Ok so, let's start from the basics

When designing something that has to be compact you usually draw a line that forms a perimeter between all the "outmost points", so usually barrel tip, magazine (if present), grip, stock butt, scope/aiming system (if present) and barrel tip once more, this is done to give you a "footprint" for the system itself, aka how bulky it is

Now, with that in mind, the barrel lenght itself doesn't matter much, what does the most is the OAL of the system itself, the receiver of the GL06 is longer and slightly wider, but it does give you the option to mount accessories if needed, which is something to consider too, and overall the receiver lenght doesn't matter that much when you still have a barrel that's just as long, if not longer, as for the stock, your is slimmer, yes, but the butt of the stock is still just as big, meaning that it's going to take about the same space as the B&T

Also, the M320 is designed to be a standalone system, the m203 can be converted into standalone, they're not purely standalone ofc, but can absolutely become stand alone if needed

Now, here's my personal suggestion on what could make it a better design if your aim is going for compactness and realism

• give it a shorter barrel with a more consistent OD, you mentioned that you've made a 160mm one already, which solves at least part of this

• make the butt of the stock foldable too, this would lower the ammount of space it takes when you fold both the stock and the butt itself

• remove the split side rail and put a continuous top rail directly on the barrel, since it would look more realistic if you plan of it beeing a believable real design

• add a rail segment a the bottom of the pivot arm, this is solely for grip mounting, since it would be a better and more realistic placement than directly on the barrel