5D damage? Same as a blaster rifle? The RPG gave damage for a firearm/slugthrower rifle as 3D+1, in multiple sources [core rules, gamemaster screen, etc.]. Any reason to believe this kind of rifle is somehow much more advanced than that? [Where would the Sand People get advanced rifles or ammo?]

"Despite its crude design, the rifle could hit targets reliably at long ranges, but the fire rate was typically slow"—this sounds reasonable, but the provided stats don't actually *mention* the Fire Rate. I suggest a Fire Rate of 1.

Completely agree with the fire rate, and I've added it.
As for the damage, the RPG is really inconsistent about Slugthrowers, while the main rulebook states them as being 3D+1, if you look through the technology books, you can find ones which do 6D.
I just tend to consider standard deadly damage to be 5D, lighter pistols 4D, tiny concealable weapons to be 3D, and large weapons to be in the 6D range, and don't like making Slugthrowers weaker than that, as Armour against them is generally 1D higher anyway, so they're already less effective.

So just a personal choice, which isn't outside the range that West End Games set for the game.

The d6 mechanics and stats for projectile weapons really ensure that PCs don't want to use them, and, unless the GM is representing a primitive culture, the GM won't either. As mentioned, any slugthrower is essentially 2D worse than a comparable energy weapon against a target in armor (typically they do 1D less than an analogous blaster, and the aforementioned armor bonus against physical attacks).

While West End games managed to generally avoid power creep with their sourcebooks, almost anything which came later was demonstratively worse than than the original equipment. (i.e. no Rebel/Republic starfighter from a later technological period was ever overall better than the iconic X-wing). Similar thing with weapons - few weapons introduced later were overall better than those included in the original rulebooks.

My memory was that there was some power creep, but now I think about it, it might just be one book, Gundarks Personal Gear
As it's got the
Enforcer Slugthrower Pistol (page 19): 6D+1 damage
but then again it's also got the
Thunderer Heavy Blaster Pistol (page 28): 6D+2 damage

I'd need to check through other books, but I do feel as if there were a few gems here and there that greedy players loved to get their hands on.