I don't believe bolt-actions count as a weapon for this achievement by design. I've had two batman worthy sniping streaks one with the Railjack and one with the LA80, but neither netted the infantry only achievemnts. It's considerably easier to get such a long killstreak with a sniper rifle than it is with other weapon types so I can understand why this is the case. Of course, it is a little misleading that the achievement doesn't exclude snipers in the description, but that's a small detail.