An AR mag will typically allow for a loaded .223 round plus max. a few thousands for clearance; 2.250" min. plus a bit more room (~2.260" - 2.265").
The problem with the AR box magazine is there is no taper back to front (in section), and the tapered round of the 7.62x39 has no support either to the side to side at the front of the mag, or top to bottom.
I've never seen an AR mag reliably feed 7.62x39 consistently, either equipped with a properly designed follower or not.
I've got no experience with the Ruger Ranch, never mind one equipped as you say, but any AR I've had or shot in that caliber was a frustrating experience.
Had a Robinson arms XCR-L years ago in 7.62X39 that fed flawlessly from a number of 10rd pistol magazines, can't remember who manufactured them but they were steel and labelled "XCR Micro Pistol 10rd" or something of the like. They were STANAG and there was never a hiccup with them through hundreds of rounds of Czech surplus.
I have run over 100k 7.62x39 through the XCR mags without feeding problems. In AR15s and mag well converted SKS rifles.
You can’t just stuff x39 smmo into a 5.56 magazine, of course.
There isn’t much room to seat bullets out really long but I don’t really see the appeal of that with the x39 cartridge anyway
I have run over 100k 7.62x39 through the XCR mags without feeding problems.
You have shot over 100 000 rounds (in any caliber, never mind 7.62x39) through AR mags (never mind the brand or maker)
Without one stoppage attributed to magazines?
I'm still wet behind the ears, but I have been shooting ARs for over 35 years and you have to be the first human I'm aware of that has fired even a fraction of that number and not had a stoppage attributed to magazines.
Years ago I bought a 12 pack of XCR pistol mags with the 7.62x39 followers (Can-Ammo deal? - can't remember), but running those in my AR - I could only load 7 or 8 rounds for any semblance of reliability.