Can you explain why a pistol would be more ambitious then a SMG? Even if we can't make a semi-auto pistol in a reasonable timeframe I'm sure we could figure out a revolver as a stopgap measure.
So this is very much my limited understanding, and extrapolating from what I know about larger weapons. I am, probably, wrong.
Also, a revolver would be more simple; that is indeed true. I hadn't been considering revolvers.
Anyhow, as far as I'm aware most semi-auto pistols in this period that fire a "powerful" round use a locked breech of some sort, usually short recoil. This means your pistol needs to have:
- Some way for the barrel to recoil separate from the grip and fire control group and then return to proper alignment when it's in battery
- Locking surfaces, on both the bolt and the barrel (or some extension of it) which are capable of, by themselves, holding the breech closed until a safe pressure is reached; these need to be relatively well manufactured because if they don't lock up in a consistent manner, the gun jams or worse
- A spring to close the bolt
- Since pistols are usually closed bolt, a separate firing pin with its own spring inside the bolt
- The fire control group, which normally includes not only the trigger/sear but also a disconnector to prevent it from running away in full-auto like it would want to do otherwise and often a moving hammer to hit the firing pin
- The magazine
The two big sources of complexity are the locking mechanism, which necessitates several moving parts that need to be machined to tight tolerances and out of strong material so that it keeps the breech closed until you want it to open, then smoothly and reliably opens; and the moving firing pin, which necessitates a separate, small spring, as well as the added complexity of the fire control group trying to interact with something that's supposed to be on the inside of the bolt and the extra moving parts/machining steps this requires.
Meanwhile, a straight-blowback SMG with a fixed firing pin is:
- barrel and chamber, which are fixed in place and don't need to move
- chunk of steel for the bolt that moves backwards slowly simply because it's heavy and has a firing pin that's fixed in place (or just a raised bump)*
- spring to push the bolt forwards again after it got pushed backwards
- trigger and sear to hold the bolt open until you want to shoot
- magazine (this bit is tricky to get right in both cases, although slightly harder here due to the higher rate of fire)
- toob to cover it all
And that's about it. If you exclude the bullets, the bare minimum SMG has four moving parts (trigger, sear - the thing between the trigger and the bolt, some sort of safety latch, and the bolt). Three of those are already things we have experience manufacturing for our rifles and light machine guns, and the fourth is a matter of purchasing a single SMG of your choice from abroad, measuring its bolt and chamber somewhat accurately, and then machining a lump of bar stock until it looks and weighs the same. Yeah, high rates of fire do require a bit of care and reinforcing parts that are found to break in testing, but if things are getting super complex, that points to a deeper issue - ideally it should be as simple as possible.
Someone mentioned the Owen gun upthread, and that's an example of a
teenager deciding he wanted to make a SMG and building one in his garage. Sure, it had to be tweaked by a pair of engineers to get it to work reliably enough for widespread service, but an excellent gun came out the other end. From what I gather, most of the reliability factors involved in a SMG are how the bolt and magazine interact, and that's a case where we could just steal someone else's design and probably not face much of an issue.
* I realized I omitted ejectors and extractors from this, but one's a metal stud on the receiver, and the other one is a piece of bent spring steel that gets press fit into the bolt, and they're common parts to pistols, SMGs, and rifles anyways. Likewise, I'm ignoring the feed ramp that extends from the chamber, because that's a common feature on basically everything in this kind of role.
Edit 2: this is also not talking about straight blowback pistols, because scaling up a blowback pistol to take, say, 9 mm ammo and removing the semi-auto disconnect turns it into an underbuilt submachine gun.
