Idle Q of the day - Spandau jackets

[Lavochkin]

I don't really NEED to know, but I was wondering as I was wandering, why? Specifically the jackets on the Spandau guns. My understanding was that the guns were originally water cooled and then for aircraft use, they ditched the water and put holes in the jackets. Why not just leave them off entirely rather than go to all the trouble of making those cooling slots? Bored factory workers filling up idle time? I know that the leaders on both sides of the conflict predicted the rise of scale modeling and thoughtfully provided us with myriad but undocumented details to amuse us. Could this be one of them?

I did do some google searches, but either the answer is not out there or I didn't know the magic words that would unlock the secrets!

[Maxim08]

Lavochkin,
The barrel jacket on the mg/lmg08/15 provides several functions. The one specific to your question is that the front of the jacket supports the front of the barrel and provides part of the baffle for the booster gases which provide some of the force which cycles the complete mechanism.
The booster/flash suppresor screws into the front section which is screwed onto the jacket which screws into the trunnion/rear jacket section.
Regards,
John

[Ransom E. Olds]

That's way the Austrian Schwarzlose, being a delayed blowback with a fixed barrel, could dispense with the entire jacket, leaving that big old receiver with a slender little barrel sticking out of it. Ransom