Years ago, against my better judgement, I had an opportunity to buy a barely used 6.8 SPC AR-15 upper for a pretty good price, so I took it. My first clue there was trouble should have been was how badly the case head was marred after extraction. I always made my reloads lighter than factory, but reality is I could only ever push about 2100ft/sec at the muzzle without creating notable symptoms of overpressure. Factory ammo all exhibited overpressure signs.
Turns out that manufacturer’s supplier of that upper over-chromed their barrels, and so they all had pretty significant chamber pressure issues. Not knowing how safe it really was to shoot, and not liking the anecdotal evidence that suggested I could only avoid overpressure symptoms at positively anemic velocities, it’s sat in my safe for years unfired.
I’ve been debating whether it makes sense to re-barrel that upper in 6.8, or just repurpose the upper for a standard 5.56x45mm AR, sell the 6.8 brass, bullets, etc, and move on. The only reason I’d even re-barrel in 6.8 is because of just a little inner gun hipster. But I don’t really know how much of a future 6.8 has, and I can’t abide by modern guns I don’t ever shoot taking up safe space.