I did see this on the wiki, but it mentions 1 HP/turn. Still doesn't quite balance the bit of basic math I worked out above (which, I realise didn't even account for the P-mod).

I think you're over estimating the double shotty's damage.

As I understand things (and I'm willing to be corrected if I got anything dreadfully wrong here), the gross average for each shell of a P-mod DS is 20 - 2 * distance. So corner shooting at a range of 2 means that the first shell is expected to do 16, knocking the Shambler back 2, so the second shell only does 12... for 28 average.

Then the Shambler gets to apply it's 3 AC... double for shrapnel, and double again because there are two shells, so subtract 12... so we're back to 16.

Then the Shambler has to walk the three/four squares back into position, so we'll just estimate that you'll lose around 3.5 of that progress to regeneration... giving 12.5 damage of progress on average. Which means you should expect 8 shots to kill a Shambler in this optimal position... except that it also it can be expected to teleport more than once every other shot (2 walks of 3.5, and it teleports about 1 in 6) and regenerate even more, so you should really be prepared to take a few more shots.

I also notice that you were only firing "relatively close", which suggests that the Shambler might have been 4 or even 5 away on some of those shots, and that 10% damage reduction will strip things further very fast... you might have only been doing half as well as the repeated corner shooting situation presented, if that, on many shots.