How would mallard ducks be able to handle guns from say around World War 2 tech level? I assume with clawed fingers on their wing that it would be likely easier for them to handle a rifle, but not being able to lock their shoulders, basically requiring to maybe use bipods. How would they handle against say humans with the same guns? Would they be at a disadvantage? How would these ducks become less disadvantaged as time goes on? And would ducks being able to control the recoil of their guns like an human still have some sort of disadvantage?
Bipods would definitely be necessary, and even then, there are a few disadvantages your mallards would have in wielding guns.
I'm guessing your mallards' fingers would be similar to a hoatzin chick's. While fine for scrambling around branches, these fingers would not be as conveniently jointed for curling around a gun, though pulling a trigger should be doable if it doesn't require too much strength (someone familiar with various guns would be better equipped to tell you what WWII trigger pull was like). The hoatzin-style finger would also only have metacarpals to oppose it, as opposed to fingers, which would further limit grasping capacity and strength. A bird with fingers is still going to be better at holding things with their feet and beak, although mallard feet and beaks would require modification to do that as effectively as, say, a parrot.
Guns are heavy, and birds are not. While birds have reinforced bones and very powerful muscles to move their wings, able to bear their whole weight (and inflict bludgeoning damage if one whacks you in the face), birds are still fairly lightweight creatures with hollow bones. If a small human has difficulty holding a gun up or dealing with recoil, a bird will definitely have problems. So bipods would be very helpful. Tripods would be even better. But your ducks would experience whatever mobility limitations come with a bipod. Stronger bones might help your ducks, but at the cost of flight. Though if you go with domestic ducks, flying will already not have been very effective because of body fat.
Carrying guns and bipods to set up would also be difficult. Humans tend to carry heavy equipment on their backs. Birds, especially females, tend to be opposed to having anything touch their backs, because that's where their ovaries and testes are located, and that's where the male bird mounts the female bird for sex. Carrying heavy items in front would likely throw off walking balance, unless you had a bird with a remarkably fat butt (see later point about domestic ducks). If your ducks had feet adapted for grasping, carrying [small] firearms in flight might be doable, but at the cost of effective paddling in water.
A gun with a long butt will also not work well. Those powerful wing muscles are attached to a very deep sternum, which means your ducks' wrists will not be much farther forward than the outside edge of their chests. Domestic ducks will have an advantage over wild ducks, because they are remarkably fat, and most of that fat is carried toward the rear, which forces them into a more upright stance, so there will be less body mass in the way of their forward reach.
In short: Your mallards could pull a trigger, but the guns would have to have short butts, and the ducks would have difficulty carrying or grasping guns unless their anatomy was modified further. The bigger and heavier the gun, the worse the problem. Domestic ducks would probably adapt better to gun-wielding than wild ones. If the ducks could learn to build their own guns to fit their anatomy, they'd do even better.