I can offer you an opinion based on years of marksmanship experience and training. Ask youself the following questions:
1) What will be the primary purpose of this rifle? (Target shooting, hunting, plinking, etc...)

2) What is the total budget for the rifle? (How much can you afford to spend. Don't forget you will most likely want an optical sight requiring bases and rings, this can add up to be more than the rofle itself)

This may seem simple but can answer many questions if viewed in the proper perspective. Use your answers to eliminate options rather then trying to select one option from a sea of others. If your primary purpose for this rifle is to practice long range target shooting you should eliminate ANY semi-automatic gas operated rifle. I'm sure folks will argue with me but it is an established fact that bolt action rifle are inherently more accurate than their semi-automatic / gas operated counterparts. This is especially true if you are on a limited budget. Sure I have seen semi-auto's based on the AR action shoot 1/4 MOA or less @ 100 meters, but the guy paid about $2000 for the rifle. For the $600 buget that you are working with, I would not recommed any semi-auto rifle, you simply will not be able to purchase a quality product at that price. My opinion, in this particular case, eliminate the semi-auto or save for a while longer.

You mention that you would like a sniper style rifle, take a look at what military and police/SWAT snipers are using. 99.9% bolt action rifles. There is an abundant selection of acceptiable quality bolt action rifle out there. You can then narrow the selection in this field further first by deciding what is most important to you, appearence or accuracy? If you stay within a price range you will notice this; a company will spend only so much money and effort building a production rifle. Their time is either spent on appearance or accuracy for the most part. I is in my nature (and traiing) to forget the "look" of a rifle and focus on how it feels when I shoulder it, how smooth is the action, how bad is the factory trigger, does the company make any accuracy promise? If you choose this route (which I recommend) it is easy and failry inexpensive to put a cool stock on a gun if you so desire, it is extremely difficult and possibly very expensive attempting to troubleshoot and correct a poor shooting file. The rifle is a tool that delivers a payload (projectile) to the exact spot you desire; keep this in mind when a shop owner trys to sell you the high polished low-end Howa for the same price as a high end (accuracy wise anyway) porrly finished Savage. If your guns looks nice, but your are not hitting your target, what good is it? If you want decoration buy a reproduction Sharps and hand it on the wall.

There are many bolt rifles that have great accuracy potential right out of the box. My personal suggestions, well again folks will give me what for but, Savage is generally a very accurate rifle out of the box, you pay the price with some of the worst metal finishing I have ever seen on a rifle, the blued rifle rust when exposed to the eliments very easliy. All that said I own a Savage, it does the job well.
Winchester makes overall well made rifles. The fit and finish is much nicer than the Savage and the accuracy does not seem to suffer, I would place Remington in the same category, I own several of both and they do well. Ruger, I have had trouble with Ruger centerfire rifles, owned two. The only two guns I have ever sold. Neither met the major requirement, accuracy. Fit and finsh were above average, the barrels were not. I tried every home gunsmith's trick to resolve the issues, glass bedding the receiver, ensuring the barrel was free floated, tried as many different loads/projectiles/power combinations as my arm would let me reload, no positive results. Even tried lapping the barrel, little to no improvement. Maybe it was bad luck but I can not recomment them. There are other rifles out there to consider but I have no experience with them.

I know this is a lot and feel free to ask me more specific questions I will be happy to share the knowledge. I can and will give you the straight no BS factual answers. The last thing to consider, .223 is a fine caliber but it does have its limitations. If you would like to try your hand at say Whitetail deer hunting you will want a little more bite than a .223. If you plan to only target shoot and maybe shoot the occasional ground hog, .223 is excellent. Again from what you said (a sniper type rifle) look at what the pro's use, mostly .308, which happens to make an excellent deer cartridge. Actually it is adequate to take all game on the face of North America with the exception of the large Bears. Both .223 and .308 ammunition is among the most reasoniable to purchase and both have an excellent variety, bet you money that your local Wal Mart has at least two different loads in either caliber. I will not talk calibers specifically, that is a never ending battle. Folks defend their favority caliber like it was their daughter on prom night! Among the most popular by gun and ammunition sales are 30-06, .270 Win, .308, .223, personally I would stick to one of these calibers, all those folks purchasing them can't be wrong, they are among the most tested and proven calibers on the market.

That is all I can offer for now, please feel free to ask me anything. I look foreward to contributing to this message board. I hope this will at least give you a place to start sorting through the confusion.