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Most Imperial characters can get Melta Bombs for five points but Demolitions Doctrine for IG vets gives ALL 10 vets MB's and even a Demo Charge.
Costs 30 points - makes them pretty redundant at range if they want to charge but...
MB's can target MC's as well as vehicles - some (possibly incorrect) maths says:
Knight charged by vets - 3+ to hit (2 hit) 2+ to wound (about 2 wound) no saves => 2 vets die (ST(D) wounds don't carry over)
Vets make 8 attacks 4+ to hit (4 hit) 2+2+ to glance (armourbane) => 1 glance 3 pen 1 explodes! (AP1 +2 to VDT) - Explodes! causes 1 +D3 hull points loss avg 3 (no ion saves in CC) meaning...
Knight goes from 6 => 0 HP - Destroyed!

The fact is:
Are your guards gonna make it base to base with an opponent's vehicle ?
They are vulnerable, few and don't have assault vehicle.
But I'm very interested in this thread !
In IA2, 2nd Ed you can have veterans with demolition doctrine and tank hunter, that's funny ^^

...Good to know?

Anyway, you're not taking into account how difficult it is to get in charge range. What Assault Vehicle do Guard have that will get them in 9" reliable charge range alive.

As others have noted, unless your opponent is fielding nothing but Gallants, you're T3 5+ models are going to get shot but battlecannon/thermal lance/gatling cannon fire and get shredded.

One way to achieve what you want however is to tie the knight up with another squad to tarpit it, then disembark your dudes ad charge next turn. But then you're looking at 2 squads, a transport and a bunch of support elements and it's not quite the deal you think it is.

Same deal for Tau and EMP grenades. If a knight player sees Fire Warriors, guardsmen or vanguard moving towards them, the first thing they should be asking is if the squad has grenades which can hurt the knight, then backing up 12" and blasting them.

The ideal way to face knights with that tactic is let them come to you while hiding in cover with the camo cloaks.

It's that or still camo cloak and flit between covers and use distractions and make it look like the demo squad is after objectives.

But can you still get cammo cloaks with demolitions?

Anywayz, they're a bit too vulnerable. They CAN work in specific situations. And they can probably explode an imperial knight here and there if they get around average rolling and don't fail fear test. But they basically can't get delivered into combat by codex means.