I'm not exactly sure how an adamantine firearm would handle. On the one hand, it's got light weight and nice rigidity, you'd think it to be durable, and it would be for traditional fire. But once you get into large bore and higher calibers, then you hit adamantine's shatter point and you deal with recoil. Any force imparted on an adamantine object goes directly into anything it's touching. It has no sway, so a crowbar would not flex at all and instead transfer all the force to the pivot. Similar,y a space elevator demands that the stalk be able to flex slightly in orbit and accommodate small tidal shifts, while adamantine would simply gouge out whatever it's anchored in and float away, or else crash down, depending.
An adamantine firearm would transfer 100% recoil into whatever is catching the recoil. In pistols that's your arm, flexing slightly as you fire to adjust for the force. Adamantine's hilariously low weight and perfect rigidity means that any small arms fire would have ridiculous knockback. It would prove good for a revolver, as it can field large rounds while being thin and lightweight, but for semi automatic or full auto, you'd have so much recoil as to make consecutive shots impossible, and any hefty rounds would risk damaging the user. One reason a desert eagle can fire large rounds is that it's a heavy gun and helps to absorb and dissipate the force more fluidly. An adamantine desert eagle would fly back so hard it'd probably break your wrist.
For a mounted cannon, it'd fair well if it had springs and a heavy casing. An adamantine barrel wrapped in lead would have the weight to distribute recoil and would have the strength to endure heavy shells without warping. The largest artillery ever fired would actually fire progressively larger shells, as the barrel would enlarge under the force and warp. Adamantine would have no such issues. The issue is that recoil would be a bitch.
For that matter, an adamantine jackhammer may perform exceedingly well, and a large adamantine rod being pneumatically driver would ensure a 100% force delivery to the point of impact. It would make a great wedge for splitting lumber, or for cracking open bunkers by ramming tanks against a wedge.