Yes indeed! This is fantastic stuff! I have to say this is one of the best documented builds I've seen in a long time--and thanks a lot for sharing your learning process with us!

I usually try to go with hammer builds, just because heavy homemade strikers seem like they'd be tough to keep from slamfire if something goes wrong with the disconnector. You can shape the back of the bolt so it slowly eases a hammer back down in case of disconnector failure, which isn't very easy with a striker.

I've noticed most commercial striker-fired pistols I've looked at don't have a full-length spring on the striker, and the striker is milled down to be very lightweight. I wonder if similar tweaks could make this version similarly safer?

Well,

I am seriously impressed by this build!  Good job on this project!!!

Last test firing was June 14 of last year. Since then I have been covered up in development, manufacturing, and selling stuff for the upper receiver. With that under some semblance of control, the next product needed is the FCG - hopefully a builder friendly one. Lately I've been busy redesigning and firing testing the striker system shown in previous posts and yesterday April 11 finally achieved a reasonably successful series of tests. The first pic below shows my son in law playing a menacing Finnish soldier with the test gun in it's current configuration.

It now has a forward denial bar as my receiver repair sections do. This is shown in the next 2 pics along with a 4th pic showing the bolt with the milled slot in the RHS to accommodate the bar.

This shot also shows a section milled out of the bottom rear of the bolt. This is to clear the redesigned striker. Note that that the aft denial plate spotwelded on has been cut back. This is because the new striker allows the bolt to ride back an additional and much needed 3/4 inches. The last pic shows the new striker in position behind the bolt. The old striker had it's spring in the bolt. This caused more problems than it solved so the striker spring was moved aft to the pushing position like everyone else does it. I keep reinventing the wheel! It now helps the recoil spring instead of opposing it. This required a spring guide which extends through the striker and into a clearance slot in the bolt when the bolt and striker are in the aft recoil position. The guide is held aft by a buffer - a piece of 3/16 plywood temporarily. All these new parts drop into place and quietly stay in place without any fasteners or guides. Next post, more detail on the striker system a new firing pin and the firing tests.

 I've been looking forward to this for months (my customers too). The first pic below is another view of the striker in position. I considered anchoring the guide in the original recoil spring plate but opted to add a new plate (buffer)instead. Easier to remove/install and adds some buffer material.

The next 2 pics show the new striker components and a new firing pin and spring. The new striker is full bolt diameter and full weight and here I will digress.

 

Why a striker? Why not a "drop in hammer system". I would love to "drop in" an AK FCG. They don't "drop in". They require major surgery on the lower receiver to adapt and originality is lost. Different trigger, different safety. These have been some of the major factors holding the KP-44 (and PPS-43) back. It's beyond the capability of the typical home hobbiest. The goal was an FCG that could fit with as little modification as possible and this pointed to a striker. The original gun was a striker of sorts - the bolt itself being a striker. The new striker is a simulated aft bolt, with 2 hooks for the disconnector and the original sear. A problem with strikers is that the bolt travel is reduced to make room for it. The original bolt travel was about 5.6" and the new travel is about 3.6". That doesn't mean that the bolt always traveled that far. Differences in springs, friction, temperature , etc made travel a variable. It moved as far as it wanted to. It had to make at least 4.3" to hook the sear. A problem with SMGs was that the bolt would short cycle in some cases, not catch the sear and empty the magazine even with the trigger released. The new striker and modified bolt combination is 13% heavier than the original bolt alone so this helps slow it down (heavy is good). The recoil spring is unchanged. The striker spring helps too. Still, test firing the rig shown resulted in the plywood buffer being permanently compressed .012" so the impact is still considerable though not noticeable to the shooter. This can be tailored by tweaking the striker spring though that involves another factor discussed later.

The striker has 2 feet up front to maintain contact with the bolt rails. These plus the recoil spring and the receiver inner surface seem to provide adequate guidance. These feet now slip under the aft end of the bolt via the new milled notch mentioned before. The bolt shown has a new 1/4" milled slot to clear the striker spring guide though this will be a drilled hole later. The new firing pin shown is full bolt length and has a flat on the aft end to engage a newly relocated stop screw. This screw limits rearward travel of the firing pin. Forward travel is limited by a flange on the aft end which fits into a new pocket in the aft end of the bolt. All this is shown in pics 4 and 5. The old 3/8" bore in the aft end of the bolt houses a hefty firing pin spring. Why do we need a "hefty" firing pin spring? Here I will digress again.

