I like it very much actually

My best friend has an H&R Ultra Hunter in this cartridge and it has performed exceptionally from day one. He hasn't been able to bag any game with it yet, but I have no doubt that he will soon; I also don't doubt that when he pulls the trigger, the gun will hit what he's aiming at & bring it down just as effectively as any other big game cartridge. He reloads for it, so ammunition cost isn't a problem; there are enough factory loads out there to work with, but they are quite expensive and not as common as they should be in some places. If I had to buy a big bore rifle/cartridge tomorrow, it'd be a Marlin or an H&R in .450M without a doubt; it's a good cartridge, my buddy reloads for it, and it its loads of fun to shoot

I only wish they still made an Ultra in this caliber, but as you found out, the barrel is still available

I'm still trying to con my buddy out of his rifle and ammo, but I don't think he'll ever sell it; you can't buy another one, and he loves it, so there we go

I've heard some people (very few, since I don't think many people ever bought Ultras in .450) say that their H&Rs weren't accurate in this chambering, but I would stake my life on the 22" tube of this Ultra Hunter any day--it's that good. I'm actually hoping to use this gun on a few of the pesky coyotes we have around here if I can borrow it for a spell; I cannot think of a better gun to use honestly

At any rate, I have nothing but good to say about the .450 Marlin, and if it works well for you, definitely stay with it. I hope you're barrel shoots as well as the one I've used...you can't go wrong if it does :-) Sorry for the rambling & good luck with your .450.
Kind Regards,
.450Marlin
P.S. My buddy has been using a Simmons Blazer 3-9x40 & Leupold rings on the Ultra since he got it & has had no problems; that combination has taken a worse beating than our shoulders have (the rifle does not kick very harshly at all in my opinion) have and has held up perfectly--not bad for a sub-$100 setup :-)