Thanks for all the kind words.
Replying to as many as I can :

I have found that with the big hard kicking handguns, using the biggest grip you can will help with the recoil.
I almost bought a Desert Eagle BFR in 444 but instead put that money toward the double rifle.

One of the better decisions of my life.
I deeply enjoy the gun even when I am not shooting it which is most of the time.
It's in a small custom rack where I can see it while I synthesize and model loads in QuickLOAD.

QuickLOAD has a feature that is especially useful to those trying to ballpark a non-standard load for a double rifle.
One of the results is barrel time.
You can play with the various powders to try and match both the velocity and the barrel time to the regulation load.
Graeme Wright suggested as how a little faster powder works best for lighter bullets and slower for the heavy ones.
This is born out when studying those velocity and barrel time numbers.
Maybe I am a little slow but that seems counterintuitive to me as one would think the lighter bullets would
want to be accelerating at the same rate as the heavier regulating bullet.
I am beginning to accept that much is mysterious when it comes to double rifle physics.

The Rizzini is CIP proofed at 64 kpsi and is rated for the CIP 444 pressure of 51,488 psi (3550 bar)
so I have few worries of handling USA ammo or my own loads.
I am not quite sure how they (CIP) arrived at that number as it's quite different from the SAAMI numbers.
The rifle was regulated for the Hornady "LeveRevolution" 444 round which uses the 265 gr. FTX bullet.
This may have been the only factory 444 ammo available in Italy at the time.
Since this bullet is quite long they cut the brass shorter which lowers the velocity compared to the full length
"Superformance" load which uses the regular 265 FNSP Hornady bullet (at around 2330 fps).
This was necessary to allow the cartridge to load into the side of a Marlin.
You can load the FTX in full length brass one at a time into the top when the bolt is all the way back.
The velocity (out of my 22" 444S) for the LeveRevolution is around 2260.
Hence this is the ballpark velocity for regulating other loads.
Being 70 fps slower than the "normal" 265 load gives me some headroom to try regulating a heavier bullet.

I was measuring velocity with the 444S and a Magnetospeed before I got a LabRadar.
This worked fine as when I matched velocities in the single tube, they shot fine in the double.

The fact that my clone of the original Remington 240 gr 444 cartridge regulated right off the bat blew me away.
The grin on my face would have made me look like a South Park character.
This is the load that caused the very slow start to the 444's acceptance as the bullet was a handgun bullet and
sometimes blew up when hitting an animal. They added another cannelure near the bottom of the bullet but it didn't really help.
The 444 did not get reliable until Hornady came out with the 265 SPFP which has a thicker jacket and harder lead.
It was THE standard 444 bullet until just recently when Hornady discontinued it as a reloading supply.
The SuperFormance round which uses that bullet is evidently still being sold.
The main substitute is the Speer 270 FP which also works well.
I have not tried it yet in the double though.

The obvious bullet for a 444 DR is the Woodleigh 444 280 SPFP weldcore.
Definitely fills the role of the "blue-nose" as John Taylor called them.

They are as rare as hen's teeth here in the USA but I have gotten 100 of them and will try to regulate them next.
Going lighter was easy (doesn't get any easier than finding out stuff you already made regulates perfectly).
Going heavier will require some work especially as I want to fire as few of those precious Woodleighs as possible.
If the Swift 280 A-Frame ever becomes available again that will be a good substitute.
I have about 100 of them as well.
The Beartooth 280 WFNGC will be my main alternate while getting close to regulation and for practice as I have several hundred of those.

I am completely satisfied with the Rizzini BR550E.
The stock fits me perfectly after adding a butt-pad (I have long arms).
Most intelligent off-the-shelf stock I have ever encountered.
It's the best value in a double I can imagine.

The load I plan on pursuing Bambi with in a couple of months is that 240 gr. load that regulates so well
but with a Swift 240 gr. A-Frame instead of the Hornady handgun bullet.
Should leave a nice big hole all the way through.
Our whitetails aren't that big. Neither are the black bears around here.