How does the .260 Rem fare in shorter (16") barrels? Lots of muzzle flash (hard on cans, as I understand it- not terribly intersested in droppin' near $1K on a custom can, only to tear it up quickly) and substantial loss of Muzzle velocity, I'll bet.
Don't know, but I wouldn't think it would be any worse than from a .308, they are the same case, and a quick glance at my load manuals show they use a lot of the same powders. Even loaded down a bit, it would be head and shoulders better than the 6.8SPC
If there was a company that made reproductions of WWII-era rifles from all sides, they would get a lot of my money. Rifles like the Garand, StG44, MP40 (semi-auto SBR, maybe?), etc. Taking away the collector's value and allowing one to enjoy the design and reflect on the history it represents would make for some highly interesting firearms.
There is no money in the Garands (or any of the WW2 bolt guns for that matter), there are simply too many originals out there for reasonable prices. A new manufacture M1 would also be expensive, they are very labor intensive with a bunch of machining required.
It would be awesome if someone made an affordable replica of the STG-44 that fired 7.62X39 (or 5.45x39 or even .223 for that matter). There was a company making a semi-auto reproduction of the StG44 in 8mmK, but they didn't make many, and were very proud of them. Like $5000 proud, If I remember correctly.
A MP40 would not be difficult, it would just need to be configured so that the folding stock would not extend so it would be classified as a pistol. SOG is selling a PPS 43C like this.
TNW Firearms
http://www.tnwfirearms.com/index.shtml makes semi auto version of the Soviet PPSh-41 and Finn Suomi M-31 submachineguns as well as some other cool stuff, like semi MG34, M1919 and M2HBs.
As to interesting things I would buy? A pistol caliber carbine (that doesn't look like a prop from a Starship Troopers movie) using existing off the shelf magazines. Sten or Uzi for 9mm and possibly .40S&W with some mods, Thompson or grease gun for .45ACP. Either that or that use pistol magazines, probably Glock mags would be the best choice. (Ruger and Marlin both made carbines that used pistol mags, but both are out of production)
Two years ago I would have said an AR Upper that shoots 7.62x25, but unfortunately the supply of dirt cheap ammo dried up. Now is is just fairly cheap, and there is no telling how long it will remain that way.