There are two possible ways to reconcile the verses:
- Luke’s purpose in Acts may have been simply to report what Peter said at a point in time when the apostles’ information on Judas’s death may well have been sketchy. After some of the Temple priests converted (cf. Acts 6:7), they may have given further details on Judas’s death that were later incorporated into the Gospel accounts.
- It is also possible that after Judas hanged himself the rope broke and he fell onto rocks that disemboweled him postmortem. Matthew’s emphasis then would have been Judas’s actions in taking his own life, while Peter’s emphasis was on what happened to him after his suicide.