it should be stichable. I stiched the haywire cannon with my snow cannon so I don't see why the normal cannon is non-stitchable.

Any weapons with unique attack cards that require unique animations (Phrygian Frostlock, all Nefarious 5 weapons, all Staffs of Power, etc) are non-stitchable. While I agree that it'd be nice to be able to stitch the look of these unstitchable weapons to the stats of a stitchable weapon (e.g. Haywire Thunder Rifle's stats to the Phrygian Frostlock's look), KI has decided not to allow that. However, your situation of wanting to stitch the stats of an unstitchable weapon to the look of another weapon is not doable, and rightfully so. The Autocannon has an exclusive attack card that requires a unique animation. Having to put an extra unique attack animation on every single gun in game to accommodate for that weapon's unique attack would necessitate too much coding.