I purchased a new Ruger AR Elite complete lower receiver.  I attached a used LWCR M6A2 upper receiver and went to the range.  After six rounds the bolt and bolt carrier locked to the rear position.  I disassembled the lower receiver and got the rifle apart.  I could not determine the cause of the lockup.  I assembled the LWCR upper to a different Ruger SR lower.  After 60 rounds there have not been any issues.

I attached a Ruger SR upper in 6.8 SPC to the first AR Elite lower.  After about 25 rounds the bolt and bolt carrier again locked to the rear.  I disassembled the lower and got it apart, again without finding the cause of the lockup.  The top front of the buffer tube was peened enough that the buffer and spring were held captive when I removed it.  The peened area matched the size of the carrier key section of the bolt carrier.  I read to FAQs and determined the largest diameter section of the firing pin was somewhat chewed up.  The gas block on the upper was set to position 2.

I compared the height of the hammer on both lowers and they appear to be about the same height.

Suggestions?

Sorry, no pics.  The bolt face is to the rear of the ejection port.

Is the buffer in the Ruger as long or longer than the buffer in the lower that works?

How does the Rugerbuffer tube length compare to the buffer tube on the working lower?

It sounds almost like your bolt carrier is being allowed to go too far back....

The Ruger SR lower (the one that works): Buffer tube 7 7/8 inches long, the buffer weighs 2.94 ounces, and the buffer spring weighs 1.83 ounces.

The Ruger Ar lower (the one that doesn't work): Buffer tube 7 7/8 inches long, the buffer weighs 2.98 ounces, and the buffer spring weighs 1.94 ounces.

Both buffers and buffer springs are the same length.

Good buffer tube 7 inches inside.  There appears to be an insert at the bottom of the tube.

Bad buffer tube 7 3/4 inches inside, no insert at the bottom.

Both buffers 3.25 inches long, and both buffer springs at 10 7/8 inches long.

I installed a replacement buffer tube, buffer, and spring.

The bolt carrier was hitting the front lip of the buffer tube, but no marking on the lower receiver.

I will get to the range as soon as I can to test fire.

When did they start offering a warranty?

Check to see if the Ruger upper has an SP1 type carrier with the cut out ramp exposing the firing pin. If you use the wrong type firing pin with the BCG or the wrong type hammer in the lower with an sp1 type carrier it can hang up the BCG.

Got the range today, 40 rounds without any incident with the buffer assembly.

Sent an email to Ruger explaining the issue.  Now I wait and see.

I put another 60 rounds through the rifle today without any issues.  Nothing from Ruger yet.

Talked to Ruger Customer Service today.  They are sending me a new buffer, buffer spring, and buffer tube.  Excellent service.

As mentioned above, the operable tube is 7 inches deep while the bad one is 7 3/4 deep.

I received the replacement parts from Ruger yesterday.  I have installed them but it is kind of cold and snowy today so I didn't get to the range today.  The replacement tube is 7 inches internally.

If your gas key (or whatever similar part a piston gun has) was hitting the rear of the lower, you may have done serious damage to the lower and the gas key screws. I got a F'ed up POS Chinese buffer tube that was to deep once, now the first thing I do on a new buffer tube is measure the inside depth.

No contact with the lower receiver, just the buffer tube.  The rifle was working properly with the replacement parts.  Now I just need it to warm up enough to shoot it with the Ruger parts.