help please on my r10 mk1 ive had it a month or so now and its been fine as it was a new gun had 20 pellets or so through it , ive shot loads of rabbits ect with it , anyway i checked my zero this morn set it at 30 yards all sweet then put the gun away four hours later got it out to show the old man it i shot at the same target and there was a 5 inch drop this happend for 10 shots then it went back to bang on t 30 yards how i zerod it :-s HELP ADIVICE ANYTHING PLEASE !