It doesn't take long, particularly with subsonic ammunition, for the operator to notice that the first round fired is considerably louder than subsequent rounds. It is only 2-3 decibels on a good sound meter, but since decibels are logarithmic rather than linear, to the human ear, this is a lot. If there is a "cooling period" of more than a few minutes, FRP rears its ugly head again. Operationally, this can be a very undesirable thing, so here are two methods of combating this phenomenon.
1. If your suppresser is of the "Quick Detach" type, perhaps the easiest and most effective method is removing the can and spitting in the rear (blast chamber) end.
2. A more technological solution is canned Nitrogen gas. Available at any computer store outlet, this is the stuff with which you dust your monitor screen. The method is to chamber a cartridge to seal the rear of the barrel, and BEING CAREFUL WHERE YOU POINT THE LOADED WEAPON fill the bore and suppresser with the Nitrogen gas. A squirt of several seconds does the job. Put a piece of tape over the muzzle to hold the gas in. This method is good for several hours, until the Nitrogen slowly leaks out the back around the unexpanded cartridge case.
3. As well as the phenomenon of first round pop, the first round out of a cold barrel is often slower than succeeding shots due to energy - that would otherwise be used to propel the projectile, being soaked up and used to heat the steel barrel. This is exacerbated by a long barrel, as discussed in the article "Subsonics Do Not Like Long Barrels".