It seems I misread your post. I m well aware how two types of crit work. I think it should be additive to the final as in the case of base form. I think it would be more of how much crit would be appropriate 100% or 150% I don't know but as a flavor to how ability works it should be additive. To restrict it to snipers would be cool by me, I added other primaries to the discussion
mostly since that is how DE tends do things. I think with primaries the real problem would be weapons with large AoE and would be fine having them left out. Possibly snipers only, but maybe assault rifles and shotguns could be fine too. I know 250% crit Kuva Ogris would be absolutely nuts.
I quess combo counter synergy could be left out, but personally I think that 3x/3.5x increase is rather trivial to get on many snipe rifles that have minimum combo of 1, in particular Rubico(Prime), Perigale and Sporothrix that you already mentioned. It would substantially help out Komorex and Vectis Prime with cumbersome 5 minimum combo. In my opinion Primed Chamber is extremely Powerful mod since it brings one extra layer of multiplicative damage on top of combo and everything else. Also I would not compare Primed Chamber on warframe ability or augment.
We'll see. Anyway brainstorming wont hurt and I would like this augment since it would give Kullervo more of a reason for little gun play, not that it would outperform his melee dps anyway. Besides I'm not expecting him to get augment anytime soon, but when he does I think something like this would be an interesting candidate.