I haven't checked ballistics tables, but .303 British is not as powerful a round as a .30-06.
I've always thought of it as more like .308 Win., and maybe a hair less.
I may nadin the ballistics later.
The .303 is only slightly less powerful, but in the same class...however the military loadings of .30-06 and .308 are ballistically identical, the bullet is exactly the same weight (Exactly the same in every other way for that matter in the ball loading), the only difference is that the .308 (Nowadays known as the 7.62x51 or 7.62 NATO in the military version) used more modern propellants at higher pressures to achieve the exact same muzzle velocity as .30 M2 Ball, the whole purpose of the round being to get the case as much shorter as possible by applying 40 years worth of development in propellant technology to the problem. That was really the whole reason the .308 round was developed around the end of WW2.
The '06 was about the longest standard military cartridge in use in the world and nothing in use in other countries with other standard calibers could be converted to '06 easily and with sufficient reliability for military purposes. Army Ordnance experimented with the MG42 converted to '06 during the War, but it didn't work worth crap due to the longer '06 cartridge, and so they turned their backs on the gun. In other countries, however the MG42 with only about three significantly modified parts (Feed tray, pawl, and .308/7.62 barrel) became the MG1/MG3, and had a long record of successful service (Still going on, in fact). Of course we had hungry engineers and ordnance officers to feed, so we didn't go with that solution but instead developed the M60 (Incorporating the MG42 feed system and the FG42 gas system) shortly before Viet Nam and the horrifyingly-unreliable and complex M73/M219 MGs, the latter of which won an exceptionally-crooked shoot-off competitions managed by the same Ordnance Corps pukes who were going to be making them over better weapons like the MAG and MG1/3 (An eyewitness described what went down to me, all I can say is it's a case study of what happens when you have a government-run enterprise as a competitor with private industry, and the same government entity that made its entry is judging the competition).
The M60 proved only moderately reliable but complex and expensive to make (Somewhat complex to reassemble when fully stripped, and prone to have an important part fall off as well), and finally lost an
honest shoot-off to the M240 when there was a competition for a new AFV MG to replace the M219s in the late 70s, the 219s having been the bane of tank gunners since their adoption, and after-action analysis from Viet Nam finally providing the catalyst to dump the damned pieces of crap. A vehicle-mount configured FN MAG proved to have only 1/3 the MTBF of a similarly-configured M60, and so it became the M240, simple and reliable (Still not as quick to change a barrel on as an MG42/1/3/53, but a bit better at long range since it is gas operated rather than short recoil operated, yielding a more rigid sight plane). A decade later the rest of the M60s went away with the adoption of the M240B, the infantry/GPMG-configured FN MAG (Heavier than the MG42/1/3/53, but at this point there was a parts commonality strength for it since the M240 tank coax was already in the system).
