West Midlands Police executed a warrant at the Birmingham property after receiving intelligence that Wesley Carter, 39, and Keelan Simpson, 24, had access to firearms.
The officers contained the block of flats in Monmouth Road, Bartley Green, and made their way up to maisonette, where they were about to force entry.
Watch the raid here:
But the pair opened the door by chance as they were about to head outside, and they were arrested and the flat was searched.
Police found about half a kilogram of cocaine, 50 cannabis plants, three revolvers and a black handgun in rucksack, alongside nearly 80 rounds of ammunition.
Officers also uncovered lead shot, casings, ammunition-making equipment and an instruction leaflet on how to make your own bullets.
But there was no evidence the pair had actually manufactured their own rounds.
The cocaine − valued at around £19,000 − was found in a black leather bag and a branded red carrier bag.
Detectives trawled CCTV in the area and found footage showing Carter, from Barn Piece, Quinton, carrying the bags in public.
He went on to admit possession of illegal guns and ammunition, Class A drugs and cannabis cultivation, while Simpson, from Stonehouse Lane, Quinton, admitted the firearms offences and cannabis cultivation.
At Birmingham Crown Court yesterday they were jailed for nine years and six months, and seven years and six months respectively.
Detective sergeant Dan Tang, from the West Midlands Regional Organised Crime Unit, said: "They certainly got a shock when they opened the flat door. Maybe they’d heard footsteps and had expected to find a customer - they couldn’t have been more wrong.
"The revolvers were examined by ballistics experts and identified as being manufactured in the late 1800s. However, they were all fully functioning, lethal weapons.
"The 9mm handgun was capable of firing in both semi-automatic and fully automatic modes and had three unfired bulleted cartridges in the magazine.
"Some deadly weapons and we’ve potentially saved lives by taking them off the streets."
All of the guns were test-fired by ballistics experts and the examined. None of them was linked to any recorded shootings in the West Midlands.