V-2: Walter Dornberger
Panzer Leader: F.M Heinz Guderian
Taschenbuch des Deutschen Panzer: F.M. von Senger und Etterlin
The Machine Gun: George M. Chinn You NEED Volumes 1 and 4: 1 for history & development, 4 for working diagrams
Small Arms of the World, 11th Edition, no later: Smith and Smith
There are a dozen or so more, but those will get you started.
Be sure you understand thoroughly any term you come up with: Stecke System, Barnitzke System, delayed-blowback, straight-blowback, short-recoil operated, long-recoil operated, short-stroke versus long-stroke pistons, flap locks (Degtyarev did not invent it; it is much older). Know how to describe accurately any particular firearm: K-43 is a good example; it is a short-stroke piston, gas-operated, flap-locked, magazine-fed semi-automatic battle rifle equipped with a Tego System compressed-lamination stock and manufactured almost entirely from green-sand castings and lathe-turned parts.
Know how to recognise a firearm for what it IS: the famous FG-42 is a Lewis Gun gas/piston assembly with a Johnson-type Lee box magazine and a Johnson searing mechanism; it was later modified to belt-feed with a modified MG-42 feed syatem and became the M-60 family of MGs.
Stress in part the continuity of development through the inter-war period in which Germany was not allowed to develop MGs; they were developed in Holland and Switzerland by German-owned companies instead (emphasis here on FF and AA guns), tested by Germans in Russia. Same thing with tanks: designed as "agricultural tractors" with chassis faster than necessary for combining, tested in Russia in joint German-Soviet tests. Kar 98k was a Short Rifle development of the Gew 98 through the Czech VZ-24 and the FN-24, both of which were made on German WW1 equipment, adopted as the Standard-modell and named Kar 98k in 1935.
That should get you started.
In my opinion, there is no satisfactory cross-reference of design and development influence as integrated with mechanical and physical features. That is because I haven't written it yet, still have a number of Mediaeval tales to do before that one, but it is in the works.
Hope this helps.
.
.
BTW, it's "free rein", as in horses running free of the influence of the reins. Dock you 2% for spelling!
Yes, I am a miserable old B*stard..... and I'm GOOD at it!
.