Call it 100m. Pistol cartridge will not be effective beyond that, again, as I'm sure the russians found out.
And again. Sorry but modern european and Russian militaries disagree. Assault rifles on their own are not up to snuff and long range engagements happen often enough that not having squad level weapons that can reach at that range is a serious hamper. The 7.62 MG is a bitch to carry that much is true and I can attest to that but so is the heavy duty multi purpose AT weapon like the carl gustav and that is making a comeback as well. We recently traded the MG-3 for the minimi and less than a few years later we're looking to get a MAG variant back at the general squad level (because a lot of the old MG-3s date back as far as WWII complete with waffenamt on the bolts and the turkish MG-3s are crap), danes are doing the same and the french and brits did it a decade ago.
6.8 etc has been a meme for a decade now and has its share of drawbacks as well and I don't think this round of trials will produce anything new in a while just like the previous 5.
The PPSH41 manual told gunners how to aim out to 300m. The MP18 and MP40's effective range is listed as 200m both out of a 10 inch barrel.
I think you're misunderstanding the point of what modern armies are doing. Especially as they have only small caliber, high velocity rounds as the only squad weapon and retain only two MMGs in 7.62 at the platoon level. Of course in modern armies they have a lot of optics around which means they can actually use range out of their weapons in a way that armies prior to the 1980s simply could not. The switch to the SAW after Vietnam was due to the M60 (and it's replacement) being too heavy for squad use, hence the M249. Getting a MAG variant at squad level is probably not going to happen for most armies, as the US is really working hard to make the LSAT happen in 6.8mm:
https://en.wikipedia.org/wiki/LSAT_light_machine_gun
https://www.tactical-life.com/news/us-army-6-8mm-weapon-systems/
The above link shows they officially adopted the 6.8mm round in October of this year.
Which NATO armies will most likely then adopt. The CG is not really that practical as a weapon long term due to the impact it has on the user's body, despite it's combat utility. But in WW2 armies had the bazooka (or Panzerschreck...or PIAT).
Britain, Denmark, and Finland are probably only interested in pushing the big guns back down to the squad level due to their involvement in Afghanistan, which is a completely different animal to WW2 Europe. Since I'm proposing the 9-7.62x25mm SAW for a WW1 and/WW2 army, not a modern army fighting in Afghanistan, the needs and uses are different.
I do not advocate the bipod long sighted SMG instead of the LMG but merely that it can supplement or be a makeshift substitute where necessary. The SMG and LMG are a team. You would never use an SMG beyond 200 metres if you have an LMG to hand. Nor is an intermediate rifle a substitute either. The 7.62x25 is not hugely far from a 7.62x33 Kurz but will operate as a simple cheap blowback and has done. There is always the small infantry platoon mortar as an alternative with a well practiced user and this is neglected when speaking of the 200-700 metre zone. The French long had a love affair with the rifle grenade albeit to only 400 metres. Still, in WW1 on the Western Front, it all only affects the immediate trench action or mobile warfare after a breakthrough but cannot make a difference to entrenched MMGs in quantity nor artillery which define battlefield parameters.
Perhaps SMGs could play a history changing role on other fronts?
Right, I'm only suggesting it as a supplement for squad use, not a replacement for an LMG/MMG at the platoon level and above, just as the SAW is not replacing the long range full MG in a modern army. Though looking at the history of the RPD when in service, it did a pretty damn good job despite being relatively short ranged compared to a full powered round. Rifle grenades are like a light mortar minus the mortar itself and 400m range for the rifle squad is plenty for their objectives; in WW1 they learned the benefits of using rifle grenades to knock out MG positions suppressed by the Chauchat. Also BTW SMGs in WW2 proved themselves useful even in open field conditions due to being able to get close to an enemy before they received effective fire enough to suppress/eliminate them. In fact the Brits themselves in one study found that Sten gunners were more likely to actually hit their enemy than SMLE users and that the limited range tended not to be an issue in combat conditions due to the difficulty in actually spotting someone using cover and in drab colors beyond 200m or so. In the desert or in mountains perhaps, but in general field conditions especially with camo sighting without optics is very tough.