theres a cutaway pic of the 6.5 LSAT on google, and they managed a very VLD bullet, which apparently had good feeding/seating, because the case had a thick internal shoulder for the bulllet to run along. Whereas textrons round appears to have no shoulder at all, its just pure powder.

this leads me to think you could have a 257 or 6.5 with an almost-fabrl quality bullet, which would be incredibly efficient.

why did the LSAT approach not see further success do you think

I know of only two cutaway pics.  Both are actually from Textron.  Neither has a very VLD bullet.

And "Photo 18" is obviously an artist's concept drawing, not a photo of a real cartridge or bullet.

Some reading that may be of interest:

BACK TO THE FUTURE? Textron Displays Newest Polymer-Cased Telescoped Firing Rifle -The Firearm Blog

An Analysis of the Soldier's Load with 6.5mm Cased Telescoped Ammunition (Part 2) -The Firearm Blog

ARES Research Report No. 7 - Cased Telescoped Ammunition (armamentresearch.com)