Meson guns were an attempt at hard science, based on the misunderstanding that unstable particles' half-life meant that their decay time could be tuned be relativistic time dilation. Actual unstable particles decay exponentially, and all the relativistic time dilation changes is the distance to the half-decayed point, not the exponential.
There are some real things that mimic some of the characteristics of a meson gun. Medical proton accelerators have the interesting property of delivering relatively little energy to surface layers, gradually increasing with depth, and then quickly dropping off. That's very similar to what a meson gun is supposed to do.
One obvious difficulty with a proton weapon is that ordinary electromagnetic shielding would work. Another is that that explanation loses the distinction between a particle accelerator and a "meson" gun.
Another problem is that because planetary shielding would count toward the depth where energy delivered gradually increased, deep proton sites wouldn't work.
Getting farther away from known physics, if there are long-lived unstable particles that can be catalyzed to decay (similar to how excited atoms can be stimulated to release their excitation energy in a laser, or how neutrons trigger fission of some atomic nuclei), maybe one pulse of relativistic particles, followed a moment later by a second, faster pulse, could produce a timed interaction with characteristics similar to the Traveller meson gun. The stimulated decay aspect may violate real physics, but not as conspicuously as mesons with fixed decay times.
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Psionic Assault is not nearly as effective as bullets, or even a hefty rock, but it's subtle, and bypasses conventional armor. And sometimes you want someone unconscious, not dead or brain damaged.
And there's no technological substitute for Probe, Teleportation, or a few other powers -- at least not in the tech levels that are well-defined in Traveller. They're fantasy; the only concession to hard science fiction is that they have clearly defined limits.
