Quoted:
The exposed lead base on a 9mm is larger, the suppressors normally operate cooler, and the pressures are lower creating more optimal characteristics for leading.
5.56mm suppressors shouldn't foul as badly or nearly as quickly, simply because to some extent temperature, pressure, and particles are going to blast some fouling away.
I think what ends up happening is that copper fouling is more likely to build up in 5.56 suppressors because it has a higher melting point and isn't as soft as lead. Obviously copper fouling is greatest when the rate of fire is most excessive and barrel temperature is elevated.
There are a lot of people with suppressors that have run 6000+ rounds and are working well. One Gemtech customer had around 100,000 rounds on a M4-96D at which point some significant buildup had occurred. It's a horse apiece at that point- you either build a can that comes apart and has potential for mechanical issues that can make it in-operable, or you weld one and fouling eventually claims it. Suppressors don't last forever but they do last a long time.
I would be careful with blanket statements on rifle suppressor fouling.
In our experience and from talking to other industry folks, a properly manufactured, quality suppressor, will most likely meet its end due to fouling than actual baffle wear or erosion. But of course there is variation based on the particular suppressor design.
Best Regards!
Tuukka Jokinen
Ase Utra sound suppressors