Internal length of pistol buffer tube is 6 3/4 inches from rear of tube to the top side flush with the receiver.  Length of spring is 10 1/2 inches long, 40 coils.  Buffer is 3 1/4 inches long.

Manually cycling bolt, with or without a magazine inserted, bolt does not lock back.  Bolt rides over bolt catch by about 1/16th of an inch.  Tried bolt from another upper, same thing.  Took original bolt and upper and attached it to another pistol lower (different buffer tube) and it locks back as it should.  

So what next?

Replace defective pistol buffer tube? Should be closer to 7" than 6 3/4".

If you must continue with what you've got, shave 1/16" off the buffer's urethane bumper at the rear to get sufficient dwell time for the bolt hold-open to engage.

I was thinking about shaving the silicone from the buffer.  Opened it up (rust powder had a weird smell between the buffer weights), then looked at the buffer tube.  I had had the tube notch snug up against the buffer retaining pin post.  Backed it off one revolution so that the tube is holding down the buffer retaining pin but not resting on the post - that fixed my problem.  Managed to mangle the takedown pin spring (had an extra so I replaced it) in the process though.  Staked the castlenut, now I just have to take it to the range.

The 6 3/4 " measurement was within the tube itself.  Externally its 7".

the depth of the hole in the buffer tube should be 7.0 on the money

Measured from the lip with the notch or the shorter edge that is flush with the top of the receiver? If I measure to the lip, it is 7", if I measure from the shorter lip, it's about 6 3/4".