In addition to the reason given by Pisigma, there could be technical benefits as explained here in the case of bullets, which should apply to artillery shells as well:What I want to know is why PLA decided to adopt a new standard for their 155mm (155 with brass casing). What's wrong with adopting 155 NATO if they want to standardize on 155mm? is this another case of national pride like with their 5.8mm rifle cartridge? (I think their performance advantage claim is negligible at best); because I can't see what is the advantage of making another new and comparatively unproven standard when there're 2 perfectly functional ones (again, just like with 5.8 when compared to 5.56 NATO and 5.45 Soviet). If you know the reason then please enlighten me.