Let's suppose you have SoaB 3 (but not Necroarmor - it'd probably be really rare to get two specific uniques in the same game)
2d2 = 3 average, so 6*3, 18 damage average, or 36 dps average
Now a regular vanilla chaingun with SoaB 3 - 1d6 = 3.5 average, so 6.5 * 5, or 32.5 dps average.
So compared to a chaingun, your weapon is marginally better (and yes the armor piercing adds a bit more too), but drains a rather large chunk of health. Draining 1-4 health is actually pretty large, and the possibility of gaining 1 back doesn't offset the cost by much.
The way you've set things up I wouldn't even use it if I found it, a health penalty is a huge drawback, it needs a powerful benefit to make it worthwhile. Furthermore, uniques are supposed to be superior to ordinary weapons, this one clearly is not.
And really, comparing it to a plasma rifle is the final straw.
1d8 = 4.5 average, so 7.5 * 8, or 60 average dps!
Even the new nerfed plasma at *6 is still 45 average dps, better than your design and with no health loss to boot.