To be honest I think the opposite is the case. Taking out knights doesn’t require low-strength multi-shot guns. It requires very high strength guns. Remember that knights aren’t infantry to be gunned down in a hail of fire – they are large and heavily-armoured war machines protected by ion shields.
This is because ion shields are actually very good against low-strength guns. And you still need a 9+ to actually hurt knights. Bear in mind that knights have no side or rear arcs and you never get bonuses to hurt them, which is what lets you use these guns effectively on titans.
Consider a banner firing gatling guns at an equal banner of 3. 24 shots. 12 hit with BS4+. 6 4+ saves are passed, so 6 go through. You need to roll a 6 with your strength of 3 to do damage, so that works out as a single point of structural damage on a knight. With battlecannons you have 6 shots, 3 hits, 1.5 failed saves and 0.66 damage.
Even on a Questoris knight you need a minimum of S7 to have any chance of a devastating hit. That’s also the point at which the ion shield save drops to a 5+.
A far better way to get rid of them is with things like plasma blastguns, volcano cannons (either kind) or melta cannons on reavers.
One of the reasons I don’t like equipping titans like reavers and warhounds with double Vulcan/gatling is that I think you’d find the load-out virtually useless against a knight army. If you had a pair of warhounds it’d be much better to give each of them a plasma gun and a bolter, rather than having one double-plasma and one double-bolter titan. The knight player can focus on the plasma titan and ignore the bolter one.
These guns are great for knocking down shields and for aiming into damaged locations to finish off a damaged titan. They aren’t for killing knights.