I have a 1:9 rifle built , by Jim Dugan, with a Gaillard barrel, for 5.56 NATO in 1995… Still shoots IVI 62 grain into 3/4 minute, and 65 grain handloads into 1/2 minute. The 62 grainers will stay supersonic out to 900 m IFF you're at an elevation similar to the Heritage Range, outside of Calgary, and you'll still be able to shoot reasonably well. Take that down to (say) Camp Dundurn and try shooting, and the heavier air will slow the bullets down enough that they'll be unpredictable past 700. (found that out in a hurry)…. A different Gaillard 1:9 barrel on someone else's rifle fired a 14 inch group with Sierra 80 grainers at 1000 - off elbows in "fullbore" style (I wasn't shooting that one)…
An acquaintance who was on the DCRA's Bisley team thought he'd get a Brno sporting/hunting rifle (12 twist, IIRC) to use the issued IVI stuff (those were the days) for some fun plinking, but found that the bullets couldn't even hold a 6 foot group at a mere 200 yards, so he sold the rifle to someone who uses 55 grain bullets.
Wheee!