It is not very urgent for me, that is correct (and I also do not need exactly the M115, it also might be another cold war howitzer which is form this time, maybe "Long Tom" - but yes, M115 ist best). But if there is a useable, maybe already animated model, I would be glad to use it right now (and maybe later replace it).
https://www.3dcadbrowser.com/3d-model/howitzer-203-mm the simplest model. But this site has a fancy free download system. With a free account, you can freely download only one model. Then the restrictions begin.
«Using a free account you can download all free 3D models. To download non-free 3D models you need to upload your 3D models or order a paid account. You'll be able to download one 3D model per each download of your uploaded 3D model by other user».
At the same time, they have only one artillery model marked free.
In general, it makes sense to look – maybe something here
https://www.3dcadbrowser.com/3dmodels.aspx?category=65&page=1
you will need more, and there will be no alternative. And will have to make additional efforts to plunder the site.
There is one for the M115, alternative is doubtful.
Clearly problematic mid-polygonal models lie in many places.
https://open3dmodel.com/ru/3d-models/3d-model-203-mm-howitzer-2_124997.html
https://open3dmodel.com/ru/3d-models/3d-model-field-howitzer_41852.html
If you need a separate "Long Volume", then it is also quite popular. For example
https://open3dmodel.com/ru/3d-models/3d-model-155-mm-gun-m1-field-artillery_124999.html
In general, there is not much post-war artillery, but there is. At the same time, I came across the second generation – that is, with an auxiliary engine, automation and an increased firing range. There the main question is the limit of polygonality/ "weight". The question is key, because...
I have in my computer, for example, a model of a modern Russian howitzer – MSTA-B (not a self-propelled gun, but a towed one). But it is large even in a zip file, 2.82 MB. And there are a lot of such things.
But this is for a possible future, of course.