Renegade334

That's not the point being made - that a 120mm gun would be better for the GDLS MPF because it has access to a wider range of ammunition including programmable rounds like the AMP and combustible propellant cases, which would eliminate if just mitigate the toxic fumes issues the prototypes encountered during testing.
As of yet, there is no 105mm AMP/equivalent available in the US armor panoply or in development -- at least officially.
 
 
^--- MPF firing HE.
 
For the time being, the MPF is fine with a 105mm main gun because the smaller caliber allows a larger loadout and the vehicle is not meant for antitank warfare, just infantry support. In the future, however, we could possibly see up-gunned MPFs, should USAR decide the threat ecosystem, doctrine and/or combat requirements have significantly changed and increased firepower is a must. The XM360 would be the go-to candidate then.

I have to admit, I'm kinda bugged by that egress hatch on the left turret side. Would have preferred armor instead, but it seems it wasn't too much of a concern to warrant its removal from the design.And, yeah, Nick thinks GDLS' offering was the right one. We'll see his take on the BAE MPF soon, but the vehicle has apparently failed his "easily wiggle yourself into/out of it" test.
 
EDIT:
 

More tests in store for farther-shooting howitzers (defensenews.com)

Army announces improved test results in light weight active protection systems | Article | The United States Army
 

 

USAR is gauging having 230 M1299 self-propelled howitzers with a possible first unit equipped in FY2026.
 

A rare picture: an overhead shot of the M1 Thumper, used to test the XM291 ATAC gun (sometimes referred to as LW120) in conjunction with the XM91 cassette autoloader.
 

 
The Thumper, which was built out of a M1A1 at Anniston Depot, used a 120mm XM291 in three different barrel lengths, one 265in/6.75m long (the standard, "long tube" XM291), a second one cut the same length as the M256 ("short tube", 5.3m) and a third, intermediary one ("medium tube") that stands somewhere between 6.75m and 5.3m.

On-the-move test fires done at speeds of 10, 15 and 20mph showed that the target impact dispersion (TID) was bigger (read: lower accuracy) with the "long tube" XM291 compared to the "short tube" or EVEN the M256, which was problematic. To support the increased barrel length, Benét Labs also had to create a 415lbs and 22in-long extension for the gun cradle...which was later reused in the proposed installation of a XM360E1 inside the notional M1A3...and, quite likely, inside the M1 AbramsX demonstrator showcased at AUSA 2022.
 
 
 
The 1988 M1 Thumper was only fitted with the 120mm ATAC gun, but the XM291 installed on the 1993 M1 CATTB, on the other hand, went on to trial a 120mm and a 140mm barrel. At one point, the ATAC prototype pool was noted to have successfully fired 1,300 rounds, 150 of which were of the 140mm variety. The latter (at least the KE part) was even judged capable of defeating the armor of what was tentatively described as the FST-3 (Future Soviet Tank 3, essentially a Obj.477/477A); there are no known numbers for the 140mm XM964's performance, but the 140mm ATAC itself was expected to produce a great maximum of 25MJ of muzzle energy. The ammunition for the ATAC was developed by Valentec International and Hercules.
 
 
 
While the Thumper was originally meant to trial the 120mm ATAC, some of the official literature indicates it may have also undergone the 120-to-140mm tube conversion and done static test firings at Aberdeen Proving Grounds, where it showed precision equal to a M1A1 but "with greater penetration".
 

>> Source document for the two above scans <<
 
Also, a common mistake (one I've also been guilty of, mea culpa) I often see around in forums and milblogs is that the Thumper is a derivative of the CATTB, which is false, as the Thumper (AKA the ATAC Demonstration System) predates the CATTB Phase I by at least five years.
 
M1 CATTB Phase I (using a M1 hull as basis), in 93/94, about to be shipped to Aberdeen Proving Grounds for testing:
 

 
Picture of the CATTB Phase II (ATTD?) hull under construction, completed in 1994. Note the closed back with no exhaust grille, the absence of apertures for torsion bars (replaced by a hydro-pneumatic suspension system) and the smaller engine compartment designed to receive a Cummins XAV-1000 AIPS instead of a Honeywell AGT-1500. The freed-up volume between the XAV and the turret basket was reclaimed to accept two vertically-stacked non-ready cassettes housing a total of 22 rounds (either single-piece 120mm or two-piece 140mm), which would have given the CATTB Phase II a maximum loadout of 39 rounds (either 120mm or 140mm), just five less than a M1A2. The huge tradeoff, of course, was that all this made the CATTB massively overweight.
 

 
Apparently, at least one of the two CATTB testbeds is currently sitting at the Sierra Army Depot in Herlong, California, but one has to wonder if it's not a case of mistaken identity; the Thumper was, after all, spotted in 2010/09/30, in Ohio, as it was being relocated by train to parts unknown. Here's to hoping any of these big bois shall make their way to a museum one day, as did the TTB, the Crusader and many others before them.
 
XM291 ATAC with its 17-round Benét Labs XM91 bustle autoloader:
 

Also:
Army approves Milestone C and awards LRIP contract for the Mobile Protected Firepower program | Article | The United States Army
 
 

 
Sauce: NATO JFC Naples.

And after Textron, here comes GDLS with its take on the USMC's ARV:
 
EDIT:
 

Also:
Army approves Milestone C and awards LRIP contract for the Mobile Protected Firepower program | Article | The United States Army
 
 

 
Sauce: NATO JFC Naples.

From tank-net:
 
A 2001 precursor to the XM1299, the M109A6+ with its 52-caliber, 155mm M284 gun.
 


 
EDIT: the M109A6+ was also dubbed "International Howitzer" and boasted several improved capabilities. Aside from its 52-caliber, JBMOU-compliant and Watervliet Arsenal-built M284 gun, it also sported a semi-automatic rammer, an automatic fire control system and a modified suspension.

