It seems that most of the current generation of sub-guns are closed-bolt designs. The reasons behind this are many, from what I've heard/read, and include greater potential accuracy.

So why were most of the first-generation sub-guns open-bolt designs? Was it simply technologically impossible at the time to build a reliable closed-bolt system? Was it merely economics (though it couldn't have been cheap to make those Thompsons!)? Or is it something else? (Would you believe I woke up with this thought in my groggy head and couldn't get back to sleep until I wrote this down? Arrgh! TIA for any info.)

One advantage is that an open bolt lowers the risk of cook-offs.  This is an inherent problem with full autos after sustained firing.

Regards,
Gary

I would say the open bolt design was less expensive to produce and more reliable since fewer parts to break.

I feel open bolt guns are more controllable on sustained fire than thier closed bolt counterparts.

Relating only to submachineguns, open bolt designs tend to be easier to manufacture. Consider the old Solothurn 100..While beautifully made, it's still a blowback breech with it's sear engagement surface as part of the bolt's body..

There were close bolt early SMG's. The Reising 50/55 comes first to mind. Yes the USMC had problems with them, but in civillian hands, they are a lot of fun, very accurate and easily controllable in full auto.. (Also consider how far lubricants have come.. Lube a dry '28 Tommy with WW2 spec oil, fire it, then do the same with something like CLP.. The change in cyclic rate is noticeable..)

The cookoff concern had more to do with sustained fire weapons, rather than SMG's Consider that NONE of the Browning belt fed MG's fire from an open bolt, while his M-1918 does..

Basically, it boils down to ease of manufacture..

Meplat-

Advantage - you can resume firing more quickly from an open bolt.
Disadvantage - loss of accuracy.

(I'm sure there are many more pros and cons but these two come quickly to mind.)