Depends on how old it is. If its an older one its 1:12 and the newer ones are 1:9 or 1:12. All the Marlin made barrels are 1:9 Best bet is to check it yourself. If you do not know how, drop me a message on my message board here at Answers.com
50-165
1/10
Just guessing- New England Firearms shotgun? If so, the answer is no. There ARE NEF rifles that can be fitted with a shotgun barrel, but not the other way. The frames for the shotguns will not withstand the higher pressures from a rifle cartridge.
H&R Handi-Rifle
Made 1989 to date, sold for about $150 new, No serial number data published.
No published sn data. You will have to call the factory.
Gunsmith or maker.
Marlin Firearms Co.
About $100.
Can't be determined from the markings you list.
100-500
The one issue with 5.56 in a handi rifle is the harder primers. You will almost absolutely have to install a heavier hammer spring. I have some Malaysian 5.56 and it often won't fire in the handi. I have heard the same about the Rossi single shots. As far as your concerns about pressures, the Handi handles all sorts of calibers on the same frame and barrel blank. Many of those push 65k or so. I don't worry about any difference in 223/556.