Well, I always thought that I could screw up a wet dream.
Yep, I can.
I decided to assemble a Colt upper to a new Wilson .300 AAC barrel, since they went on sale. I already had a Noveske pistol length gas tube, a new gas block and a DD free float forearm, plus all the pins, screws and a crush washer for my new Wilson muzzle break. Put it all together, and took it to the office to try the headspace.
Imagine my surprise when the bolt wouldn’t close on an empty chamber.
The only part that I had never used before was the gas block. This style had the gas tube hole bored completely through. (You may consider this a hint).
If you don’t know, cause after disassembling this upper to fix my idiocy I certainly do know now, gas blocks can be put on backwards by numb nuts like me. Yes indeedy.
Reversing the gas block pushed the gas tube back too far into the receiver, which stopped the bolt from contacting the chamber face. Not to mention it covers the gas hole in the barrel. Duh!
I had already convinced the customer service rep at Wilson that my barrel was short-chambered, and he sent me a return UPS slip.
Luckily, I turned the receiver over, compared it to my SBR upper, and the dim bulb got a clue. I reversed the gas block and all is well. Re-using the Crush Washer on the muzzle was the hardest thing I had to do. This will be another part that I keep extras of, for my next case of inattention.
May your gas block always be of the one-way to install type. I guess using adjustable gas blocks have prevented this screwup on my past efforts.
Consider this a PSA

Well, at least you can admit to it.

I'm taking my effups to the grave.

Carry on.

Personally, I prefer the gas block to be drilled all the way through.  If you’ve ever had a gas tube seize up in the block you’d know the open ended gas block makes it relatively easy to use a punch to knock it out without ruining anything.  Open ended gas blocks have saved me from the headache of removing a stuck gas tube a couple of times.  

One just has to be smarter than the gas block.  That is, putting the gas block on backwards, installing the gas tube pin in front of the gas tube instead of through it, or installing the gas tube upside down.  Let’s not forget those that ram the gas block slam up against the barrel gas block shoulder when the shoulder has been cut for use with the front handguard cap, thereby blocking some of the gas port.  If you’ve been here much over a couple of months then you’ve seen all these mistakes made, and not always by new builders.

I’m with the above poster, my muck-ups are going to the grave with me.

On the bright side, at least your MD wasn't P/W'd

Doesn't block gas port and a larger shoulder may not be completely indicative of endcap use, but I'll admit to mounting the gas block all the way back againt the gas block journal shoulder and having the gas tube bind inside the gas key causing bad accuracy after a couple rounds. The gas block journal being long enough to accomodate a FSB should've clued me in, but it's been so long since I've dealt with that, I completely overlooked it until it was pointed out in my did I buy the wrong barrel thread.