I'm pretty sure all 300mm and 370mm rocket artillery rounds are guided. The longer your range the greater the CEP for unguided rockets, so large calibre rocket artillery round would all have unusable CEP given their range if not guided.Is this their first try? if so, it sure is impressive. Now that they are sure it can work, the remaining task is to finetune the targeting. Either the engineers finding ways to improve the rocket CEP and/or improved training for the troops responsible for the MLRS.
also, are they using guided/unguided rockets?