Chrome lined barrels have greater longevity because they are better able to resist conditions which erode the bore (combustion heat during shooting, moisture/rust, powder or copper fouling and general wear & tear from handling or cleaning).
Chrome lined barrels don't lose accuracy any more than non-chrome lined barrels do. With proper maintenance & cleaning, they'll probably retain accuracy longer.
While it may be possible to have a barrel chrome-lined after purchasing a rifle, it is not cost-effective. It's better to purchase a new barrel which was chrome-lined at the manufacturer and install it into a rifle. (This would apply to mil-surp rifles too.)
Here's some material from a FAQ page (sourced below)
"Q: What is the difference between Chrome/Moly and Chrome Lined? How does Stainless fit in?
A: All manufactures use Chrome/Moly steel to make barrels - its a type of steel that has some chrome in it to help with wear and corrosion resistance IT IS NOT the same as 'Chrome Lined'. There are two types of Chrome/Moly that are used to make AR-15 barrels. The most common (an most inexpensive) is 4140, its a decent steel that is used on many firarms. The other (more expensive) type is 4150, its got more chrome in it and is more durable and corrsion resistant; only Colt, Bushmaster, and FN use 4150 steel for their barrels. A 4140 chrome/moly barrel is the least expensive barrel you can purchase - yet they can be quite accurate.
Chrome Lining is a process where chrome is 'welded' to the steel of the barrel. This produces a coating that is twice as hard as the steel. It prevents corrosion, makes cleaning much easier, and extends the barrels life. There is also a slight drop in accuracy (compared to an identical quality non-lined barrel), about 0.5 MOA worth from what I've seen - something 99% of shooters won't notice. Chrome lined chambers are a reliability enhancer. Only chrome/moly barrels can be chrome lined. Bushmaster and Armalite chrome line their barrels, Colt chrome lines their chambers (and the bores on their military profile barrels), and RRA has produced limited runs of chrome lined M4 barrels. Chrome lined barrels have the longest life expectancy.
Stainless barrels are used on match rifles. The Stainless steel is easier to rifle consistantly, and consistancy is the key to an accurate barrel. Once broken in stainless barrels are almost as easy to clean as chrome lined. Stainless barrels are also very corrosion resistant. Some companies (like Olympic Arms) have a process that blackens the stainless, others (like DPMS) leave the barrels 'in the white' (silver color). Generally the most accurate rifle have stainless steel barrels."
Q: How long will my barrel last?
A: (with data from Bushmaster and LTC Santose)
The U.S. Army recently (Oct 2000) reported the Mean-Time-Between-Failures (MTBF) for M16A2/M4/M4A1 barrels is 9600 rounds. These are chrome lined barrels with a 1:7 twist made out of 4150 steel (just like Bushmaster & Colt use), and they see full-auto use. Also note the military is using full power M193 & M855 rounds, while most commercial .223 runs 100 to 200 fps slower (with the same bullet weight). The 1:7 twist barrels do burn out quicker than the 1:9, especially under full auto stresses. For a chrome lined barrel on a semi-auto only rifle 20,000 rounds (or more) can be expected. [Warning: More civilian barrels are ruined to over (or careless) cleaning than to being "shot out".] Stainless barrels won't last as long, 4140 steel barrels wear longer than stainless; but not as long as the Colt 4150 barrels (w/o the chrome lined bores). nor as long as the chrome lined bores (given the same shooting)."