Lee Enfield rifles were believed to "compensate". The vibration pattern of the barrel and action "compensated" for inherently lower quality ammunition at long range. This was believed to occur because as the bullet was passing through the bore, the barrel was vibrating in a vertical plane. Because of varying barrel times, bullets would exit at different points in the muzzles' oscillations. This was believed to reduce vertical dispersion at long range. Bullets with lower velocities tended to be launched slightly higher than faster ones. It had no effect on lateral group size. With quality ammunition, compensation was no longer a factor. It was not unusual for target shooters until about 15 years ago to have two rifles - one with a modern action for ranges to 600, and a Lee actionned rifle for 800, 900 and 1000. But once issue ammunition was no longer supplied and mandatory for competition, the Lees disappeared practically overnight.
A rifle that has total group dispersion of 3 minutes at 100 will not shoot 1 1/2 minute groups at 600. Lateral group size is not affected by compensation. At long ranges, vertical stringing was thought to be reduced. Groups became rounder.
There was a formal study done in the '30s, which confirmed what target shooters had observed.
The so-called "DumDum" bullet was a hollow point variant of the pre-Mk. VII cartridge. The name came from its manufacture at DumDum, an ammuntion plant outside, IIRC, Calcutta. This pattern of ammunition gained a reputation for effectiveness on the NW Frontier; the earlier pattern of 215 gr. roundnose bullet had a reputation for poor terminal ballistics. It was a standard pattern of cartridge, and was made in ammuntion plants throughout the Empire, including in Canada. The Hague Conventions, 100 odd years ago, did outlaw the use of expanding bullets for military purposes.
The Mk. VII bullet does have an aluminum or fibre tip under the jacket, which reduces the weight of the bullet, and changes its balance. I do not know if the primary purpose was to produce a bullet that would destabilize once it struck something. I suspect that this was not the case. Most countries went to pointed, higher velocity bullets at about the same time, and it was observed that these bullets did tend to tumble on impact, while the earlier heavy roundnosed bullets had greater penetration.