Dandelion Slap (Gaia's Wasp Part 2) mentions that the Wyrm personal weapon fires a 1.5-gram jet of silver at a velocity of 2.5 kilometers per second. Is that wacky?

Do we even know how to fire a 2.5 KPS silver jet? As it happens, we do it routinely, when anti-tank projectiles reach the end of their flight. A HEAP (High Explosive Armor-Piercing) warhead uses a shaped-charge explosive to squirt a jet of metal at speeds up to 13 KPS. (In reality, different parts of the jet move different speeds, which we will ignore.)

Conveniently the "shape" in a shaped charge is just an open cone lined with a ductile metal such as copper or silver. Rather like a jacketed hollowpoint bullet with the lead replaced by, say, TNT.

Very well, but how would it feel to fire?

Survivable, maybe, because shaped charges are directional. Perhaps the best-known examples are door-breach charges, which slice through a steel plate without slicing through the SWAT team standing three feet away holding their ears.

What about recoil?

From our discussion of proton rain, recall that a smaller, faster bullet delivers more kinetic energy with lower recoil, because recoil (I.e., momentum) grows only linearly with velocity while kinetic energy grows as velocity squared. So a 2.5 KPS silver jet could, in principal, deliver more kinetic energy per unit of recoil than an ordinary pistol bullet traveling at, say, 0.25 KPS.

Specifically, a typical .45 ACP slug fired from a Colt 1911 travels at 250 meters per second and weighs 15 grams. So its muzzle energy is E=MV2   = 1.510‑2Kg  (2.5102M/S)2   = 9.4102KgM2/S2   = 940 Joules, while it's recoil (I.e., the momentum imparted to the shooter's hand) is MV  = 1.510‑2Kg  2.5102M/S  = 3.75KgM/S.

For comparison, the alien's sidearm fires 1.5 grams of silver at a velocity of 2,500 meters per second, so it's muzzle energy is E=MV2   = 1.510‑3Kg  (2.5103M/S)2   = 9.4103KgM2/S2   = 9,400 Joules, while its recoil (I.e., the momentum imparted to the shooter's hand) is MV  = 1.510‑3Kg  2.5103M/S  = 3.75KgM/S.

In a startling coincidence, the Wyrm's shaped-charge sidearm produces exactly the same recoil as a Colt 1911, while delivering 9400/940 = 10 times the muzzle energy.