OK, there's actually several issues.
First, the transfer bar that started with the New Models (2 screw, 1973 and forward) is an automatic safety similar to what's found on modern DA wheelguns. In other words, you can load it six-up, drop it, and it won't go "boom".
Downsides: there's a slight boogering of the trigger feel as a result. Also, the loading sequence changed a lot: on a new model, you leave the hammer down, open the gate, and an interlink between gate and pawl allows the cylinder to spin. Problem: as you spin it, it clicks, but the clicks don't match up with the cylinder bores actually appearing in the loading gate window lined up ready to drop a round in. And since it can't spin backwards, spin it too far and you've gotta go all the way round to load that cylinder bore.
Which is why Power Custom sells buttloads of $35 "freewheel pawls" with good installation instructions - you can then spin the thing both ways with the loading gate open.
On an old model, you half-cock the hammer, then open the loading gate - at which point the bores line up with the gate on every click.
In other words, the "old model" (3 screw) is basically a Colt with improved metallurgy and coil springs instead of flat.
Still with me?
Good, it gets weirder
.
After 1973, Ruger realized they had a problem. By selling guns that had the safety, they had basically admitted that transfer bar guns ARE safer (they really are, in that idiots would sometimes FAIL to load old models five-up and lower the hammer on the empty). So their lawyers explained that they could be sued if some damnfool blew his foot off with a fully loaded old model.
So, sometime shortly after 1973, Ruger developed a transfer bar retrofit kit for old models, and did a free recall and upgrade program. It's still in effect - send them an old model, you'll get it back with a new hammer, trigger and other stuff in there and the old bits in a baggie. Said old bits CAN be re-installed, so the collector value of the gun is OK so long as you keep the baggie. Almost nobody does. And when an OM goes back for ANY repairs, or even re-bluing or whatever, it gets upgraded and they won't even listen if you tell 'em not to.
So most of the OMs you'll see out there HAVE a transfer bar. On a NM, pull the hammer about halfway back and jiggle the trigger (UNLOADED GUN, please!) and you'll see the transfer bar. It's a flat piece of metal. When the trigger is pulled, it rises. The hammer is then able to hit it, and it in turn hits the frame-mounted firing pin. The hammer is "stepped" such that when lowered and the transfer bar is down (trigger not pulled), the hammer can't hit the firing pin.
Now check the old model - if it's unmodified, the hammer face will be dead flat and there'll be no transfer bar. That gun is safe FIVE-UP ONLY, hammer on an empty. Pretend it's 1873. Modified, well, you'll see the diff.
The loading sequence is the same regardless of the safety present or otherwise - in other words, Old Models always load on the halfcock as described.
Next: all centerfire NMs are build on the same basic frame. It may have different sights, grips, barrel length, SuperBlackhawk Blackhawk, Bisley, Vaquero, whatever - doesn't matter, the primary frame IS capable of 44Magnum or stronger loads. Take that 30cal NM, swap barrel and cylinder (or re-bore both), and you've got another caliber up to 44Mag/45LC.
With the old models, there were two frame sizes. They're usually called the "357 frame" and the "SuperBlackhawk" frame. The latter is capable of 44Mag horsepower. The former is NOT. It's smaller, handles better, but isn't as grossly overbuilt as all NMs are.
I'm 95% sure the OM 30cal was built on the 357 frame.
So which to get?
The NM is stronger in many ways, and there's a lot more parts availability (esp. aftermarket). But OMs, esp. the ones on the 357 frame, have a near-cult following, many people swear by 'em. That frame is often rebuilt as a 44Spl and in theory could handle 45ACP. You COULD set it up for 45LC but most gunsmiths would quite properly refuse - you'd end up with a 45LC that could take "cowboy loads" or a bit stronger, but NOT the "Ruger ONLY!!!" 45LCs that are often hotter than 44Mags. Some of those such as the Bufallo Bore 325grain @ 1,300fps
would probably blow an OM based on the 357 frame to Kingdom Come.
Hope this helps.
