Post #283 explains why tuners enable smallest groups at only one range. Such is life with different trajectory curves.
The British fullbore marksmen knew that over a century ago; their 303's had better accuracy at long ranges compared to mid ranges. I'll find the link to the web site explaining it.
This is 'positive compensation', a well known feature of rifles using the Lee action. Put simply, the barrel flex sees the muzzle rise during bullet travel down the barrel. So, a low MV round has a slightly longer bullet barrel-time and when it exits the muzzle the rifle is in effect being aimed slightly higher than one with a mean MV in the range; a fast one sees the opposite effect, the bullet exiting at a lower elevation and shooting lower at short range.
At longer ranges, 600 yards and especially beyond there, this 'positive compensation' largely cancels the effects of a large MV range on elevations on the target. This was a VERY GOOD THING given typical .303 arsenal ammunition MV ES values.
When UK all-range NRA prone / sling shooting moved from 'Service Rifle' and the .303 which primarily used the Enfield Number 4 rifle, backed up by the Mauser system Pattern 1914 as a short-range model, to 'Target Rifle' using the 7.62mm NATO cartridge in 1967/8, there were big problems for some years with various factors, a major one being ammunition (lack of) consistency with rifles initially built on WW2 era military actions. For those who could afford it, two rifles became the norm for use at different distances - a Mauser system build for 200-600 yard matches and an Enfield No.4 based one for 800-1,000 yards, the latter's 'positive compensation' giving a marked advantage at these distances, even with very heavy barrels compared to those in the 303 service rifle, also free-floating unlike the stock pressure-bedding that was part of the 'art' in making the No.4 perform. This situation lasted for at least 10 years into the TR discipline era with the Queens Prize winner in the annual Imperial Meeting using a No.4 based rifle for all the long-distance stages. What brought this to an end was the arrival of the stiff SWING action and its very rigid four-lug bolt which shot better than the Lee at all distances. Even so, many club shooters kept their No.4s going as their long distance piece for many years until the supply of heavy barrels ran out and they ended up being sold off at low prices as 'starter rifles' in the discipline mostly to tyros - many are still in service today, but nearly all shot-out despite the very hard hammer-forged chrome moly barrel material.
My first TR rifle back in the early 80s was an ex club 7.62 gun bought for the grand sum of £40 (50 odd US $) which was cheap even then. This was one of the many budget conversions done by G E Fulton of Bisley which simply replaced the barrel and extractor and cut the military forend and handguard right back free-floating the barrel. The club had had eight of these rifles which it had previously used in 303 form converted in 1968, this being the last example still retained but hardly ever used. It would have had a lot of rounds down it in its early years as a 7.62 and obviously nobody had ever done a de-copper job on it, probably just using '009' bore solvent - good for carbon, but which doesn't touch copper. Anyway, this rifle with very mild 7.62 handloads with 146-150gn commercial bullets produced a very strange range behaviour initially from a 'clean' barrel in the normal 2-sighter + 20-score match even at 200 and 300 yards. Sighter 1 would be very high - in the white at 12 o'clock, likely 5 or 6-MOA high; Sighter 2 just got in the 'black' at 12 o'clock some 3 to 4-MOA high, then score shot 1 would have approximately correct elevation and it would be pretty consistent thereafter. Nobody could explain this behaviour, then I read an article by Dr Geoff Kolbe of Border Barrels on the importance of decoppering. It took 10 days of continuous application and soaking of aggressive anti-metallic fouling solution to remove all the fouling, so bad was it - these barrels were VERY tight to handle undersize 7.62 arsenal bullets and raise MVs - but it changed the rifle's behaviour completely. It was obviously a combination of already low MVs with a coppered but carbon fouling free bore reducing them even further until a layer of carbon fouling was laid down that produced wildly exaggerated 'compensation' effects even at short ranges. At the time, nobody I knew had access to a chronograph, so I can only surmise what was happening, not prove it.
