I just assumed that you could find them with chrome lined barrels.
You assumed correctly apparently. The Palmetto State link that pilpens posted had a ton of 18" chrome-lined barrels available as pre-assembled uppers in a variety of configurations.
As for chrome-lined barrels, it all depends on what level of accuracy you need. When the Marines and Army were exploring the Designated Marksman/Squad Advanced Marksman concept, they both experimented with stainless match barrels and reached the conclusion that very few of the people in their study had the level of skill necessary to make use of the stainless barrels. Basically, the shooters didn't see any accuracy difference between a good chrome-lined barrel and a stainless barrel. However, they were also using M855 ammo, which was probably the limiting factor on accuracy and explains why the barrel made no difference.
When SOCOM looked at the same concept (an accurized AR), they did go with the stainless barrel, even though they have a much higher training rate and the stainless barrel isn't going to last as long. Of course, they also were using the 77gr Mk262 match ammo and had high standards of training and more regular refresher training.
So some questions to ask are:
1) What are my accuracy requirements - how big a target do I need to hit and at what distance do I need to hit that target?
2) If you give me a rifle that can meet those requirements, do I have the necessary skill to actually make it hit at those ranges?
3) What kind of ammo can I afford to feed it? If you buy a $600 AR15 and feed it cheap 55gr FMJ, you are going to get 3-4 MOA accuracy. If you buy a $3,000 top of the line precision AR15 from Knights or Noveske and feed it cheap 55gr FMJ, you are going to get 3-4 MOA accuracy. If you aren't going to handload or go with premium ammo, there isn't much point in spending money for an accuracy capability you'll never use.
I plan on putting a low 1-4 power optic on it. I just like the thought of a do everything SD rifle.
Every rifle is a compromise. For years, I tried to create a "do-everything" rifle that could do anything you could think of. In the end, I had a rifle that I liked a lot; but it weighed around 12-13lbs loaded and while it could do almost anything you asked of it, it couldn't do any one job as well as a more specialized rifle could have. The "jack of all trades, master of none" result.
That approach also overlooks one of the great benefits of the AR15 - they are extremely modular. Instead of trying to cram it all into one rifle and make that one rifle do everything OK, it is much easier to take advantage of that modularity. Have a red dot and a scope in quick detach, return-to-zero mounts and swap them out as necessary. Don't need the <insert piece of gear here> for today's shooting event? Take it off. Want a precision AR15 and an AR you can use for blasting without burning out the barrel? Get an extra upper and swap them out.
Having said all that, an 18" chrome lined barrel with a 1-4x optic is going to be extremely versatile and be able to do a lot of tasks well. I'll just tell you right now to save weight where ever you can because it is real easy to add it back on with various accessories.
