As discussed in the section on the World Trade Center (see Part Three, Section C, pp. 132-134, supra), Williams' method of determining weight is impressionistic and depends on his VOD estimate, which is itself speculative. As noted in that section, other members of the EU do not routinely estimate the quantity of the explosive from a damage assessment because the placement and confinement of the explosive have a significant effect on the amount of the damage. Nevertheless, we concluded in the Trade Center case that Williams' size estimate of 1,000-1,500 pounds, which he characterized as a ballpark figure, was not, as such, an unreasonable opinion because he offered such a broad range.
With respect to the weight of the explosive, Williams' Oklahoma City report differs from his trial testimony in the Trade Center case in two respects. First, the Oklahoma City report does not offer a broad range but limits the estimate to approximately 4000 pounds of ANFO.
Second, it appears that Williams' opinion was based in part on the recovery of receipts showing that defendant Nichols purchased 4,000 pounds of ammonium nitrate.
No practice runs...nothing?
McVeigh claimed to have done practice runs (scale unknown) to determine how to direct the blast. IIRC, he ultimately used 8 55-gallon drums arranged in a J shape: that works out to about 400 gallons, and thus roughly 4000 pounds, arranged to increase the effect in one direction. 2 tons at 2 yards causing OKC damage matches pretty well to 1000 tons at hundreds of yards causing Texas City damage, esp. with the former intentional and the latter accidental.
We may not have exact figures, but all the data fits together pretty well.