ive been contemplating designing my own .277" caliber on the 7.62x39 case head and preserve the taper for AK/VZ/SKS magazine compatibility
Supposedly the Grendel was exactly this at one point (I suspect very early on, when it was basically a necked-down, set back x39), but the the whole "x39 in an AR" thing hadn't been sorted out yet.
Think about it; you've just designed a new intermediate cartridge optimized for ~500-700yds accuracy (only regime where it's distinctly different enough from the competition to be worth the trouble), and now you've got to market it to owners of
SKSs and AKs. A rather expensive & intensive permanent operation, also. Talk about your fireproof paper hat.
I know that both platforms can be made accurate with proper care, but long range shooting is not what they were built for, not what they've come to be known for, and not what many are even capable of. Surely not what most owners are expecting, compared with ARs or M1As. He was on the right track choosing the x39 case head, as it's the next larger diameter from 223 but still super cheap and ubiquitous like 223, but I think a couple years ahead of his time in that there weren't good 7.62x39 AR options around to grow off of.
However, by straightening the case so as to be usable from AR mags (albeit with modification, which I would love to see the details on; I suspect it ain't much) your hot-shot sniper cartridge is now easily usable from one of the most consistently accurate platforms available. Marketing practically writes itself, at that point.
So to say it was a mistake to 'pander' to the AR is very short-sighted; the 5.45,5.56,6,6.5,6.8,7,7.62,8,9x39 cartridges have all existed for a very long time, yet not one of them got very popular in rifles not initially chambered for it (and even then, only two of all those calibers). It's simply too expensive, difficult, and permanent a proposition for most folks to experiment with their AKs like they do ARs. Now, AA could have instead pandered to the AR10 size platform, but considering the whole point of Grendel was a step DOWN from 308 (size/recoil/muzzle energy but not trajectory) the marketing was just as unrealistic as for the AK. And any sort of hybrid AR10/AR15 or AR47 type thing to get a stronger bolt in a small package was wildly beyond the scope of the project.
It would seem the homesmith tinkerers are interested, though, since there were apparently VZ52, SKS, and a whole lot of AK barrels in Grendel sold in recent years. Sadly, the way most homebrew tinkerer parts-makers run their businesses, it is likely those barrels are still yet to be completed, but it remains clear there is a lot of interest in the Grendel in non-AR platforms at the cutting-edge of development. I think whenever the RDB comes out in Grendel it will be a huge success.
To be honest, I think it'd be worth exploring whether a taper
between the 223 and x39 might be compatible in both after very minor follower/feed lip adjustments (if that). I suspect Alexander's needs were more urgent at the time, so the 223 taper was chosen as a 'sure thing' to proceed on, otherwise more experimentation with cross-compatible forms may have been done. But still, with the various AK-mag AR lowers, x39 AR magazines, and AK-STANAG conversion kits available now, it's kind of a moot point. At least with a straight taper you get another grain or two of volume
110 grain projectile with BC of .370 with a muzzle velocity of 2675fps from a 16 inch barrel, or 2535fps with a 120 grain SST with ballistic coefficient of .400.. the 110 grain would match the drop of the 123 grain 6.5 grendel at 700 yards, it would be flatter shooting inside this distance and it surpasses 6.8SPC
That actually sounds a lot like SPC, itself (or is it PPC? Talk about confusing), only with enough extra case volume to make up for the slightly-worse BC of fatter bullets vs Grendel. Just don't go trying to duplicate 7mm Mauser performance, or anything (kidding, of course; it's exactly analogous to trying to duplicate 6.5 Swede from Grendel, or heavy 30-06 from 308 for that matter --there's a time & place for each, and far downrange the only determining factor is how heavy your bullet needs to be since the trajectories are similar). Only other thing to keep in mind is recoil, at least as a sanity check; Grendel already has about 2/3 that of 308, so pushing bullet weight or velocity much farther in a much smaller/lighter firearm will be self-defeating to a certain extent. That's why I am glad to hear that Grendel is apparently at its most consistently accurate a step or two down from the max load spec for most bullets; puts it even more squarely between 223 & 308 but with a trajectory & long range ballistic effect much more towards the 308 than 223.
The main thing I like about Grendel and similar cartridges is that I think they could provide the basis for a realignment of our NATO catridge set that ends with our infantry having more capabilities at the end of the day. Bumping 5.56 up a skosh from the very bottom-end of 'intermediate cartridges' would give ample justification for bumping 308 up a bit to something more truly effective past 1000yds for LMGs, be it Creedmore or even 300WM. These larger rounds are sufficiently big to not be tempting for a standard rifle platform (except perhaps Creedmore). And lastly, since the power gap between the service carbine round & pistol round will have widened, it becomes more practical to consider something like a PDW to supplement the pistol set for those not actively engaged in pursuing the enemy but who would be better served with a long gun (meanwhile pistols become shrunk to more useful concealable size rather than the honkin and mostly unused hoglegs they are now). Specific-size rounds are used in specific types of guns for specific combat roles, and there is less temptation to use weapons for improper roles (i.e. M240 in a tight apartment sweep vs. an SBR 5.56 or 6.X, or a slung PDW for busy mechanics vs. a small pistol on the belt)
TCB