Service rifle of a Modern-day Third Reich?

Well, that's not easy to answer this question. There are some depending factors in weapons' history that we have to care with.

The following few events are surely would happen in the case of an Axis victory in WW2:
-1944: Stg44 production halted at a number of 500.000.

-1945: Stg45 goes to production (as an equivalent of the real world AK47).


After that if the Axis did not invade Great Britain the optimum 7x43 mm (.280 British) round and the EM-2 rifle with the Taden gun would be adopted by the British and other commonwealth armies (Canada, Australia, New Zealand).

Facing with this the Germans maybe trying to develop a similar multi purpose cartridge like the West German 6.5x43 IWK XPL round in the real world in the year of 1962 (IWK was the former DWM in West Germany. But this depends on the German industrial capacity and on that they would even take an effort to change the comfortable-to-manufacture 7.92x33 calibre kurz round.

You can see the 6.5x43 round in this picture:


Another great depending factor was the United States. After the war without NATO the USA did not have to change its service rounds. There were no Korean nor Vietnamese War so they stuck with their old M1 Garand and M1 carbine rifles till the 60's. that is sure that they would have taken some effort for the new generation assault rifle to fill the niche between the powerful .30-06 and the tiny .30 Carbine . Something like flechette firing SPIW program and/or a small calibre program would have happened. So the further timeline could be the following:

-In the mid or late '50s:
The Germans updating the Stg45 with a new more battle-ready, accurate, cheap... etc. rifle. Let's call it Stg55 or Stg57. It looks like a HK G3 or a HK32 rifle still chambered in the 7.92x33 kurz round. This weapon is the Axis equivalent to the real world Soviet AKM.

The FN FAL or the FN company not sure to exist in the case of an Axis victory in WW2, or just manufacturing munition.

-In the '60s:
There might be a conflict between the USA and the Axis powers where the Germans facing with the experimental or service small calibre rifles. After that they stop to develop their multi purpose cartridge program, and begin to develop their small calibre weapons, conventional and case less also. At first something like the HK36 born, with a tiny 4.6x36 spoon tip bullet.



This was an equivalent to the real world Soviet AK74 with 5.45x39 round, which was the Soviet response to the US small calibre M16 and 5.56x45 rifle and round.

Another option is that they re-barrelling their Stg55 or Stg57 rifles to a small calibre necked down kurz round. The roller delayed blow-back action rifles are very suitable for re-barrelling because of the lack of the gas system. But I don't know that does a necked-down kurz round tend to jam during feeding or not? This idea came from the the real world Soviet method of re-barrelling their 7.62x39 AKM rifles into the small calibre necked down 5.6x39 AKMR rifles, but these rifles did not worked well due to some reasons (maybe due to the gas system?). Some options for the necked down kurz rounds:




The mentioned cartridges can be seen in this photo:


In this decade they also begin to develop their small calibre caseless rifle program, which were completed in the '80s or in the '90s with developing the electric ignition that allows to use heat resistant caseless powder pellets.


So today the WW2-winner Third Reich maybe using a small calibre roller-delayed blowback action assault rifle like the HK33 or HK36 and/or a small calibre caseless assault rifle like the HK G11.

Or they might use a flechette shooter rifle like the Steyr ACR:


And nowadays maybe they would be ready to adopt a kind of Gaussgewehr (coilgun), Gausskanone or hand held and mounted death ray guns.


Furthermore I think that the development of the weapons and munition does not stop in peacetime, just look at the and of the XIX. (19.) century.