akodo: Tastes vary. IMO, the fewer and smaller the areas in direct contact with the barrel the more consistent the accuracy is likely to remain. If you'll look closely at most any tube feed design, there will be some gap visible, and part of the reason for that is to help prevent random contact from affecting the harmonics of the barrel's vibration cycle.
A Timberwolf in nice condition isn't exactly what I'd call "inexpensive" anymore, assuming that you can find one for sale. The nicest examples of the very few that I've seen on Gun Broker over the last couple of years have brought prices averaging around $650. The single LNIB w/papers example I can recall sold for almost $900.
Beagle-zebub: The Timberwolf had every feature you described as being desirable except the PG stock. It also had a scope/optics mount integral with the receiver. Not pretty, but hellaciously tough and practical.
Jack 2427: My late father-in-law was one of the survivors of the long walk back from Chosen Reservoir. His gripe concerning the .30 carbine wasn't that it wouldn't penetrate those quilted jackets, but that you could hit the guy wearing one four or more times solidly in the torso (he described them as being close enough that he could see the cotton stuffing puff from the exit holes in their backs) and not put them down.
Speaking of the M-1 carbine design, I recall the late Col. Cooper describing a conversion being done at the time as part of an experiment toward realizing his "Thumper" concept. This was what he called his idea for a self-loading carbine and/or SMG utilizing a cartridge more powerful and effective than standard pistol rounds while remaining of the same general size.
There was an outfit (I regret that I can no longer recall its exact name or location) that was converting M-1 carbines to fire 10mm and even .45 Win. Mag. ammo. To the best of my recollection it used highly modified GI magazines which reduced the capacities from 15 to 7 and 30 to 10 in .45 Win. Mag. I may be mistaken, but I believe that he mentioned that the difficulties involved in getting rimmed revolver cartridges to feed reliably from a box magazine in that platform was a major reason for the decision to concentrate on the most powerful production rimless handgun rounds then available.
The conversion supposedly worked well, but was very expensive. The "Thumper" concept didn't catch on in either military or LEA circles either, and lack of demand killed both of them off.
Percy, I believe that the old Winchester 1905, 1907 and 1910 SL carbines used an operating system which could be broadly defined as being "delayed blowback" very similar to what you describe. It was relatively complex and could only operate safely and reliably with one specific loading. The .401 Win. SL proved to be about the most powerful that the basic platform and concept could handle. Even if such a system could be designed to handle the pressures of .357 or .44 Mag., I seriously doubt that a practical solution to the problem of making it usable with a variety of loadings can be found within its inherent limitations.
Gas operation can be regulated to some extent via some sort of variable valving or the like. But I still don't see how one could use more than a very few lead bullets in any existing system without messing it up in short order.
Just a thought, but I'd like to hear some other opinions: What about an inertial system a la the Benelli shotguns?
