Just food for thought but wouldn't the CS/LS 7 SMG (Above) make a good platform for an assault rifle? Something like the G3 is to the MP5. Ambi-charging handle that isn't reciprocating. A bolt release on the left. Similar mag release to the QBZ 191. And both collapsable and folding stock options are available. I feel like this could've made a great AR platform with some internal changes. Also helps to have a family of weapons that are similar to each other to ease training and uses.
It’s more than “Some” internal changes. The CS/LR7/QCQ171 as a SMG was designed as an Pistol caliber weapon it uses a straight blow back operation. To modify it to an intermediate caliber would demand a complete reengineering of the method of operation. In essence you would be designing a completely new rifle with ergonomics copied from the CS/Ls7. Now that’s not impossible it’s just not as easy as you lay it out.
The MP5 and the SIG MCX were designed in the reverse, as to were 9mm ARs and AKs. They took a rifle and redesigned it into an SMG. In the case of the MP5 we have the G3 as the basis it’s operation tracks back to the WW2 Mauser STG44 prototypes. It was always rifle caliber weapon. The roller delay blowback operation was designed for full caliber cartridges to protect the user from out of battery events.
The Sig MPX is little more than a 9mm MCX using a rotating bolt short stroke gas piston system. Again the rotating bolt was designed for rifle cartridges to prevent out of battery. as the higher pressure point of the cartridge requires a method of ensuring that the opening of the chamber happens at a safe point and not at maximum pressure causing a catastrophic failure.
the QCQ171’s blowback though is dependent on unobstructed travel of the bolt propelled via the firing of the cartridge. In a pistol caliber cartridges out of battery really isn’t a big deal. The pressure points are far lower the safety margins are higher. All you need to delay premature opening of the bolt is the mass of the bolt. Blow back bolts are heavier as a result.
Designing our hypothetical “QBZ171” would require a complete overhaul of the receiver ( not just to accommodate the longer cartridges and magazines), increasing the mass of the bolt isn’t an option as the bolt mass needed would weight would need to significant to counter the pressure of an Intermediate cartridge. You would basically have a rifle weight of 50 lbs. 45 lbs of that being the bolt alone. Clearly not a practical solution.
Shifting to a delayed blowback system like the HK roller system could solve this but recoil is higher, the system runs dirtier and cost of manufacturing is higher to. This is why HK switched to piston systems with the G36.
Obviously the piston system would solve this gas operated guns are softer shooting weapons and cheaper to make.
AR and AK conversions like the Colt SMG and Vyntaz basically replace the rotating bolt and gas systems with a blowback system. Which is a lot easier to do. Basically you are putting an SMG bolt bolt into the body of an AR.
Now you are correct that in training of handling having a family of weapons makes sense. Generally though the adaptation of rifle to SMG is the preferred not just as the engineering but also as the rifle is far more common than SMG. With The PLA they chose to retain aspects of their established rifle general ergonomics over a complete rewrite of the manual of arms (Obviously not to QBZ95 but rather to QBZ03 QBZ81/QBZ88). The Russians with AK 12/AK15/AK19 and the US XM5 similarly show this preference.
