It actually isn’t. Depending on the individual circumstances, attacker and defender might face different base Difficulties for their respective tests.
An attacker who fires into a smoke filled area, might have D3 to hit with an attack, while a defender using a MSV2 has only D1 on the defense test.
As I understand, you want firearms to be more dangerous than bows, automatic weapons more dangerous than single-shot guns.
That is already modelled by the Burst stat of every weapon
If you shoot a bow, it has Burst 1 and rather low damage code.
If you shoot a Combi-Rifle, it has Burst of 2 and a higher damage code.
If you shoot a HMG, it has Burst 3 and an even higher damage code.
The Burst rating is, how to make dodging much more difficult. Firing several loads means rolling more d20s in your pool, generating more Momentum.
In a Face-to-Face test the generated Momentum of attack and defense is compared. Being able to generate lots of Momentum on an attack helps succeeding to hit your opponent, by making it harder to dodge your attack.
Having an Expert quality does add to the Momentum, making dodging your attack even harder.
So there is no need for setting different base difficulties for different classes of weapons to dodge their attacks in a Face-to-Face test. That is regulated by Burst rating and by Bonus Momentum, from Expert quality and other sources (like Surprise).
If the attacker does not use the Burst rating, so only firing single shots, then it does not matter if it is a bow, a handgun or a full auto-capable gun. That does not make it harder to dodge.
One could, and rightfully so, argue, that dodging ranged weapons is from a realistic point of view not possible at all. Even considering a bow, not a weak LARP bow or for sports archery, but a war bow, you will not dodge an arrow fired at close to medium distance. You wouldn’t be able to dodge any kind of firearm at all.
So you could make a case to drop the Dodge Reaction altogether from Infinity. That would make it VERY deadly, but a bit more realistic.
Though, Infinity uses Vigor stress for damage - how realistic is that? Not at all, of course.
Infinity uses external hit locations, but does not consider which internal organs were hit - how realistic is that? Not at all.
Infinity allows for dodging attacks by ranged weapons where under real world considerations no dodge would be physically possible.
Infinity is a very cinematic action-movie like setting. That means, realistic weapon usage and - even more important - an equally realistic wound ballistic model is not at all appropriate.
You get shot at by a squad of the Celestial Guard. All five are firing like hell at you. And you get not only a chance to dodge that barrage, as a player character you have a good chance to dogde that hail of projectiles and not even get a scratch. Because it is an action-movie.
This is, as I understand it, the intention of the Infinity rules. Action-movie shoot-outs, high-octane action scenes, big damn heroes pulling off stunts that are not at all possible using real world physics and physiology.
Changing this action-movie cinematic premise by introducing “realism” would make Infinity a very different game - and a very deadly one at that.
If you and your players want it that way, you can easily change the rules to your liking.
I do like the cinematic approach Infinity takes for action hero scenes.