Ruger Only....
If I'm not mistaken the , moniker Ruger-only was born out of the non-Colt safe .45 Colt rounds.
You aren't mistaken, but you need a little more background information to fully understand.
Today, the term "Ruger Only" is often misapplied. If you wish to be correct, "Ruger Only" loads are ONLY in .45COLT. Not in any other caliber.
And while at one time completely accurate, today there are Rugers that will not safely handle the "Ruger only" loads.
"Ruger only" loads came along shortly after Ruger introduced the .45 Colt BLACKHAWK revolver. At that time, there was no REDHAWK, no, Super Redhawk, and no Vaquero or New Vaquero. There was only the (new model) Blackhawk.
Handloaders quickly found out that, compared to the Colt SAA, the Blackhawk was hell for stout, and would safely take loads well above the safe max for the Colt gun (and clones).
this data began showing up in magazines and reloading digests in the mid 70s, and by the 80s was actually being put in its own section in some reloading manuals. (Ruger & T/C Contender).
This happened because of the combination of two things, the .45 Colt cartridge had the physical room in the case to be uploaded above its black powder pressure origins, AND the Ruger Blackhawk was the first gun, bigger and stronger than the Colt single action to be offered to the general public in .45 Colt caliber.
These loads probably should have been called "Blackhawk only", but its a bit late for that now. At the time they were developed, the only gun that would take them was a Ruger, and there was only one Ruger that would do it, the Blackhawk.
As time went on, Ruger introduced new guns, the Redhawk, (later the Super Redhawk) and the Vaquero. The massive Redhawks could also handle the Blackhawk only loads, and so could the (original) Vaquero, which is essentially a new model Blackhawk with a Colt style fixed sight frame. The (original) Vaqueros use the same large (.44) size frame.
So now you have 4 different Ruger .45colts that will handle the "Blackhawk Only" level loads, and "Ruger Only" becomes fixed in the lexicon.
Move forward a few more years, and Ruger throws in a ringer, the "New Vaquero" in .45 Colt.
Ruger dropped production of the Vaquero (44mag size frame gun), and replaced it with the NEW VAQUERO, a smaller, Colt frame sized gun. The New Vaquero will NOT safely handle "Ruger Only" .45 colt loads!!! Even though is it a Ruger, it is not as big and strong as the (44 frame) Blackhawk and Vaquero.
The New Vaquero is considered to be in the same strength class as the Colt SAA (and clones). You can push a 250gr up to a bit over 1000fps (depending on barrel length) but that is about the MAX for that gun.
Ruger caused a LOT of confusion when they replaced the Vaquero in their line up with the New Vaquero, because while the names are very similar, the guns are quite different, something easy to see when they are side by side, but difficult to tell which is which in conversation.
Vaqueros say "Vaquero" on the frame. New Vaqueros say "New Vaquero" (both words) on the frame. But in conversation, many people only say
vaquero, which, without more context doesn't tell us which gun they are talking about, for certain.
As to the .45 Colt matching the .44 Mag and doing it with lower pressure, this is one of (what I call) "tricks of physics". Slightly lower pressure per square inch / cm (or what ever unit of measure you use), over a bigger surface area (the base of a .45 vs. a .44 bullet) yields equal (or possibly even greater) overall force.
SO the bigger bullet (same weight) goes as fast, with "less" pressure. The "pressure" (total force or energy) is actually the same, but because it is spread over a larger surface area, it gives us a lower pressure number
per square inch.
Hope this helps.
(and if you aren't confused enough now, I can keep going,
...)