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.

Lined up gas rings, in a properly functioning AR, will NOT cause short stroking.

In a marginally functioning AR (one that has low gas delivery) lined up gas rings MAY cause problems.

It appears to be short stroking.

Do a short stroke test to confirm.

Load one round only in a known good magazine.  Hold the rifle loosely while retaining safe control.

Fire it.  Repeat say, 5 times.  The BC should lock back every time with only one round in the magazine.

If it does not, check the gas key to make sure it's not loose.  Check the gas tube retaining pin at the front sight base to make sure it's installed and that the gas tube is secure.

report back with results.

I did the short stroke test with one round in several different magazines, and sadly, the bolt would not lock back 7 times out of the 10 times I tried.

While cleaning, I shot some brake cleaner down the key and noticed that there was a small amount of brake cleaner seeping out between the key and the carrier on both sides.  I now believe that my issue is caused by a gas leak due to a bad fit between the key and the carrier.  

This is the first J&T upper I have had that looks like the carrier was parkerized because it has a rough finish instead of the smooth finish on the older ones.  It looks like I am either sending it back for further diagnosis OR re-staking the key to the carrier after some lapping.  Regardless, it is disappointing to have problems with a new upper.

ETA:  If I can sneak away for a little while tomorrow, I will try my Bushmaster carrier in this J&T upper to see if the problems go away.

Quoted: Is it normal for a new upper to short stroke after approximately 100 Yes, during this break in period, the upper bearing parts are mating together (read higher than normal Anodizing/Parkerizing powderized fouling), and the chamber is polishing out (read higher than normal gas blow-by before the case fire forms to the chamber gunpowder fouling). Just to add, had you just pulled the B/C when the rifle started to go problematic and applied a fresh coat of CLP to cut the accumulated fouling (read applied to the B/C, but allowed to migrate through out the upper during live fire), the rifle would have run without incident.

I am not giving up on this upper yet.  I did want to add that after a good cleaning, the action is a lot smoother than when this upper first started giving me problems.  I believe I need to take a day off of work to test fire it some more

Also, I want to thank everyone who has taken time to reply thus far.

lap the key, torque down, and stake

Quoted: lap the key, torque down, and stake

I am thinking of bringing the upper back to the place I purchased it from to show them the leak at the carrier.  FWIW, I pulled my Bushmaster carrier out and I see NO seepage when blasting brake cleaner down the key.

Dont condem them give them a chance.Now you get to test there customer service.
Check with who you purchaced it from first if not J+T.
Dont take it apart they will most likely want it back as is.

Quoted: Dont condem them give them a chance.Now you get to test there customer service. Check with who you purchaced it from first if not J+T. Dont take it apart they will most likely want it back as is.

You are right.  Years ago I worked as a comptroller for a manufacturing company, and even with all the quality control standards in place, a bad product can slip out there every now and again.  If you keep your customers happy, you will have a constant revenue stream.  You piss them off, and competition will bury you.  

The sad thing is I can actually fix this issue, but, I think I will test out their customer service just to be on the safe side!  I did purchase the upper from a J&T/Double Star dealer in town and I know they will take care of me!

EDIT:  I brought it in just after lunch and the dealer took it apart before my very eyes.  The gas tube at the FSB appeared to have WAY too big of an opening.  You should have seen the carbon build-up around the FSB end of the gas tube.  He said that it will be fixed ASAP.  As a result of being taken care of in such a quick manner, I want to take back my comment about being disappointed in J&T.  Anyone can produce a lemon on any given day, and the out of spec gas tube probably wasn't even J&T's fault.

Quoted: Can you post a photo of the key staking? The reason that I ask is I have order more than few kits direct from J&T and yet to have one of their keys that leaked. Not saying that J&T may have not gotten it wrong, but do wonder if this kit started off stripped, and the dealer didn't install the key correctly when putting the parts together.

The key appeared to be staked on well because there was no play in it and the screws were pretty dang tight when checked by the local shop.  Sadly, I can't post a photo because the dealer has my complete upper.  He speculated that there may have been a burr or two that made its way between the key and the carrier when assembled.  He said that he would pull it apart, check it out, and re-stake it for me due to the leak.

Also, my main problem appeared to be due to a faulty gas tube at the FSB end.  The local shop owner pulled the gas tube from the FSB and noted that there was way to much carbon build up for a rifle with that low of a round count.  He also showed me that the hole in the gas tube at the FSB end was a tad bit out of spec (i.e. too big).  Sadly, I could not tell from visual inspection alone.  

I was hoping to make this 20" chrome moly HBAR my "tack driver".  Although it got off to a bumpy start, I know that my local dealer will take care of me because he believes in customer service.

On a funnier note, the shop guys were picking on me saying that my problems were due to the fact that my rifle was too clean.  They also dubbed me the Warranty Warrior

Update:  I called my dealer today, and he said that a brand new upper is inbound.  Now that is what I call customer service!  The only reason it is taking so long is that 20" non-chrome lined barreled uppers were on backorder for the past week or so.