38 tons according to this, ~40.7% heavier than its BAE competitor:
 
I can only surmise the numbers being up to date and reflecting any modifications made during the selection process </disclaimer>

NGC refers to this:
 
 
Also:

Textron: we named it after a snake!
Everybody else: is it a reference to smoking pot?

EDIT - source for the CATTB construction images: EME Journal 1994 issue #2
 
AAI RDF-LT on display at Nokesville, Virginia, in September 2021:
 
 
Blast from the past: RDF-LT with unmanned turret and two quad Stinger launchers, at AUSA 1984.
 

 

A rare picture: an overhead shot of the M1 Thumper, used to test the XM291 ATAC gun (sometimes referred to as LW120) in conjunction with the XM91 cassette autoloader.
 

 
The Thumper, which was built out of a M1A1 at Anniston Depot, used a 120mm XM291 in three different barrel lengths, one 265in/6.75m long (the standard, "long tube" XM291), a second one cut the same length as the M256 ("short tube", 5.3m) and a third, intermediary one ("medium tube") that stands somewhere between 6.75m and 5.3m.

On-the-move test fires done at speeds of 10, 15 and 20mph showed that the target impact dispersion (TID) was bigger (read: lower accuracy) with the "long tube" XM291 compared to the "short tube" or EVEN the M256, which was problematic. To support the increased barrel length, Benét Labs also had to create a 415lbs and 22in-long extension for the gun cradle...which was later reused in the proposed installation of a XM360E1 inside the notional M1A3...and, quite likely, inside the M1 AbramsX demonstrator showcased at AUSA 2022.
 
 
 
The 1988 M1 Thumper was only fitted with the 120mm ATAC gun, but the XM291 installed on the 1993 M1 CATTB, on the other hand, went on to trial a 120mm and a 140mm barrel. At one point, the ATAC prototype pool was noted to have successfully fired 1,300 rounds, 150 of which were of the 140mm variety. The latter (at least the KE part) was even judged capable of defeating the armor of what was tentatively described as the FST-3 (Future Soviet Tank 3, essentially a Obj.477/477A); there are no known numbers for the 140mm XM964's performance, but the 140mm ATAC itself was expected to produce a great maximum of 25MJ of muzzle energy. The ammunition for the ATAC was developed by Valentec International and Hercules.
 
 
 
While the Thumper was originally meant to trial the 120mm ATAC, some of the official literature indicates it may have also undergone the 120-to-140mm tube conversion and done static test firings at Aberdeen Proving Grounds, where it showed precision equal to a M1A1 but "with greater penetration".
 

>> Source document for the two above scans <<
 
Also, a common mistake (one I've also been guilty of, mea culpa) I often see around in forums and milblogs is that the Thumper is a derivative of the CATTB, which is false, as the Thumper (AKA the ATAC Demonstration System) predates the CATTB Phase I by at least five years.
 
M1 CATTB Phase I (using a M1 hull as basis), in 93/94, about to be shipped to Aberdeen Proving Grounds for testing:
 

 
Picture of the CATTB Phase II (ATTD?) hull under construction, completed in 1994. Note the closed back with no exhaust grille, the absence of apertures for torsion bars (replaced by a hydro-pneumatic suspension system) and the smaller engine compartment designed to receive a Cummins XAV-1000 AIPS instead of a Honeywell AGT-1500. The freed-up volume between the XAV and the turret basket was reclaimed to accept two vertically-stacked non-ready cassettes housing a total of 22 rounds (either single-piece 120mm or two-piece 140mm), which would have given the CATTB Phase II a maximum loadout of 39 rounds (either 120mm or 140mm), just five less than a M1A2. The huge tradeoff, of course, was that all this made the CATTB massively overweight.
 

 
Apparently, at least one of the two CATTB testbeds is currently sitting at the Sierra Army Depot in Herlong, California, but one has to wonder if it's not a case of mistaken identity; the Thumper was, after all, spotted in 2010/09/30, in Ohio, as it was being relocated by train to parts unknown. Here's to hoping any of these big bois shall make their way to a museum one day, as did the TTB, the Crusader and many others before them.
 
XM291 ATAC with its 17-round Benét Labs XM91 bustle autoloader:
 

EDIT - source for the CATTB construction images: EME Journal 1994 issue #2
 
AAI RDF-LT on display at Nokesville, Virginia, in September 2021:
 
 
Blast from the past: RDF-LT with unmanned turret and two quad Stinger launchers, at AUSA 1984.
 

 

EDIT - source for the CATTB construction images: EME Journal 1994 issue #2
 
AAI RDF-LT on display at Nokesville, Virginia, in September 2021:
 
 
Blast from the past: RDF-LT with unmanned turret and two quad Stinger launchers, at AUSA 1984.
 

 

EDIT - source for the CATTB construction images: EME Journal 1994 issue #2
 
AAI RDF-LT on display at Nokesville, Virginia, in September 2021:
 
 
Blast from the past: RDF-LT with unmanned turret and two quad Stinger launchers, at AUSA 1984.
 

 

Dunno if it's ever been posted here, but I'll leave these as is - PDFs from Benét Labs, ARDEC and Tank Main Armaments (Picatinny Arsenal) on the ATAC gun project.
 
https://www.docdroid.net/BNPwd3j/20184655mnbt989107254f026170i002-pdf
https://www.docdroid.net/9uIlRZe/20181815mnbt989112214f167098i011-pdf
 
ff6a7899ba6d421e2d9e4bc222fdd340d6d6.pdf (semanticscholar.org) <-- study on barrel vibrations and stabilization; XM291 is featured.
https://archive.org/details/DTIC_ADA320105/mode/2up <-- has a bit on the XM291 testing.