It's not a 4 tonne gun (unless you are talking about leaving it at an Army Barracks or air mobile - but I'll get to there in a second). It's a 4 tonne gun with a 32 tonne tow truck. It's longer than a SPG, just as heavy and considerably less mobile off-road (it is not as tough as a M198). So, just ground mobility wise, how is is it better than a tracked vehicle? How does a tracked vehicle go less places than a HX77? The length means it takes up more room on an LHD too - and will probably cut a C-17 down to 1 gun (the same as a SPH). So what mobility does it give?
@Raven22 talks about the ability to pop up at unexpected times around the Asia Pacific? The M777 can't (unless there is a road or nice firm terrain); the SPH can tactically and operationally. Hell, even just moving a SPH on Australian roads in peacetime is easier than a M777!
Ahhh....airlift I hear. Yes, a CH-47 will carry a M777 a lot further than an SPG. Great. And does what? Unlike an L118 it can't carry ammunition, so that'll be another CH-47. So 2 per gun; that's 8 CH-47 for a Bty or 4 for a Tp. And gives you a few minutes of firing. Note that the anaemic rate of fire of an M777, especially at high angles, means a Tp doesn't really offer any weight of fire and a Bty is likely going to be the minimum needed. So 8x CH-47 for a few minutes of firing. How many CH-47 do we have in theatre? I suppose they could do ferry flights - but that is giving up surprise and undermining the concept of multi-domain operations that Army is looking at tackling where short temporal windows are all that can be assured.
Now tactics. The extra range of an SPH means that it does not have to move as much, nor as far forward, as a M777. For those who can, there is more than 1 DSTO / DSTG study that shows this clearly. So a force with M777 may have to consider airlift while an SPH force just drives to a different spot not available to a M777 and operates from there. Speaking of operating, it takes about 30 - 60 s to get a SPH from stopped to in action. On Chong Ju (the most friendly and kindest scenario you can generate for a gun crew) it took 6:32 min to go from the first gunner touching the gun on the back of the HX77 to first round down range. In that time, a K9 has fired 30 rounds - or a Bty has fired 120 - 180 rounds (either a 4 gun or 6 gun Bty. From now on I'll assume it's a 4 gun Bty. Urgh). Without the super MRSI mode. The K9 doubles the RoF of the M777. In every case you have artillery, it's the first few seconds that matter. The Americans proved this in 1944 and it remains true. So in 10 minutes from stopping my M777 Bty gets 24 - 28 rounds off (depending on elevation), the SPH (assuming K9) Bty gets 180 rounds off. In 5 minutes from stopping that drops to 0 for the M777 and 80 for the SPH.
Now, against a reasonable threat (one we are throwing a lot of 155 mm at), there is likely to be a counter-battery threat. You don't need to be fighting the Fulda Gap - you just need to be fighting any army built on the Soviet model (of which there is a large number in the areas Australia may operate). Russian C-Bty units in the Ukraine are getting down to 80 s from "Blue" firing to the first "Red" shell landing on them (that's unclass OSINT figures too). Any threat we face is going to be similar, but gosh, lets call it 120 s (that's 50% worse than the Russians). After the first shell fires on the M777, how much time to get the gun hooked up (hint - at Chong Ju it was over 3 minutes). The SPH? In fact, assuming you can get your SPH and M777 loaded and ready to fire without detection, in 120 seconds you can have 4x shells fired and 44 dead soldiers from a M777 Bty or 32x shells and some diesel fumes from a K9 Bty.
Resupply is also easier with an SPH, especially the K9-K10 combo. The flatpack addition to a HX77 does speed it up compared to the older Mack; but it still is slow. Note that the M777 combination will require another HX77 to resupply ammo; so moving a K10 is no more a load than another HX77.
Operationally there is little to discuss that hasn't already. The lack of inherent mobility of the M777 means it needs an HX77. It is also restricted to only operating against threats without a counter-battery capability. Any FOB mission can be done by an SPH as much as a M777 - so that isn't an issue.
Strategically, it comes down to personnel and money. M777 chew up people - 10 - 11 per gun. A K9 uses 5 (a 50% saving), with the new kit in the turret that drops to 2 (an 80% saving). If they can be built in Australia (probably only applicable to the K9) then that adds to our sustainability and economy. Noting that a SPH is better tactically and operationally, why would I pay the money to run two fleets? I haven't run the numbers, but I wonder how many more SPH we can buy if we delete the M777 entirely and shift that funding across (it'd add up, especially taking into account the SoA and supply chain).
So, a SPH is better than a M777 in almost every respect at every level. Those it isn't, it's equal. The one niche capability of airlift demands more online CH-47 than we have - and hence is not cost effective. The M777 capability (because it's more than just the gun) is slower, clumsier, longer, less agile, more manpower intensive, less safer and less capable. It's a wonderful tool - for the 1940s. But this is the army of 2025 - 2045. The M777 is obsolete. SPH is the answer, especially for a small Army like ours. I love the Centurion and the Charles F Adams class DDG, and tolerate the F-4) - but I would never, ever suggest we bring them back, let alone use them into the 2030s. Towed guns of 155 mm are the same.
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