So I found a promising way for locking the striker that's not unlike the bullet/cartridge lock mechanism. It will use a single pin and a technic hole to properly lock as well as force vector that differ by 90°. That'd be the perfect case for proper locking - isn't that what I everyone wants for an unlockable lock? However, this method wouldn't be that great if there weren't these small claw-alike parts I discovered yesterday. I believe they can fill exactly the gap for small lock mechanism I'm always looking for so badly. Discovering the english word "sear" for the first time in English (also: in a firearm context), I think I've finally found the perfect to describe my mechanics a tad better. Sears, locks and springs - that's what most gun tech is all about and also the only stuff that always interested me in mechanics beside gears and all similarly related stuff.
Anyway... this lock "implementation" is certainly the most spiring lock ever since I started making Lego guns. I've used it before in my earliest models and didn't realize how to make it smaller. Now that I know it better, It'll be awesome I assure you! And here comes the output of this inspiration: I said I wanted to have a muzzleloading, very long Lego gun after the current double-barrelled variant. And well, it has to be some kind of musket with scope and other stuff that's not possible with my cartridge designs due to their sheer size. The problem with all muzzleloaders I did was that their bullets would fall off when tilted below a horizontal base line. I believe that I finally found a solution how ot overcome this problem by using a pressure-sentive bolt at the barrel breech which release a lock otherwise holding back a sear locking the bullet release mechanism. So after using the ramrod, a the bullet-backholding sear will snap right into a hollow in the ramrod end and will lock from this point on. Because the pressure-sensitive bolt was pushed, the trigger is now ready to release the bullet. After the shot release, the trigger will snap to it's pulled position until the barrel breech's bolt will be pressed again. So it'll always be cocked when loading and you only need to ramrod the bullet all it's down. Unfortunately I didn't noticed that the same effect can be done with the lock I'll use for my double-barreled cartridged rifle, too. That means that however awesome this concept may be, it's more complicated and even more error-prone than the most simple of all available solution. So it's more of a no-brainer to make this musket one, I guess. However, depending on the barrel length and rubber band used, you can get quite a massive range and power for a Lego I. Sure, it has heavier bullets and will probably need to be veeeery long, but that's definitely worth testing it.
PS: Check out this site about swing locks. It's an action/lock used for percussion cap muzzle loaders only that gives a really awesomely reliable design. The idea is amazingly simple, just check it out and give it a go. I really love the compactness of this design.