The Slivergun has been wrong since 2nd edition. At least since 3rd edition.
In 1st Edition it was a LIGHT Pistol, NOT a Heavy Pistol.
Also, this weapon is based on Molly Millions Needler from William Gibsons Neuromancer.
Well, lots of things changed from 1st-2nd (or from 2nd-3rd, or 3rd-4th). That doesn't mean it's been wrong ever since, just that it's changed.
And even in 1st it was pretty messed up. They told us it was a Light pistol in its fluff, but it was listed as a Heavy on the actual weapons chart, and it specifically stated it used the Heavy ranges. It was just kind of...weird.
Maybe it's just a heavy pistol with a compact frame, for greater concealability...
In 1st, each gun had an individual concealability rating. There were tendencies for Lights to be smaller than Heavies, and that sort of thing...but it wasn't a blanket rule. The Viper was just weird. They called it a Light, but it was a Light that had the range and damage of a Heavy, and was listed as a Heavy on the chart of guns, and a Light that had the same concealability as a Browning Heavy.
It was just...yeah. Weird. SR1 was basically just the guy saying "It's a Light Pistol, guys! Honest!" So I didn't really
mind when in later editions they just went ahead and called it a Heavy Pistol.