A .220 Swift fires a bullet the same diameter as the .22/250 ie. .224 inches.
In 1972 at the time of your legislation Norma ammo for the Swift was 50gr in bullet weight but the muzzle energy was 1877 foot/pounds.
It had surplus energy but lacked the weight.
At that time Remington .22/250 ammo was advertised as having 1770 ft/lbs with the 55gr bullet.
Norma advertised theirs as 55gr SPS bullet at 1637 ft/lbs.
In 1982 the Remington 55gr ammo was listed as having 1699 ft/lbs.
Norma reduced their bullet weight for the .22/250 to 53gr.
The whole thing was a mess !
Many knowlegeable people at that time turned to the 5.6X57 RWS cartridge which had been developed in Germany for shooting Roe and Chamois.
It fired a 74gr bullet known as the `kegelspitz`with a muzzle energy of 1910 foot/pounds.
I had a 5.6X61 Vom Hofe Super Express on my Irish licence for a few years, this was the ultimate at that time with a 77gr bullet and 2350 foot/pounds of energy.
Currently Remington advertise their .243 ammo as having slightly less than 2000 ft/lbs of energy, only slightly more powerful than the 5.6X57 RWS !
So much for progress as many on these forums think the .243 is the ultimate.
Believe me it is not, I have two .243 rifles [ shortly one only ] yet I always pick up the .270 when I go out to shoot a Deer.