Automotive Brake Kleen spray with the long, red nozzle attachment gets the inside clean enough using needle-nose/regular pliars to hold it. Keep it at an outreach & use short thrusts of spraycuz' it comes outhard & fast... thusoverspray may get on you/clothes. Q-tips & pipe cleanersdipped in solventwork the 209 primer hole & a sewing needle will keep the tiny hole at the breechplug face clear.

For the outside, I usefull strength Simple Green spray with a toothbrush -- thenpick upthe plug with needle-noze pliars again & hokl the plug under steamy-hot faucet water for a minute rinse.

I always do the outside first because steamy, hot water will loosen the fouling inside the plug & the Brake Kleen spray afterwardsself-dries itself faster than water.