CimaGarahau

Along with the XM291 there's 3 photos, the first one is of the gun itself, the 2nd one is the XM8 with the XM291? and the 3rd photo is of the ATAC System Demonstrator aka Thumper, perhaps indicating that the gun tube which the Thumper is armed on the photo is the 120mm L/56 gun tube.

XM360 and the XM291 with the 120mm L/56 gun tube in the background

Saw some photos of the XM291 with the 120mm L/56 gun tube few months ago, it's also stored at the U.S. Army Watervliet Arsenal (Benét Laboratories), the gun also had the "famous" Thumper photo along with it.

ATAC System demonstrator also known as "THUMPER" remotely firing the 140mm gun tube which got installed in 1992. 

Photo of the finished XAP-1000 AIPS used on the CATTB.

ATAC System Demonstrator also know as Thumper, to receive the 140mm gun tube and a functional rearm port, 1992.

Warren Jones, right, project officer for the U.S. Army, instructs Oak Ridge National Laboratory staff on how an M1 Abrams tank operates in September 1992. From left are Dong-Soo Kwon, Carol Scott, Sam Meacham, Jim Hannah, and Katie Vandergriff.

Katie Vandergriff and Jim Hannah helped develop the ATAC System Demonstrator's rearm port, they also developed the Future Armor Rearm System (FARS). 

Compared to it's first appearance (1988), Thumper received weight simulators in the turret, they also changed the mudguards? Or the turret was assembled on another M1A1 chassis. The XM291 gun got removed from the turret to change the gun tube, from the 120mm L/56 (6.75m) gun tube, to the 140mm L/48 (6.75m) gun tube to test the FARS with the new two-piece ATAC 140-mm ammunition.

The ATAC System Demonstrator and the FARS are sitting together at Sierra Army Depot.

Location: 40.2051086922686, -120.13611546011911

ATAC System Demonstrator with the 140mm gun tube and the FARS.

  RED = CATTB Phase ll                                                                 BLUE = CATTB Phase l

CATTB Phase l

Very rare CATTB Finite Element Stress Analysis document in color and higher quality.

https://www.docdroid.net/f6S4v1w/ada228389-pdf#page=10

I'm pretty sure the CATTB Phase l is the one using a normal M1 hull, while the Phase ll uses a completely new hull  for the AIPS and for the hull ammo storage, while also applying the modifications suggested on the CATTB Finite Element Stress Analysis, they also installed the IN-ARM Suspension on the CATTB Phase l and Phase ll.
It seems like the M1 hull only got used because they hadn't finished the CATTB Phase ll hull.

Some info about the tracks.

Requirements for the hydropneumatic suspension.

Cadillac cage suspension.

Teledyne suspension.

I belive this photo of the ATAC System Demonstrator shows the 120mm gun tube.

While this one shows the 140mm gun tube, you can see how thicker it looks compared to the first one, it also seems to very similar to the gun tube that appears on the video below.

Jane's and Soldat und Technik says it's the XM291 with the 120mm gun tube, Soldat un Technik says the photo is from 1988 and Jane's says it's from 1987.

Indeed, it looks very identical to the ATAS ATD I, even the bore evacuator looks similar compared to the ATAS ATD l drawing and way it's smaller than the CATTB's bore evacuator

I've found a very rare document, the Finite Element Analysis of the ATAS turret, also called ATAC System Demonstrator or Thumper, the gun used is the XM291 with the 140mm gun tube, the same gun is also used in the CATTB Finite Element Analysis, but in the CATTB's case they renamed the XM291 140mm gun to L.W. 120.

How do i know they're the same gun? the stresses due to firing load of the CATTB gun (L.W. 120) is (375,000 lb), while the ATAS gun which is called (ATAC 140) share a very similar number, 370,000 pounds. This also reinforce that the CATTB used the 140mm gun tube.

https://apps.dtic.mil/sti/pdfs/ADA275853.pdf

XM964 APFSDS-T 

Height Comparison between the XM964 (Only forward part) and a M829A1, the XM964 seems to be a bit longer than 40 inches if they're draw to scale.

Analysis of the CATTB's modular armor attachments under impact loads, no idea if it got posted before.

Also, Is https://emu.usahec.org down? I can't access it anymore.