It looks like you can create light mesons like pions (I'm assuming they're pions) with x-rays (produced by electrons fired at a platinum target) hitting a carbon meson source for 335 MeV (2.146917582e-11 Joules) per.
A typical gunshot has a muzzle energy of 500 Joules to 1,000 Joules. Assuming the process above scales linearly and can be miniaturized, you'd get a beam of 50 trillion to 100 trillion mesons for the same amount 500 -to- 1,000 Joules.
Pions can be charged for the sake of accelerating them towards a target, and have a lifetime of $1 \times 10^{-8}$ seconds. At such time, they decay into a charged muon and an uncharged muon neutrino. Even at the speed of light, this would only have a range of about one meter (3 feet).
But what you are probably looking for is kaons, which I can't find a source for, but have a mass about 4 times that of a pion (so maybe you get 250 trillion for a rifle shot). Kaons decay into neutral pions (which are probably the armor penetrating particle you are looking for) in $1 \times 10^{-8}$ seconds. Kaons are charged, so you could accelerate them up. But, like pions, even at the speed of light, these only have a range of about 1 meter (3 feet) until the kaons decay, plus about another meter (3 feet; 6 feet total) as pions. These neutral pions decay into photons, so all of this energy is absorbed by the target as heat.
Per this source (https://melindajolleyacademia.weebly.com/uploads/2/1/3/3/21333742/muon_lifetime_and_time_dilation_effect.pptx) time dilation could arbitrarily extend the lifetime (and range) of these particles as you near the speed of light.
To get a typical rifle range of around 200 meters, then, you’ll need to accelerate this mass to 0.995 c (0.99 c if you're using kaons). Per here (https://www.vcalc.com/equation/?uuid=85b315c3-baf3-11e3-9cd9-bc764e2038f2) this will cost you about 200 times the rest energy of the beam you created (200 times 1,000 joules or 200,000 joules)