drover,
I would argue that using data from ONE manual is far more skewed than using an average of data from several sources, which is what I did. But let�s try some other numbers of the sort you suggest.
Loading data is complicated by the fact that the SAAMI maximum average pressure for the .204 is 57,500 PSI, and for the .223 55,000 PSI, which is no doubt why the SAAMI �suggestion� for muzzle velocity for 40-grain bullets from both cartridges is almost identical, 3775 for the .204 and 3770 for the .223. But I couldn�t find any presently listed factory loads that adhere to those numbers.
I did find two 40-grain .204 factory loads listed at 3900 fps, but have tested both in the 24� barrels (the SAAMI standard) in my own rifles and came up with an average of 3727 fps. The other two 40-grain factory .204 loads I could find are listed at 3625 and 3650 fps, for an average of 3737.5. The only two .223 factory loads I could find are both listed at 3700 fps.
There still isn�t a vast amount of .204 handloading data, especially when compared to the .223, and some I rejected for comparison purposes because the .223 pressures were CUP and the .204 PSI. Also, a few companies still pressure-test in one barrel then chronograph velocities in a commercial rifle, on the theory that this results in a �representative� velocity. I only compared pressure data derived by the same method in both rounds, with velocities from the same barrel as the pressure data.
The reason I used the same powder charge weight when calculating recoil with the Sierra formula is that in my own .204�s and .223�s I�ve used charges of around 28 for 40�s. In my .204�s I use 27.0 grains of Ramshot TAC (their maximum listed charge), partly because TAC is one of the faster powders in the .204. In three different rifles with 24� barrels, and with three different 40-grain bullets, this charge produced an average of 3752 fps.
In the .223 I used both 28.0 grains of TAC and 27.0 Benchmark with 40�s. Both are the maximum listed or very close to it. In this instance neither of the .223�s used had a 24� barrel. Instead one had a 26� and the other 22�, so I adjusted the velocities by adding or subtracting 25 fps for each inch of barrel length. They averaged 3809 fps.
I used the same barrel-length velocity adjustment for published handloading data, because a couple of the .204 rifles used for data have 26� barrels. The average for top velocities with 40-grain bullets in the .204 came out to 3730 fps, and for the .223 3799 fps.
Averaging all of this data result in a 72 fps velocity difference in favor of the .223. This is a little more than the 100 fps difference I used in calculating the recoil numbers, probably due to the small difference in SAAMI pressure between the rounds. (The formulas used were a couple I came up with empirically from loading data, but afterward they were confirmed as valid by the late Don Miller, the same guy who came up with the Miller bullet-stability formula, now pretty much considered the industry standard.)
On the other hand, the powder charge I used could reasonably be dropped a grain in the .204. Using a velocity of 3728 fps and a powder charge of 27 grains in the fps for the .204, Sierra�s recoil program resulted in 3.9 foot-pounds or recoil, a .1 reduction from the first number quoted.
