Both pistol and rifle.
“Issued” meaning anything that wasn’t a one-off or made specifically for a gimmick. Putting “Longest Suppressors” in Google gives up a bunch of results of people specifically making suppressors as long as possible as a joke which I’m not interested in. I’m curious what exactly was the longest suppressor ever taken “seriously”.
Longest in terms of total length? Or longest in terms of length compared to the barrel, or something else entirely? And that’s leaving out weapons with integral silencers that are fundamentally the entire barrel such as the De Lisle carbine.
I mean, there are big suppressors that fit pistols, which will almost certainly win in terms of the second category. And of course, suppressors work better with lower powered, ideally subsonic rounds. And length is not the same thing as efficacy.
Sticking with total length, I’d bet it would be for one of the .50 BMG platforms, so I’d nominate
https://thunderbeastarms.com/products/ultra50
As a starting point at least, 22.5 inches long, and I picked it as being supplied by a commercial silencer site, rather than a one-off in keeping with the theme of the thread. There’s also quite a few in the 15 inch range from a multitude of providers as commercial products. But I repeat, length isn’t the only factor to consider outside the narrow range of bragging rights.
When I made this topic I was imagining extra length on top of the barrel, so integral wouldn’t count. I was thinking of what the longest most unwieldy barrel and suppressor combo that was still issued.
I read an article long ago about a shotgun used to kill problem birds at a city airport. To make it quiet the barrel was very long (around 4 feet). Can’t find anything online about it today.
Quick if slightly hijacky, question.
Can individuals own “silencers”? Is it legal? Can you buy them in gun stores? Can you open carry with one?
I can’t keep up anymore. 
In order. Yes, Yes as long as you live in a permissive area/state and pay the (quite reasonable fees). Yes, depending on the store and the previous mentioned fees, though due to cost and comparatively low demand, they’re pretty uncommon unless it’s a specialty place. Probably, as long as you’re in an area with legal open carry and a cop doesn’t choose to harass you (even where legal, open carry can make cops concerned about your intent, which I DON’T object too).
Should give you an idea. Not a lot of work, some records, and a good bit of delay, so not an impulse thing. But not substantially different from the documentation, work, and fees to get a CCW for example.
I don’t think this qualifies but maybe. Q silencers come in sections that can be added to. Silencers
THis video shows the ridiculousness of the possibilities www.youtube.com/watch?v=5ehmdV80IiE
You can buy silencers for shotguns these days. Here’s one place to do so.
My recollection is that the long barrel was a way of avoiding the paperwork. Might have even been in a state where they are not allowed.