I am looking for an FCG group that works similarly to the FG42 FCG, as in that it is a dropping sear that is an open bolt in full auto and closed bolt in semi-auto. I am not looking for a hammer-fired design.

The sear mechanism does not have to function with the current oprod used by the FG42 as I plan to make a new one with a new bolt design that removes the weak link by having an RPK-style oprod with the bolt more solidly attached.

The new design also does not have to fit into the current FCG housing as the old style is known for beating itself up quite quickly due to the stamped sheet metal design only being held by 2 pins and receiving a lot of impacts from the BCG.

I am willing to compensate someone for finding a usable design* from another working firearm or to design a new one. My issue with the current one is due to the sear rotating left and right to engage on different surfaces of the oprod for semi-auto or full-auto fir the sear surface that interfaces with the oprod is quite small and easily damaged.

To be clear, there will be no permanent modifications to an original FG42, This is a replica.

*A useable design means there are readily available samples, clear drawings or animations that can be replicated

This will be the current test bed before I make a new one.

Current FGC in Full and Semi modes.

Simplified drawing of the sear pivoting from one side to the other to engage a different surface on the oprod.

The current FCG exploded view.

Oprod bottom view showing the rather small notches that mate with the sear depending on which side it is rotated to.

A simplified bottom view of the oprod showing the different control surfaces.

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https://preview.redd.it/uqwqkb6i9sec1.png?width=446&format=png&auto=webp&s=d5476192555b363ed1694d04adf59d98636b6d89

im making a kind of 3dprinted replica of an old single shot break action shotgun

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The M3A1 grease gun is generally considered a very cheap and easy to manufacture weapon, and a small part of that seems to rely on its utter lack of a charging handle. To charge the weapon you have to grab the bolt directly and pull it back. However every other SMG, even the cheapest of the cheap like the Sten, had a charging handle.

This got me thinking, would the grease gun's lack of a charging handle actually be any cheaper? You still have to make the cut in the bolt, but now you have to do it at an angle that can be readily grasped by a finger. Comparatively a charging handle would've just required a bit more depth, but in theory any random bolt or screw or pin could have worked to fill the role (texture it if you can, if not it's not a big loss). Seems like the M3's method would have required a bit more machining (and/or an additional bit size), if only fractionally.

Mind you I'm not considering other features of the guns, such as "safety cuts" or dust covers or so on, and I get how these could affect production costs beyond here. However just in this one area, the M3 seems to be "complexly simple" - simplified in a manner that requires greater complexity in the manufacturing process.

Hi I'm trying to figure out which static coefficient of friction value(s) to use when determining which Trapezoid or ACME threads to use for my projects? As I can't find a consistent value (a) for hardened steel against steel when greasy. As I have to assume user ignorance and less than ideal circumstances.

Ideally, the threads would be kept dry but considering that rifle bolts are often have a lubricant of some kind, usually light grade machine oil or something similar. Thought there could alternatives and that's one problem as source (a), which gives several different static and kinetic coefficients of friction depending on the lubricant being used. As depending of the the static coefficient of friction used can eliminate specific thread pitches (b),(c)&(d) from consideration due to the possibility backdrive.

Backdrive, is when a load that is perpendicular to the threads causes the screw to unscrew itself. Which can't happen, as bolt thrusts and max average pressures vary from 4.41 KN at 170 MPa (1000 lb at 25 Ksi) for .22 LR to 50.8 KN at 450 MPa ( 5.71 Tons at 65 Ksi) for most magnums.

In order for backdrive to not happen the back drive efficiency (f) (which is used in calcuatinting several different things (e))must be <0 with the variables being, angle of the pitch, flank angle, and the coefficient of static friction. Which depending on the lubricant, coating and other things can vary and I'm not sure where too look or which values to use.

Any information would be greatly appreciated, sincerely the OP

a. https://engineeringlibrary.org/reference/coefficient-of-friction

b. https://www.engineersedge.com/hardware/iso-metric-trapezoidal-threads1.htm

c. https://www.engineersedge.com/hardware/external_acme_thread_13360.htm

d. https://www.engineersedge.com/hardware/internal_acme_thread_13361.htm

e. https://www.roton.com/screw-university/formula-calculators/

f. https://www.roton.com/screw-university/formula-calculators/efficiency-power-screws-backdrive/

Hi, I'm wondering which material is better for a rifle receiver? Using the following assumptions: that the barrel is using a barrel extension that the lugs lock into, and carrier tilt is accounted for (with steel inserts for the aluminum and stamped steel with a steel skeleton for the polymer) as I'm thinking of going with Siminov style short stroke piston system which will introduce carrier tilt for the first few millimeters of bolt group travel.

Out of curiosity, are there any show stoppers for having a rotary bolt semiauto rifle use a turbine to open and cycle instead of pistons?