Even better than the 40 mm grenade is 57 mm ammo... with more than 10 km range and huge destructive capacity, especially against soft targets. This is almost a 3 kg shell, so no small thing at all.
Quite true, and a 57mm gun armed vehicle will likely be part of the air defence of most static and mobile targets needing defence from armed drones, but in terms of weight and size and cost, a light simple aircraft with a grenade launcher would be cheaper and simpler, and much less bulky.
They have talked about a tethered UAV for Armata that hovers above 10+m above the vehicle with thermal imagers and radar to detect targets and threats... those sort of sensors plus a laser range finder would be plenty to detect small UAVs and UCAVs... the interception UAV should have the mobility to fly out to where the swarm is coming from and to fly around firing at the UAVs from close range... preferably from behind so they don't see it coming... if it can match the speed of the incoming drone attack it can shoot from behind and above and just pick them off.
In fact you could have an area... say 10km away from your airbase where these UAVs operate... inside that range you can use other methods of dealing with UAVs... but having a 40mm grenade launcher and an EMP weapon perhaps on your UAV... because it is going to be flying close to the UAVs anyway... it makes sense to use both.
In terms of weight you should be able to manage 30-50 rounds of 40mm grenades, but the EMP weapon should be good for a few more kills from very close range... which should make it more effective against anything with any electronics in it.
Low power EW may be limited against autonomous, military grade hardened UAVs, but pure and simple destructive means like 57 mm ammo will do wonders against them and be a quite effective option IMO against UAV swarm
Actually Russia and Bulgaria developed a jamming 122mm shell that was designed to be fired into enemy territory near the front line and it was supposed to jam communications in the area for a period of a few hours... a scaled up model for a 152mm shell that sent out an EMP pulse to disable nearby UAVs could be another option.
It all depends on detection performance... a 152mm shell could be delivered to 70km, so potentially in Syria you could be delivering jammers to the launch sites... though you might want to design the jamming round so that it destroys itself during use so it cannot be captured... the volume difference between a 122mm jammer round and a 152mm round would mean careful design and material choice... lots of magnesium and big lithium batteries for instance, and even a charge or amount of naturally combustible materials that will destroy completely the round after it has blown all the nearby electronics would be an improvement.
So it changes role from 4-6 hours jamming to one near instant jamming pulse and then self destruction.
Then at 10-20km range you have UCAVs shooting and jamming enemy UCAVs, and then within 10km you have 57mm guns and of course 30mm cannon with air burst ammo too.... and perhaps even cheap command guided or beam riding missiles like Shturm or Kornet...