ADDITION 22.12.2023: Straight magazine added as a remix: https://www.myminifactory.com/object/3d-print-straight-magazine-for-suomi-kp-31-347124.

NOTICE 27.7.2022: There are two versions of bolt operation. One is spring-operated as in the Guide, and the other is rubber band operated as in a picture above. It the rubber band is too strong, use only one side. Files for the rubber band version are identified by postfix _rb:

bolt_rb.stl
end_cap_rb.stl
bolt_spring_support_rb.stl

There is also a new version for the the trigger mechanism, identified by _v2, It is operated by rubber band or by a piece of ball-point pen spring, as in a picture above. It allows activation of the bolt also when the safety is ON. The files for the new version are:

mechanism_case_v2.stl
mechanism_inner_v2.stl
mechanism_outer_v2.stl

END OF NOTICE

Update 12.8.2022: Minor modifications in drum_base (v2). Added shorter and longer latch in mechanism_outer_v2 (a). The longer latch should be the first try.

The Suomi KP/-31 (Finnish: Suomi-konepistooli m/31 or "Finland-submachine gun mod. 1931") is a submachine gun of Finnish design that was mainly used during World War II. The Suomi KP/-31 is regarded by many as one of the most successful submachine guns of World War II. / https://en.wikipedia.org/wiki/Suomi_KP/-31 /.

This real-size replica includes many features of the gun: quick-replace barrel and barrel shroud, functioning bolt, trigger and safety mechanism, adjustable rear sight, and removable drum magazine, which can be opened and filled with (PLA) bullets. However, it does not feed the bullets nor shoot them.

Short introductory video: https://www.youtube.com/watch?v=carE71aRAds.

The project is fairly easy to print and tolerances should allow effortless assembly. Supports for some parts may require some experience in printing and removal. There are some joints that may require fitting and sanding due to their very nature, e.g. the quick-release mechanism of the barrel shroud. Some additional hardware are needed.

Several internet sites have been important in the design and construction of this replica. I recommend you to take a look at two of them before starting the construction of this project. https://sketchfab.com/3d-models/suomi-kp-31-7e5a00199276485b96b4749f68a0a9a0 is a detailed and downloadable Sketchfab model, which I used as dimensioning base for most parts. https://www.youtube.com/watch?v=V9YDNWT6qUY is an excellent video displaying the internals of the real Suomi KP/-31. Bullet is taken from https://www.thingiverse.com/thing:4347609.

NOTE: Printing details and construction details are presented in the Suomi_kp31.pdf file.

I have double-checked the uploaded files and they should be final working versions. Please comment if you have problems in printing or assembly.