I built a Frankingun and have run into an issue with the gas key. The BCG is NIB and was difficult to go into battery. With lube and several hundred manual racks, it seemed fine. At the range, it locked up after only five rounds. It wouldn’t go into battery nor would it open. After it cooled down at home and I beat it down, it popped open and the live round came out. I discovered the gas key was too narrow for the tube bulb. I worked it over again and went to the range. This time I got about 5 rounds out and it locked up again. Only this time, it would not extract the spent round. It took hours to get it apart. The tube bulb was stuck in the gas key this time. After working with it for a couple days, I discovered the outermost half of the gas key is slightly smaller in diameter than the innermost half. I push the gas tube in and it is tight until half way in and then it gets loose. There is a good bit of play while fully inserted but when I start to pull it out of the key, it jams up and I have to finagle it around to get it back out. I've taken a small metal probe and cannot find a bur or anything that catches so the internal area is smooth but just narrower.
1. Is the interior diameter supposed to be graduated like this?
2. If not has anyone ever encountered this?
3. What is the most efficient way to deal with it?
Thanks!

Try a different BCG

If that part is that out of spec, there is no telling what corners have been cut manufacturing that bolt carrier. Why risk it? you can get a quality BCG now days for less than $100. You can always replace the gas key for a fraction of that also.

^This. If another bolt is doing the same, then the gas tube is your culprit. I have had gas tubes come all jacked up from the manufacturer before. Easy to slip QC, apparently.

So I take it from the responses that the inside of the gas key should be the same diameter throughout. Understand that I've never dealt with this nor have I seen a schematic so not sure what it is supposed to be like.  
What is the inside diameter technically supposed to be anyway?

The inside of the gas key does shrink slightly in size at the rear to form a seal around the gas tube flare. You can usually feel the carrier snug up to the gas tube flare as the carrier moves forward into battery.
Here is a cut out of the gas key. The decrease in the gas key size is visible but very small.

The diameter is 0.1806 +.0008 inch for the first 0.82 inch (the length that it interacts with the gas tube), then it goes to 0.172 +.005 inch for the next tenth of an inch and finally going to 0.116 +.005 inch.

The countersink is 60 degrees with a maximum diameter of 0.225 inch.

The gas tube is 0.1800 -.006 inch at the button head.

The end of the gas tube does not contact the 0.172 diameter as stated above, if you calculate all the distances it stays about 0.100 inch away from that smaller diameter.  The seal is simply due to the small size of the gap between the OD tube and the ID of the key.

Thanks! I tried to upload a pic but no luck. I have two narrowing spots. Basically I'm at about 0.1775" diameter from 0-0.15" and then again from 0.3-0.4" ish. Then it goes to 0.18"ish the rest of the way.
I guess I'll be ordering a new key.