While working on my AKSU build I had an idea as to an effective way to make a caliber conversion like the Gepard with swappable barrels.
This is for pistol-caliber conversions. Blowback is assumed, but a gas system could be added. Assume a "mostly standard" AK design.
I want you to think of an AK barrel that doesn't press-fit into the trunion. Or if you put a Bulgy barrel against a Hungarian or Romanian 23mm trunion - it just "rattles loose" inside. Hold that thought.
You start with this new trunion which could be as simple as a rectangular block with a hole drilled in it and riveted to the receiver (like a Bizon trunion?). The trunion becomes the "mounting frame" like an Uzi. The front of the trunion has an alignment notch, pin, or slot machined to interface with the rear mounting block - read on:
Again, like an Uzi, the bullet guide is modified to be taller so that the barrel comes to rest AGAINST the front of the modified bullet guide. A matching notch is added to both barrel and BG for alignment purposes.
Build a "rear mounting block" that mounts both the handguards and the top cover. This will be similar in concept an AKSU style (or M92 style) rear mounting block, and could even have the top cover and handguard notch added to it. An alignment notch must be added to be rear which should be in contact with the barrel alignment notch section. A new mounting block is required for each additional barrel in the "kit".
The barrel is headspaced by adjusting the barrel's front-to-back location inside the rear mounting block. Once headspace is set, pin the rear mounting block in place on the barrel. The barrel and rear mounting block will stay together for the life of the barrel. Repeat for all other barrels in the "kit". At this point, the top cover and the handguards should be able to fit properly.
Insert the barrel into the receiver and set the location for the bullet guide. The bullet guide can be inserted into a groove or slot that is held in place by the barrel, OR screwed in place, or any other idea that can make it removable. The bullet guide and barrel should have a notch cut that mates the parts together so the barrel stays set in a certain position in relation to the bullet guide - this will ensure it stays centered.
NOW HERE'S THE FUN PART:
Measure a CLEVIS PIN, or something similar, and drill/ream the barrel as if you were doing so for a barrel pin.
Now, insert the clevis pin into the barrel pin hole and secure in place. Repeat for the other barrels in your "kit".
PRESTO! you have a pinned barrel that is easily swapped. The calibers of choice may need a different bullet guide (e.g. 9mm vs. 7.62x25, etc) and so the need for the BG to be easily removed.
Another way to look at this is to look at an Uzi, and instead of the screw-on barrel cap, you have the clevis pin. Instead of the "shoulder" on an Uzi barrel, you have the "rear mounting block".
Remember, we are talking about 9mm as a basis, so we're not talking sub-MOA accuracy!
A captive clevis pin would be required so as to keep the pin from wiggling out.
The trunion and rear mounting block must be milled with a mating slot, or notch, or locator pins, or some other way to keep them aligned. Otherwise the barrel would not be able to stay sighted in properly beyond about 50 yards.
The described configuration will make the barrel stay in its location via the the notched bullet guide and the notched/grooved rear mounting block. That is three hard points to secure the barrel, counting the clevis pin.
The clevis pin should be made of good-quality steel... not an aluminum pin or anything like that. I do not know what the off-the-shelf pins are made of, but they should be tool-grade steel to ensure it doesn't get peened from repeated firing.
The rear of the bullet guide could extend downward to create the front lip for the magazine lugs. That would allow for an easy change of magazines if different calibers called for different magazines. Otherwise, most magazines could be swapped as is already done for Uzi caliber conversions, among others. In fact, PPSH magazines may be able to serve the needs of all the interested calibers - 9x19, 9x18, .40SW, 7.62x25
The bolt would be a one-piece design and could be a simple block of steel with notches machined, dremeled, or filed, for the rails and the ejector groove. Perhaps an Uzi bolt would be a good starting point... it is already drilled for a firing pin!
Any required barrel extensions (typical pistol barrels aren't 16" long) would be needed for each and every barrel in the "kit" that isn't already 16+" long.
The clevis pin could be replaced with any securing device - a bolt with threads on one end of the trunion would do the same task, but would require you to have the screwdriver or allen wrench as a part of the kit.
A different design would forego the slip-fit barrel in favor of a threaded insert, if the trunion was threaded. Each barrel sleeve would need to be "timed" so the headspaced barrel had the rear mounting block properly set to TDC. Otherwise, the mounting block could be deleted from the design and it would be replaced with a collar that is more akin to an Uzi design.
I'm still crystalizing the idea in my head, so there may be other considertations I have yet to realize. This, IMO, is a good start in the right direction!