At today's Snubbyfest", I finally managed a two-projectile penetration test that eliminated the possibility of drafting.
With the gel set at about 20 yards from the muzzle I hoped for enough vertical dispersion to get one projectile into the gel and the other over the top. Unfortunately, I only had five rounds of the two 105 grain wadcutter loads with me. For the first three shots, everything went over the top of the gel. So, I only got two projectiles into the gel -- the lower striking projectile of shots 4 and 5. The picture below shows the two wadcutters in the right hand block after about 16" of penetration. The two "calibration" rounds are in the left hand block after about 8" and 10" of penetration.
Attachment 242246
I don't consider two rounds to be much of a test. But we did measure the velocity (653 ft/s and 686 ft/s) and the two shots gave almost identical penetrations of just over 16". Admittedly, the gel has not been calibrated since leaving Clear Ballistics and it has been re-cast at least four or five times. But the two JHP "pseudo-calibration" rounds (a 95 grain +P Silvertip and a 90 grain Hornady Critical Defense) still haven't penetrated past the first 11" block of gel.
At any rate, I'm certain that there was no "drafting" involved here because the two wadcutters in the gel are from two different shots. Although it appears in the picture as though they followed a single path in the first block, a view from the top shows the two paths are actually separated by about an inch horizontally.