I use a heat gun but after A LOT of practice, ABS plastic is a very strange animal to heat and because it's under stress as it has been heated and pulled it can easily let go and revert back to it's former shape. A lol of people use hot water bath's which is a little kinder but still you have to be careful especially with pieces that are curved and under stress.   Practice with scrap pieced of ABS plastic is a must for the uninitiated