Tunnel Escape: How to Obtain SMG and Magnum

This guide assumes you already beat the 5th floor and unlocked Endless mode.
The dev hinted at the 2 weapons with its achievement:

The Submachine Gun requires you to see the commissioner’s event of where he is attacked by a triangular head boss monster and lost his wig.

This is a random boss, The Manager, and he can appear randomly in Endless mode every 20F. Some people got lucky and ran into him on 20F… I ran into him on 200F… After first kill, you will get the Wig…

Be careful, he can spawn 4 Nurses during the fight! They are super fast, so get your speed boost in place and AoE when they come out.

I notice the game did not auto-save after the fight, so it might be a good idea to use your white flag to get out and save immediately.

Beatrice will automatically turn in the Wig the next time you see the commissioner (after next boss kill).

The SMG uses all your Assualt Rifle skills, including passive skills.
Additionally, it adds a new skill (SMG only) called Rush B (99T CD):
Beatrice would immediately throws a Flashbang. For the next 3 turns, she gets 2x AGI & no ammo consumption for Shell Rain (it still retains the 5T CD). But you can now Rush B -> Shell Rain -> So Hot -> Shell Rain

For the Magnum, it is in a random side room in the Power Supply Zone (first area). You need to beat a ???? monster and an old friend will drop it for you
This is completely RNG, so you may need to go thru Power Supply Zone a few times. Just be sure to check every room. And if you are in a place called Switch Room and see the above monster, then you hit the Jackpot.

Before you jump for a joy, there is a little bit of nuisance for these 2 weapons. They require you to craft ammo instead of picking up dropped ammo.
At least with SMG, you can get by with the Rush B combo if you have sufficient Flashbang.