What am I doing wrong? Slammer exploding after firing rounds via trigger

[mrkeeno 10]

Hi all

More or less the 1st time i've posted here. I am creating a mission for our clan where we have to attack an artillery emplacement while it is firing. I have used the following:

artillery1 doArtilleryFire [getmarkerpos "arty_target", "32Rnd_155mm_Mo_shells", 32]; hint "firing"

Every bloody time the arty seems to do the funky chicken after the 30th round and destroy itself. There is nothing obvious that I can see so hoping someone on here can help?

Thanks in advance.