I've been intrigued by the 9x39 Russian for quite awhile, as many others here apparently are.
Been working on this for the last few months, had it up an running for deer season and got skunked (literally, as that's all I saw).
It is an Indiana DNR legal "Pistol Caliber Rifle" cartridge based on the dimensions with a .358 projectile. Thus, the name the 358 Gremlin for those living in Indiana to avoid any confusion with the 9mm moniker.
I initially started with 6.5 Grendel brass, necked up in a few steps to .358. A bit of a step forms with the Lapua brass at the old neck/shoulder junction. It is large enough it needs to be turned off, trying to cut the chamber neck with that step is asking for problems.
Shortly thereafter I tried some Winchester 7.62x39 brass, necked up in three steps to .400", it will then form in one pass with enough shoulder to fireform to size. Best thing about it - no neck turning. The brass is .0115" thick at the neck and pretty consistent. Large primer pockets though
Case capacity depending on the brass is 38.5 of H20 +/- after fireforming.
Using Lil Gun powder it has been delivering the 180 Hornady SS/SP at 2325-2350 fps out of a 17" McGowen 14 twist barrel. Accuracy has been phenomal with those bullets, rarely will a group go over 1.5" at 100 yards.
200g Hornady SP's have been up to 1950 fps, load development is continuing there.
225g Nosler Accubond have been up to 1650 fps, that bullet is such a case capacity hog due to the length. I'm unsure if we'll get it up any further than that.
Recoil isn't anything, my test rifle in its hunting configuration: medium weight barrel, carbine gas length, full float tube, Cavalry lower and a 4x Leupold and weighs 5.75 lbs. Nice for carrying in the woods.
Standard carbine buffer works fine, can't tell any significant difference in function with a heavier buffer or stronger springs - or when used on a standard rifle lower receiver.
So far, the most reliable magazine has been some "misformed" 6.5 Grendel magazines that have a shallow front rib (less than .115" deep). The 17 and 26 round "misformed" mags seem better than the 10 round magazines. For whatever reason the 10's seem to swell the mag body more than the others.
For best performance, I think the rib needs to be less than .100". Plan on researching the availability of such a product at SHOT - or may take a ball end mill to a few magazines for testing in the meantime.
With such a magazine, running projectiles up to the 310g FMJ Woodleigh won't be any problem.
I've got an order placed for 4000 pieces of basic PPC brass with small primer pockets that will hopefully be delivered in the next month or so, once the dimensions of the brass are measured - the reamer will be modified as needed to abandon the Grendel brass and maximize performance using the Winchester 7.62x39 and PPC basic brass.
Updates coming on the supplier items (magazines & brass) from the floor of SHOT in 15 days!