supersonic .177

In South Africa we are limited to .177, everything over requires a gun licence and getting one is a BIG pain in the arse and at the moment can take up to 2 years before you can pick up your gun. So what I would like to know is why is it´that airgun bullets go eratic once they break the sound barrier and is there any way of lessening or canceling this problem out ie really heavy pellets, longer projectiles, maybe a combination of longer pellets with a faster twisting barrel?

Does anybody have any experience in this? I would really like to get into PCP’s but if I need to stay sub-sonic indefintely I can’t really justify the expense to move away from my springers.