The 450/400 came about when military calibers were banned for civilan use in some countries so the 450 was necked down to 400. In the nitro version it became the standard for all 40 calibers...ie...404 jeffery etc. The 3-1/4 versions tend to run 411 diameter and the 3 inch version 408.It has high sectional density bullets and makes it a great dangerous game caliber with out un-managable recoil. Brass is readily available and so are bullets so it's not to expensive to reload, i load for mine all the time.
Sorry to disagree, but advent of the 450/400 3 1/4" nor the 450/400NE 3" had nothing to do with the ban on the 450 diameter being oulawed in India, and the Sudan. That ban was in 1909, and the 450/400 3 1/4"in it's, blk pwdr form was introduced in 1880, by Kynoch,and was converted to smokeless in the mid 1890s. The 450/400NE 3 " Was introduced by Jeffery in 1896 and was never available in anything other than smokeless powder version.
Most of the 450/400 3 1/4" rifles are very heavy for caliber with most of them weighing in at around 11 to 11 1/2 pounds, with a few as heavy at 12 lbs. Addtionally, one must be very carefull when buying a 450/400 3 1/4" rifle, as a lot of them were for blk pwdr, ans were fed a diet of smokeless ammo after it became available. The cartridge case was not changed, and the Corddite rounds would chamber the same as the blk pwdr ammo. The 450/400NE 3" Jeffery was never offered in a blk pwdr version, so any rifle cahmbered for the 3" version is automaticlly a NITRO PROOFED rifle, and the two cartridges are not interchagable. The 3' will not chamber in the 3 1/4" rifle.
The 450/400NE 3 1/4", and the 450/400NE 3" are far more powerfull that the 375 H&H, with a 400 gr bullet compared to the 300 gr of the 375H&H. The 400s went down in popularity when the 375 H&H wasintroduced in 1912, but not because it was a better cartridge for dangerous game, but that it was cheaper to buy, in it's bolt action rifles, and was better down range, so was a better all around cartridge, for general use!
Many new rifle today are being chambered for the 450/400NE 3" Jeffery, and factory ammo is again available from several makers. Chambered in S/S double rifles, and O/U double rifles, as well as the Ruger No1, it is again taking it's very hounourable place as a very well ballanced cartridge for all around use on the largest thing one might want to hunt.