That's my understanding of it , too -- "pulp."
MacPherson's speculations on wound mass (in Bullet Penetration) harkens back to the 1904 terminal ballistics testing of Cols. Lagarde and Thompson on beef critters in the Chicago stockyards. For a variety of military handgun cartridges, Thompson and Lagarde documented the number of shots into non-vital tissue required to bring a large beef critter to its knees. The take away was that heavy, large caliber, flat-nosed, deeply penetrating projectiles at modest velocities were more effective than lighter, smaller caliber projectiles. (No fooling!)
MacPherson combined the Thompson/Lagarde data with his own mathematical models for bullet penetration and wounding characteristics of different bullet nose shapes and found some agreement among the various cartridges in the total wound mass required to incapacitate a steer. So, he speculated that if it takes about 200 grams of "pulp" to incapacitate a 1000 pound steer (whether that comes from 3 or 4 rounds of .455 manstoppers or 6-7 rounds of .38s), then maybe it takes about 40 grams of wound mass to incapacitate a 200 pound badguy. MacPherson acknowledged that this is just speculation. But at least it's speculation based on some factual foundation. It's worth noting that all this has to do with wounds to non-vital tissue and has nothing to do with the fact that a lowly .22 long rifle bullet hitting a CNS target can be instantly lethal.
My own take on wound mass is that it's easy enough to calculate, much harder to measure, and even more difficult to predict what effect it might have in any particular real world scenario. But I'm suspicious even of the calculation part. For example, here's a table from MacPherson's book listing wound masses for various calibers and bullet nose shapes:
It's important to realize that these numbers represent an "effective" wound mass in a human target. They don't include wound mass from penetration deeper than 18" (as already having left the target) or from the last 3" of penetration (where MacPherson assumes the bullet is going too slow to do full damage). That part I agree with. But note too that MacPherson has a line for "Cylinders" (i.e, wadcutters) and then lumps together "All others" regardless of nose shape. Maybe he's right. But MacPherson's penetration testing was limited and didn't include, for example, WFN bullets. It seems to me that there are way too many reports from the field about the effectiveness of a big meplat on game both large and small for it to somehow dissolve into "All others." On the other hand, while it's easy enough to tell a SWC from a RN from the hole it makes in a target, it's much harder to tell one from the other by its entrance hole, path, or penetration distance in Clear Ballistic gel. Full wadcutters, by contrast, make full caliber entrance holes, leave slightly wider wound tracks, and penetrate much less than either the SWC or the RN.