Could be. Here is my raw data if you'd like to check it. All cases were filled with an eye dropper right to the base of the neck as checked under a magnifying glass and strong light. The .243 cases were fired in the previous chamber and had been neck sized only, I inserted a fired large rifle primer in each to keep the water from coming out of the flash hole and to keep that from influencing the weight differences from the fired AI cases.
All cases are Winchester from the same bag, twice fired before forming and trimmed to a uniform length after the first firing.
Formulas here are filled weight - empty weight = weight of water, all weight in grains.
.243
Case 1: 218.3 - 165.3 = 53 grains of water
Case 2: 215.6 - 163.3 = 52.3
Case 3: 217.4 - 164.7 = 52.7
(53 + 52.3 + 52.7) / 3 = 52.66666, called it 52.67 grains average
.243 AI cases
Case 1: 219.4 - 164.7 = 54.7
Case 2: 220.0 - 165.4 = 54.6
Case 3: 220.3 - 165.3 = 55.0
(54.7 + 54.6 + 55.0) / 3 = 54.766666, call it 54.76 average water capacity
54.76 - 52.67 = 2.09 grains increase.
2.09 / 52.67 = 0.0396, or call it 3.96% increase. In my earlier calculation I had used 54.7 instead of 54.76 so that accounts for the extra .16%.
Still, round that up to 4% and apply the 1 to 4 rule, which say that a given percentage increase in case capacity will give you roughly 1/4 that much percentage increase in velocity.
Figure 3000 fps as a nominal velocity for the .243 with 100 grain bullets.
3000 fps * 1% = 30 fps increase.
In the article you cite his fired cases have about 2 to 3 more grains than my Winchester cases so he could certainly have gotten a 5% increase.
5% * 1/4 = 1.25%
3000 fps * 1.25% = 37.5 fps increase. I concede that the author of the article beats me by 7.5 fps.
