I don't know the specific differences between the 208 and 208s, but my information is 208s oriented.
First, I would suggest making sure all the fired rounds have a proper firing pin mark on them. The slide face can peen on the 208/208s and close up the cutout for the case rim. With a rimfire gun, this can cause slam firing. If your "full-auto" spent cases do have the proper firing pin hit, then you might even check to make sure the firing pin isn't broken (thus floating). I suppose there's also a remote possibility that the extractor can be causing the ignition, but I'd rate that pretty low on the list. After those checks, I'd look toward the hammer/sear as the probable culprit.
The sear and hammer interface angles are cut such that the normal operation of pulling the trigger causes the hammer to travel slightly further back before releasing. This action should prevent the hammer from being able to release by itself unless the hammer hook, sear edge or both are rounded over due to wear, or the hammer has been modified to get a crisper trigger. In either event, my first suggestion is to get another hammer and sear for the gun. These are not cheap, but they are a lot less expensive than the complete gun (and shouldn't be as regulated). To remove the hammer and sear you will need a GOOD quality drift pin punch (of proper size for each, or at least for the sear pin, which is smaller and harder to drift), preferably with a concave tip. Drift the hammer and sear pins out toward
the left (side with slide stop) using a block to support the frame. To reassemble, check the ends of the pins and insert the chamfered end first from
the left side.
You might consider just sending the original hammer and sear (and/or slide) to a Hammerli Service point for review. Of course being in the US, I work with Larry Carter of Larry's Guns, but you may have another Service point easier to get to. In case you want to contact Larry Carter, you can reach him at email@example.com
. You can even mention my name and he shouldn't charge you too much extra for it.<smile> I haven't checked lately, but the US prices were pretty steep a couple years ago. The set could quite possibly run better than US $250. If the slide has been peened, it is a rather simple task for a Service Point (with the proper tool) to recut the recess.
As to the sear spring mentioned by Mike T., the proper tension for that spring is 1000 grams, but I'm not positive of the measurement point. I use the sear tab where the trigger bar contacts it as my mesurement point.
If you do get another hammer and sear set, be sure to get some moly lube (without graphite) to use in the contact area. This will help prevent premature wear. I have a page that describes "Cleaning and Adjustments for the Hammerli 208s" at:
The notes at the above link were provided to me by Larry Carter. Let us know how things turn out.