Whack instead of whomp

[gwtx]

Shot a good size boar a couple nights ago. 170 yards, 123 SST, 27.5 gr AR Comp, 18" bbl. Broadside shot. I heard the dreaded "whack" instead of "whomp". I had this happen once before with a 30:06 150gr pro hunter bullet. It sounds like it hits a fence post. It's the "shield". Either the pig moved, or I moved off target a tad. I was aiming in front of the shoulder. I shot the rifle the next day, 4 rounds into one ragged hole. Bound to have been the hog that moved,,right? Neither I nor the buzzards have turned up a carcass yet. Never found the one that was shot with the 06 either. I hate it when this happens.