The two major makers of sub-gauges tubes (Kolar and Briley) are experts in tweaking the interia triggers fitted to 12 ga. guns. They've each done many thousand's on 'em, and they'll work reliably all the way down to standard .410 loads. In some cases they can convert to true mechanical triggers at the owners request, but IMO there's really no need and it's much more expensive.

Mark

A sailor's creed - "If you can't tie good knots, tie lots of 'em."