Most people who start looking at suppressors, come across the term First Round Pop (also known as FRP) pretty quickly.
At a very basic level, the term First Round Pop is referring to the first round that's fired through a suppressor after it's been out of use. Since the suppressor is initially filled with oxygen, there will be an additional 'pop' as the oxygen burns out - which makes the first shot louder than subsequent shots.
Depending on the suppressor, firearm, ammunition, and environment; the FRP can range from almost non-existent to very noticeable. Unfortunately, because of all the variables involved, it's extremely hard to get consistent FRP measurements.
Keep in mind that, although FRP can be noticeable, the suppressed shot will be just slightly louder than subsequent rounds - so it's not something to be overly concerned about when it comes to either overall enjoyment or hearing safety.
Having said that, there are several factors that tend to impact FRP fairly consistently:
Although FRP is definitely present on higher pressure calibers, it's not as noticeable because the rounds are so much louder.
As a result, you'll hear a lot of talk about it when referring to rimfire or pistol suppressors - but nobody really cares on a suppressed .338 Lapua. :)
Some suppressors are designed with a core that comes out as a single piece: like the SilencerCo Sparrow SS. Those single-piece designs are typically referred to as Monocore Suppressors.
As a general rule, a monocore suppressor will have a louder FRP than a suppressor that uses individually stacked baffles. Some examples of individual baffle designs include the Tactial Solutions Axiom or the Griffin Revolution 9.
It's very common to use a larger bore suppressor for use with multiple calibers.
For example: a .45 pistol suppressor works great for suppressing a 9mm or .40 handgun; or, a 7.62mm suppressor will come very close to the performance of a 5.56mm suppressor.
Unfortunately, there are always trade-offs - and one of them is you'll generally have a noticeably louder first round pop when shooting sub-calibers through a larger bore suppressor.
Time Since the Last Shot
After hearing about FRP, one of the first questions we get asked is how long a suppressor can sit unused before FRP will occur again?
This is hard to answer because it's extremely dependent on the environment. For example, on a windy day at the range you'll get that First Round Pop more often than you would shooting at an indoor range.
Working Around FRP
In most cases, First Round Pop probably isn't enough of an issue to worry about; since, aside from just sounding louder, it doesn't cause any issues.
Having said that, if it's bugging you and you want to reduce it as much as possible, try adding just a teaspoon of water to the rear of your rimfire or pistol suppressor before shooting.
I would NOT, however, recommend adding water to a rifle suppressor in most cases.
At the end of the day, First Round Pop can definitely be noticeable on low-pressure calibers (like pistol & rimfire); but, it's not that big of a deal for most users.
Personally, I'd suggest using FRP as a tie-breaker if you have 2 suppressors you're interested in; but, I probably wouldn't consider it as a primary requirement in most cases...