What an interesting question. Thanks for posting it.
The answer is A, if you use the same powder and peak pressures for both bullets. However, if you pick the best performing powder for each bullet and run the two rounds at the same peak pressure, the difference will favor A less.
To understand this, first keep in mind that as a bullet goes down a barrel, the propelling gas has to chase it. This means, the faster the bullet accelerates, the more energy is used pushing the propellant's own mass down the tube after it. Since no system is perfectly efficient, it can't keep up 100%. As a result, a pressure gradient develops in the bore with pressure back in the chamber being higher than it is at the base of the bullet by a few percentage points.
The lighter the bullet, the faster a given pressure accelerates it, so the more difficulty the propellant mass, a mix of mostly gas and some burning powder mass, has playing catch up. This means, even if the chamber peak pressures are identical and the powder is identical, the lighter, faster accelerating bullets will be at a disadvantage because they have a bit less pressure at their bases. More powder energy goes into accelerating the propellant gases, so less is translated into kinetic energy in the bullet. The measure of this is called Ballistic Efficiency (B.E.), and is given as a percent of the chemically stored potential energy in the powder that becomes kinetic energy in the bullet by the time it reaches the muzzle.
Having said all that, you come to the question of what happens when the peak pressure is the same, but the powder is different. Usually one uses larger charges of slower powders with heavier bullets precisely because the heavier bullet's slower acceleration gives that slower powder more time to put its greater total energy into the bullet. This is matching the bullet to the powder, and when you do that, the percent losses get closer to one another.
This is the kind of problem QuickLOAD is useful for because, even without perfect information on the guns actually used, you can still make a relative comparison. Below are some results that illustrate the point. For the pair with each powder there is about 0.8% more velocity loss for the lighter bullet. But when you compare the H322 load for the 50 grain bullet, which produced its greatest muzzle velocities, to the IMR3031 load for the 70 grain bullet, which produced its greatest velocities at the same peak pressure, the difference drops to 0.5%. For the IMR4198 loads, the 50 grain bullet actually beats the 70 grain bullet's loss percentage with either of the other powders. Just not with the same powder. If I seat the 50 grain bullet out to 2.260" COL, the loss difference within powders increases to about 1.2% because the larger starting volume relative to the powder doesn't light up quite as quickly.
In any event, within normal velocity extreme spreads, especially for a short barrel which will show ignition inconsistencies more readily than a longer barrel does, the practical differences for 50 and 70 grain bullets is ignorable, IHMO.
For fun I also ran a 33 grain Speer TNT varmint bullet and the 77 grain Sierra MatchKing with their best powders. That opened the velocity loss difference up to 2% (15.7% and 13.7%, respectively, with H4198 and 748, respectively). Still nothing to write home about, but still obeying rule A. It should be born in mind that some bullets in some chamberings will have a more perfect powder available for them than others do, and in this case 748 was an exceptionally good match to the 77 grain SMK.
Code:
70 grain bullet seated 2.260" COL (SAAMI max)
50 grain bullet seated 2.165" COL (0.224" into case mouth)
20" 11" % MV Case
FPS, BE FPS, BE Loss Fill
IMR4198, 50 kpsi
70 gr Speer SSP 2705, 31.6% 2324, 23.4% 14.1% 90.5%
50 gr Speer SP 3165, 27.8% 2691, 20.1% 15.0% 97.3%
H322, 50 kpsi
70 gr Speer SSP 2770, 28.7% 2354, 20.7% 15.0% 94.5%
50 gr Speer SP 3206, 24.7% 2701, 17.6% 15.8% 101.5%
IMR3031, 50 kpsi
70 gr Speer SSP 2833, 29.5% 2399, 21.2% 15.3% 102.7%
50 gr Speer SP 3274, 25.4% 2746, 17.8% 16.1% 110.5% (too compressed to load in most instances)
