Yessir, Im not too inclined to believed consistent hits at 1000 with 7.62x39 without seeing it. That is certainly an anomaly. I wouldnt be as surprised if it were 5.45 or 5.56. With the 7.62x39, its not so much about having a match-tuned rifle. Theres plenty of CZ-527 bolt-rifles in 7.62x39 out there on the civvie market and most everything Ive seen has the owners claiming 1-1.5 MOA atleast, sometimes better. No matter what rifle one is using, in 7.62x39, ammo options are scant. Im not sure if the Soviets/Russians ever made any special match-grade/long-range ammo available but with the plain-jane FMJ fodder out there on the market, 1000yd is a daunting proposition indeed. The velocity as well as the BC is just too low. It will be in the transonic range around 500yd at least. Ive never tested it personally, but granted thats because with most bullet designs once you get past the transonic range, all bets are off as far as stability and therefore consistency(AKA "keyholing" in the target). However, there are some bullet designs that will transition with stability. The 168gr .30cal SMK being a notable terrible example for NOT transitioning well. Maybe 7.62x39 FMJ rounds DO transition with stability??? Again, Ive never tested it but its certainly possible. You would have to talk to Bryan Litz to get a better explanation why some bullets do and some dont transition well. From everything Ive seen, the boat tail angle plays the largest role in determining whether it does or doesnt. Hence the 13-degree boattail of the 168 SMK not doing well but 9-degree boat tail bullets and similar transitioning well. Im not sure on flat-base bullets.
With all that said, lets say for a second that common 7.62x39 FMJ rounds DO transition consistently. Even if they do, do you understand the numbers involved? If someone has the ballistic coefficient of a 123gr or 124gr .311 FMJ, Ill gladly run it through a ballistic program to get exact drop/wind numbers. As a VERY rough SWAG though, Id say youre talking about at least ~25-40 FEET of drop at 1000 yards. Hence, that bullet is coming in at a very steep angle, near vertical. Think about that for a second. What this translates into, is any velocity variation(and 7.62x39 fodder is certainly not a model for consistency) is magnified exponentially. Using a benchrest, with a perfect steady hold, and no wind, think several to several tens of inches in vertical dispersion. Though because the bullet is coming in at such a steep angle due to the low velocity, it would be more in front of/on/or behind the target. Even if it does transition well, Id be surprised to see 2/10 hits at 1000 from a 7.62x39. And thats only taking into account vertical dispersion due to velocity variation and such low velocity at that range. If theres any type of switching/complex winds, you can forget about it altogether.
Are you sure it was with 7.62x39? Approximately what altitude and atmospheric conditions?
Again, Id have to see it to believe it.
If anyone here keeps up with the NRA/CMP High Power circuit, well-known competitor German Salazar actually talked quite fondly of the .300 Blackout. (With supersonic rounds) Mr Salazar actually found the Blackout performed admirably at the 600 yard line, needing only a bit more elevation and close to the same wind as a 77gr SMK from a .223. With the best/right load: a SWAG would be supersonic-loaded 155gr A-MAX, the .300 Blackout should make it to a grand consistently.
Id be inclined to believe that one could do the same with 7.62x39 with the right handload, but Im assuming this instructor wasnt using special handloads so Im having a hard time believing it.