I first started loading 243 and 6mm in the mid 60’s.
History from my 1960 Load Notes
The 6mm Rem (244) is based on the 1895 6mm Lee Navy.
http://www.spanamwar.com/lee.htm The Navy brought out this high-velocity repeater, the 6mm Lee Navy--a rather innovative arm for the period that was unfortunately a tad ahead of its time and was withdrawn from service after only a few years. The round did have the distinction, though, of being the first rimless load used by the American military. It is the parent cartridge for the 30-03/30-06, having the same base diameter, rim diameter, and rim thickness. Although the 7X57 was undoubtedly the inspiration for the 6mm Lee Navy, the 7x57 has different base diameter, rim diameter, and rim thickness, and in addition was Berdan primed. When Remington developed the .244, their primary objective was a high performance varmint cartridge with a maximum bullet weight of 90 grains. Because 257 Roberts cases were available, they were used to form the .244. The 1-12 twist and was ideal for 75-90 gr bullets.
From
http://www.reloadbench.com/cartridges/6mmr.html For reasons that no one can explain (or prove), deer hunters decided that the .243 Winchester with its 10 more grains a bullet was a better choice. Or at least this is the traditional explanation. Possibly a factor even of greater influence was the rifles the two cartridges were available in. The .244 was offered in the Model 722, a rifle about as handsome as a fence post. The .243 was available in the sexy little Model 70 Featherweight. When it came to looks, there was no contest.
Around 1958, Remington started rifling all .244" caliber barrels with the 1-9" twist, but nobody seemed to notice. In 1962 when the new Model 700 rifle was introduced, the only 6mm cartridge it was available in was the .243 Winchester, but during the next year the .244's name changed to 6mm Remington and reintroduced in the Remington bolt action.
Incidentally:
Winchester modified the 6mm Lee Navy case a bit and in 1935 introduced a cartridge of its own design called the .220 Swift.
http://www.gunsandammomag.com/classics/3006_031306/
Practically Speaking:
Load data on 100 gr bullets lists the 6mmRem as having a 200 – 300 fps advantage over the .243. My experience loading for several weapons in each chambering shows that accuracy loads for 6mm Rem were at/or nearly at max, while accuracy loads for .243 were well under max, thus the spread in handload velocities is even greater when accuracy was included in the mix.
I have seen the argument that this 200 – 300 fps difference was insignificant. I disagree because:
1. I have taken 5 bull elk with 6mm Rem w/100 gr Nosler Partitions, one shot kills. I have taken many more with 300WinMag, and a few with 270 and 30-06. I do not suggest hunting elk with 6mm, but to point out that I have a knowledge/experience base to qualify my statements in 2. below.
2. Based on one above the 300WM puts down elk with a lot more authority than with a 30-06. The 300WM is only 200 - 300 fps faster than the 06 with 180-200 gr bullets. This velocity difference is significant. The 200-300 fps advantage of the 6mm over the .243 is significant in the same manner.
In addition, the longer neck of the 6mm yields better accuracy, less throat erosion, and longer case life.
