3D Printable Open Roller Muzzle

I'm trying to design a custom 3D printable roller gun which is slowly coming together but need some help/feedback please. Originally I was going to buy an upgrade type roller muzzle... they are all very nice but I've now decided to go the distance and design my own... cost is comparable and I'm only going to do this once... so why not.
Having read through a lot of DB threads regarding open/closed muzzles I've gone for an open style... aim, accuracy and consistent poor viz being my leading considerations. However having only ever used an open muzzle a few times clearly puts me at a disadvantage! I was very impressed with the line of sight when compared to my standard 'point and shoot' closed muzzle, euro gun... as well as the lack of rattle/noise of the open muzzle too.
Any advice, glaring mistakes or omissions would be very much appreciated... overall dimensions in cm's are 14L x 8W x 4D and I'm using an RA ally rail barrel (25.4mm ID) and Meandros (16mm) rollers.

Much thanks in advance...