I used an AT Pac early in my career (mid 80's) and was actually quite happy with it even though it was considered obsolete then. I was even happier when the obsolete BP/wings and back inflate BC concept came back into vogue. The At Pac was very much an ABS BP wing in form and function with the exception of the chamber in the plate for the shot. (see pictures)
The At Pac used a 8 inch long 2 pronged pin to hold both sides of a stainless steel door in place at the bottom of the shot chamber. (see picture)
The "dangerous" reputation came from two problems.
First was a tendency for the shot to hang up in the opening of the door as over time the relatively large shot (about 1/4" in diameter) would settle and tend to form an arch so that most of it would stay in the pack in an emergency. It's really hard to bounce up and down and shake it loose in midwater. The shot chamber was large enough to accomodate a LOT of shot, so most people only needed to fill it about half way. This caused problems with the shot shifting so Watergill resolved this by using plain old glass marbles to fill the unused space. The larger size of the marbles however would agravate the problem of arch formation, so they had to go in first with the shot underneath which made changing weight a true PITA. But once the weight was adjusted, it was an innovative and very effective system, assuming you never had to dump all the shot in an emergency.
The second problem was more serious as the over pressure relief valve was built into a fitting that accomodated the corrogated and LP inflator hoses. (see picture) It was a little small and if you got into a rapid ascent with the 45 lb wing, it was theoretcially possible to blow the seams on the wing as the air would expand fastert than the OPV could vent it. The resulting negative bouyancy from a ruptured wing then made the problem of the lead shot not wanting to leave the pack, even after you pulled the pins on the door, a very personal and alarming problem. Consequently, I did not know many At Pac divers who put all of their weight in the pac itself. We put about half our weight in there and considered it permanent.
Along the same lines, after retiring our At Pacs we also filled the plastic hardpacks on the conventional BC's of the era with shot as well and carried about 10 lbs of shot as non ditchable "integrated" weight long before the term became popular.
A third safety concern with the At Pac is that the seams on the bladder in fact had a tendency to separate once the bladder got old. One of mine died this way.
Another potential problem is that some At Pacs (I'm not sure if they were early or late production) used a plastic inflator/OPV fitting that had a tendency to crack and leak where the quick disconnect attached. This led to the early retirement of many At Pacs. My other one died this way and the remaining functional At Pac I own is a combination of the bladder off one and inflator parts off another.
The streamlined fairing that went over tank and wing is pretty cool and is very similar to what is now seen on some rebreathers.
