240 gr stuff, because it's "cheap"; but most 444 owners that actually hunt big game use the 265 FP as the absolute minimum (with exceptions of the Barnes 225 gr XPB and 250 gr Nosler Partition being considered adequate but marginal). 300 gr XTPs are quite popular and effective. 280 gr Swift A-Frames are a reliable choice. If chamber geometry and personal sizing choices will allow it to pass through the loading gate and fit in the chamber, the Lee 310 gr FP works well. And, of course, there are more bullets that are plenty good.
Most hunters end up finding 265-300 gr to be the 'sweet spot' for the cartridge; but a little jump to the Beartooth 320-335 gr WFNs is easy to make, too.
Ironically, the most notable company that you CAN'T get a .444 Marlin from right now is Marlin. They've announced the Model 444's reintroduction every year since 2014; but it has not materialized. (For 2018, it was a "New Model 1895 in .444 Marlin". ...That didn't go over well.)
Before H&R was thrown under the bus by Remington, Handi-Rifles were a very popular choice for .444 Marlin buyers. They are no longer available new, but can easily be found on the used market.
CVA, TC, and [someone I'm forgetting...] make single-shots.
Browning offered the 1885 in the chambering. Might still, but I didn't check.
Winchester Big Bores were chambered for the cartridge, and runs are still produced by Miroku occasionally.
Henry has been hinting at finally releasing a .444 Marlin by next summer.
Pedersoli usually offers their 1874 Sharps in .444 Marlin; and they do offer the 86/71 for the cartridge.
Magnum Research offers the BFR.
And, surprisingly to most people, quite a few of the "low end"/"budget" double rifle manufacturers offer .444 Marlin as a standard chambering. Apparently, it's fairly popular in Europe.
I'm sure I missed something, but that covers most of it.
As with everything in life, and especially the gun world, there are handicaps to be aware of, though.
Notably:
Marlin's 'Micro-Groove' 1:38" twist rate (standard until 1998) has a very hard time stabilizing bullets over 300 gr; and you need all the velocity that you can get, just to be able to trust the 300s. (No 16" barrels and reduced loads with 1:38s.)
On the flip side, the early Winchester Black Shadows had a 1:12" twist rate. Light bullets are 'over-stabilized' and not happy; but heavy bullets have been observed 'skidding' across the lands in the bore, before finally 'biting' and being spun up. Bad twist rate. ...Should be avoided.
Marlin's and Winchester's later 1:20" twist stabilizes everything - even 450+ gr bullets at subsonic velocities - but very heavy cast bullets (w/ or w/o gas check) or heavy cast bullets with low bearing surface may 'skid' across the first few inches of rifling, just as some bullets do in the 1:12" twist. It is not a huge issue, but has a measurable negative impact on accuracy. (My theory is that while the bullet is 'skidding', it may also be recovering from an off-axis jump in the throat, finally 'biting' the lands in the barrel while still off-axis. But I don't know how to prove it.)
Still not trying to waterboard anyone with my beliefs. Just answered questions.
The way I try to look at the cartridges, in regards to Marlins, and with a fairly open mind is:
.444 Marlin is for handloaders, and people that actually want a better chance at good accuracy from a Marlin lever gun. (And people that think the .45-70 rim cut in the ejection port of an 1895 is an abomination.)
.45-70 is for the guy that wants the classic 'cannon', but can't be bothered to reload, and/or doesn't really care if all he gets is "lever gun accuracy".