Early this week, I sent my 607 upper assembly to JT. Since I built it a few years ago, I have had short stroking and chambering issues with it. I finally decided to have it looked at by someone who is very good at what he does. He got the lower on Thursday and went to work. At first, I thought it was the gas port being too small. He check that and said it was pretty open (.104"). He tested it a couple times and it was still demonstrating the problem. He even tig welded the hole and re-drilled to a smaller size and it was still having issues. Low and behold, he put a gauge on it to check the headspace. He said it was way out of spec. That is the first barrel I have ever had that had that problem. Needless to say, the barrel is no good. I have since ordered a new 16" Ballistic Advantage 1/7 pencil barrel for John to cut down, re-thread and install the newly shaved FSB onto. Hopefully, in a couple weeks, I will have a 607 I can finally enjoy shooting.

It really is easy to get complacent with the AR platform and just take for granted that mil-spec parts produce a working gun. This is true most of the time, but just late last year I helped a friend diagnose a problematic AR; turned out to be bad headspace. It's very much worth having a set of gauges.

since he's competent enough to weld and redrill the gas hole, can't you remove the current barrel extension, get the headspace in spec and then install a new barrel extension and drill a gas hole?  should be salvageable.

Who made the out of spec barrel?

I do not recall. I bought it a few years back.

Lionel,

It is easier to have him modify this new barrel. Besides, I know that it will have a good barrel. The barrel has already shipped and is expected arrive at John's on Monday afternoon.

cool.  just sayin don't trash the old barrel.  most of the times people don't want to mess with the barrel extension b/c they don't want to deal with the gas port alignment but if you're smith is able to do that, it's salvageable.

Jrs,
   I've been meaning to ask forever how comfortable is the LOP when the stock is fully collapsed? Clearly it's operable in that position but can you get a good sight picture or is it just too close?

For me, at 5'6", the LOP is not too bad with the stock collapsed, but it is much better extended. In my opinion, with the stock collapsed, it is really a bit close to get an effective sight picture.

UPDATE**

John Thomas has finished with the 607 upper assembly. Newly shaved FSB, new Ballistic Advantage barrel (cut down to 10" and re-threaded), moderator modification, and test fired. It works, but not with the bolt carrier and bolt I have. It worked fine with the two BCGs he used. I have ordered a new Tony's Customs chrome M16 BCG for it. Now, I need to get an extractor upgrade kit for the bolt.

What kind of buffer are you, or are you planning to use?

When I first assembled mine, I used an H buffer but, it wasn't heavy enough. Cycled fine but, the recoil tore up the locking tube in the stock. Stoner25mkiv graciously repaired it for me and recommended I upgrade to an H2 or H3. I picked up an H3 to be on the safe side and it works like clockwork with decent ammo. It doesn't like my old stash of Herter's steel case though. Probably too much weight for that low powered junk.

So he welded on the barrel prior to checking headspace?

Sell me your defects.

As of now, it operates just fine with a standard carbine buffer. That is what it was tested with.

He was doing the work I had requested. It had never dawned on me that the headspace could have been an issue, so I did not have John check that. After all of the attempts to solve the issue by correcting the gas port, on a hunch, he checked the headspace. Yesterday, he sent me videos of him test firing the upper and showing me the difference on the headspace checks on each barrel. The old barrel was a huge "no go". The headspace on  the new BA barrel is dead on. I trust John entirely. He is a very capable person and does marvelous work.

I had an issue like that with my 610 clone , the carbine rubber number on the  buffer was getting pummeled and pieces were chipping off , an h3 buffer fixed my issue.

It sounds like the same problem they had with the 607s and XM177E1's - bolt bounce from being overgassed... although it's unusual in a semi-auto. Like others have said, a heavier buffer is key. As I'm sure you know, they solved this with the E1 by simply using a fixed stock, and the heavier rifle buffer (about the same weight as a 9mm) solved the issue. Of course with a 607, that would be a problem, since the coolness factor is all about the stock...