for the .204 ruger... you want to try to stay at 24" so you can squezze all that the round has out of it. the differance between the 24" and the 20 " barrel is around 50-70 fps slower. not much, but when you can go faster, why not? the best length seems to really be 23" but, its only a differance of 10-15 fps. so, you decide. also, i wouldnt hesitate buying a complete rifle from model 1 sales/ i hear really good thisng about there uppers, and there lowers are like any other. quality er shaw barrels and the finish is both matching, and good. all that i believe is needed, is a lower parts kit, and they ship the gun right to your door. or so i have herd, but overall, a good quality gun for the price. it can always be upgraded later.(i.e. better trigger, but since you will need a lower parts kit, i would start out with a good rock rive trigger.) i would continue some research, amsybe over at
AR15.COM
a lot of reallt good guys that are always willing to help.
hope this helps, brian.