Accuracy gets worse when any spitzer boattail bullet slows down to the speed of sound. They start buffeting and change directions. Just like jet planes did as their speeds were fast enough to approach the speed of sound.
It just so happened that it occured often when 7.62 NATO match ammo with 168-gr match bullets with a lower BC than the former 173-gr ones were used in long range events past 600 yards with 22 and 24 inch barrels. Bullets slowed into the subsonic range about 800 to 900 yards. This started in the early 1980's in the USA.
Military rifle teams would replace the 173-gr arsenal match bullet in M118 match ammo with Sierra 180-gr match bullets. The Sierra's were more accurate and so used, won a lot of matches and set some records. In the mid 1980's, Sierra changed that bullet's boattail from a long, 9 degree one to the shorter, 13 degree one like their 168 has as a cost cutting scheme. That new 180 shot out the same 2600 fps from 22 inch M14 barrels but it's lower BC let it go subsonic about 900 yards down range. Just like Sierra 168's did. The U.S. Army contracted Sierra to make their 180 HPMK bullets with the original boattail then they started winning again.
In the British Commomwealth countries shooting 7.62 NATO service ammo with 147-gr bullets at 2800 fps from 22 and 24 inch barrels in their long range matches in the early 1960's, same problem happened. As their rules only allowed arsenal ammo (no handloads nor commercial ammo), their solution was 30 inch barrels. They shot those light, low BC bullets out at 3000 fps. That kept them supersonic through 1000 yards.
When Sierra released their new 155-gr 30 caliber Palma bullet in 1991, people had to shoot them from 30 inch barrels from 308 Win cases. They learned what the British learned 30 years earlier. 175, 190 and 200 grain match bullets shot from 24 and 26 inch barrels left fast enough to stay supersonic though 1000 yards.