Firing of the test gun has been quite reliable lately. An issue with the drum mag was solved by filing the aft end of the cartridge port where the bolt was dragging. The gun now shoots every time the trigger is pulled. But there is another test to be passed - failed disconnector. We want the gun to shoot ony once when the trigger is pulled. If, for any reason, the disconnector fails to catch the striker, the striker will follow the bolt forward as it picks up and chambers a round. If the firing pin spring is too weak or the striker spring is too strong the inertia of the firing pin and the striker will "bust the cap" and the gun will go full auto. Needless to say this can be an embarrassment. I have done this test with an AK-47 (Tapco equipped) and It passed, but I have also had one fire 2 rounds FA with a functioning disconnector. It happens! The test gun failed the test with the springs in use (disconnector taken out) so I doubled the firing pin spring (that's why it's so long) and slightly weakened the striker spring. This stopped the FA action though the primers were imprinted (light strike). Incidentally, during this test I found it was possible to fire single rounds, even without a disconnector, by quickly releasing the trigger . This indicates that the bolt travel, fast as it is, isn't quite as fast as it seems.

Pics 4 and 5 show the firing pin in the bolt and the notch for the striker feet. Two firing pin failures in test showed that the firing pin sees heavy loads in this design and that the striker must not bottom on it, thus the pocket at the end of the bolt. The firing pin, in full aft position, has clearance of a few thou to the striker. Another consideration with this firing pin is the difficulty of drilling a straight hole end to end of the bolt. Drilling from both directions means some mismatch where the bores meet. The bolt shown is a worst case scenario of this. The solution is for the firing pin to touch in only 2 places, just behind the pin and at the aft stop.

 

After 100 rounds through it, the current FCG design seems to meet the requirements for a "builder friendly" system. I didn't mention the disconnector design. It hasn't changed from last year. The only mods to the lower receiver are for the disconnector installation. This is expected to consist of: 1) drill 2 holes in the side wall for the disconnector, 2) Dremel a notch in the safety to clear the disconnector, 3) Weld, braze, or silver solder the disconnector arm to the trigger. No machining. The mods to the bolt are not difficult and are on a par with other FCG designs. The system shown is a prototype systrem for "proof of concept" . Next comes a production prototype.

Okie.... I love it!!

Thanks for posting... and outstanding job!

Great build, I really love the striker design. I noticed the lower feed lip is still on the boltface in your pics. One of the mods done on most factory semi auto versions of open bolt SMGs is the milling of this surface flat with the recess for the cartridge base. This convert the gun to controlled feed instead of push feed. Without the feed lip, the base of the cartridge slides up the face as it feeds. If the disconnector fails or the FP gets jammed in a forward position the extended FP tip prevents the case from sliding up and you get a feed jam instead of full auto fire. With the feed lip intact, a jammed forward FP is essentially the same thing as a FA bolt without any way of stopping a runaway.

Hi kernel. Unfortunately the striker system turned out to be impractal because of the failed disconnector problem. See my new topic: Integrated hammer fired FCG. I looked at my PPS-43C and it still had the lower feed lip. I'm not up on lower feed lip-ology but I assume it was because of the fixed firing pin. If there is a firing pin jam the gun might fire a 2nd round but the disconnector would catch the hammer then. As you say though a feed jam would be better than a FA incident so cutting out the lip might be a good idea. Thanks for the input.

Thst's interesting that the PPs43c has it's feed lip intact. Back when Action Arms wanted to import the semi UZI one of the ATF requirements was a milled off feed lip. I guess they change their minds more often than I change socks!

 

It's a good idea even on hammer fired semis as the firing pin itself could jam forward, essentially becoming a fixed firing pin like on the original open bolt version. While a mag dump is fun when you are expecting it, a runaway could be very .... interesting.

Runaways are no fun at all
Had it happen with an AK one time  you just stand there with it pointed at the ground and try not to shoot yourself in the foot.
Plus all the wasted ammo.
Come to find the shepherds hook pin retainer had come loose and the trigger pin drifted just enough so the trigger could  not catch the hammer.
It was exciting and scary, even though i own and fire full autos it sucks when you have no control.
Ryche

I later checked all my Semi autos. Thompson '28 kit- full lips, movable firing pin. AK-47- same. Tokarev pistol though had lower lip gone. Maybe the thinking is that a pistol runaway is more dangerous than a military piece with a stock.