The reaper autocannon is the chaos space marine terminator's equivalent of the assault cannon.
The press that the reaper autocannon gets seems to be moderately negative. It isn't hard to see why when a quick comparison is made. For 25 points, the reaper autocannon gives the chaos player 2 twin linked AP4, S7 shots. In comparison, the assault cannon costs 30 points and gives 4 S6, AP4 rending shots. The main differences are the amount of shots (double for the imperials), the accuracy of the shots (twin linked for chaos), the range of the shots (12 inches further for chaos) and the rending (for the imperials). For 5 points more, I think I'd be tempted to ditch the twin linked and the range in favour of the rending and sheer amount of shots.
Even still, the reaper autocannon can still be an excellent choice for the chaos terminator player. With its increased range and twin linked statistic, it can readily pose a threat for almost any light tank sat on the board from a reasonably back position. If taken in a combi-termicide melta squad, it can readily back up the melta shots with a pair of its own that are much more likely to hit. Although the tank may not get as much chance of being blown to smithereens in comparison the melta hits, it is probably still worthwhile overall. Indeed, I've been toying around with the addition of a reaper autocannon in a squad of 3 other combi-melta termicide squad and have had reasonable success with it. The other benefit is the amount of attention that the reaper autocannon gets -- it tends to be a real fire magnet for opposition forces to try to take out.
The image above is of a chaos reaper autocannon terminator that I plan to paint up in Sons of Malice colours. The head is a swap for a plastic chaos warriors skull helmet, but otherwise no conversion work has taken place. The base has had multiple bits from the warhammer 40k basing kit applied to give it an urban wreck feel.