I just picked up a J&T 20" upper this past week. I followed all of the same steps that I have with every other gun that I purchased new. I cleaned it and lightly lubed everything with CLP.
Today, I brought it to an outdoor range to see how the irons were from the factory. They were pretty dang sweet, the gun ran fine for about 30 to 50 rounds, and then we packed it up and headed back home. Well, upon heading back home, I figured I would try out a used aimpoint on the 20" upper for giggles and grins. I head to the 25 yard indoor range, sight the aimpoint in, then my rifle starts to short stroke on the after I put two more magazines through the gun.
I pull the bolt, which is dirty as can be, and see that the gas rings are all lined up. I fix that, load another mag, the gun runs fine for about 15 rounds, and then starts short stroking again. I look in the action and it appears filthy as can be and now I have to use the forward assist to tap the bolt home.
So my question is: Is a new upper only good for 100 rounds before cleaning is necessary? FWIW I used both Lake City and Winchester ammo at the outdoor range, and strictly WWB at the indoor range (steel projectiles that ricochet do not feel good).
I didn't have this problem with my Bushmaster, but that "may" be the difference between chrome and non chrome lined. I am going to check the gas tube, carrier, rings, etc. when I break it down for a cleaning shortly. I figured I should easily be able to hit 300 to 500 rounds before worrying about cleaning.