Guns
The game shows you how to reload your gun. You drag the ammo to the gun’s magazine. Sometimes you need to prime the gun too. Sometimes your gun unloads itself, which is probably a bug. Small guns and melee weapons use the same inventory slot.
Don’t use small guns and strength together. You shoot where you aim. Headshots and weapon ranges matter. Shotguns do a lot of damage but only at close range. You can charge melee attacks, but it’s not better than just clicking fast. Most enemies die in two fast hits.
Faith
Magic skills are weak at the start. You won’t find any psi powers on the first floor. Expect to use guns or hit things. If you have the faithful trait, you can refill sanity at crosses, but only once per cross. You’ll find powers from level 2, but still use another damage skill.
Your powers are strong but cost a lot. Only using psi for damage isn’t a good idea. Smoking is a fair trade for sanity and health. Psi skills have a max range. If enemies are alerted, you can attack.
Everything Else
Vodka is for the merchant on the first floor. You can find what he sells in the game world. You can’t hurt friendly characters. Try to use an unloaded or melee weapon when not in combat. It’s easy to accidentally fire a shot.
You can quit hacking or keypads by hitting the power button if attacked. Holmes won’t fail you for wrong guesses on levels 1-3.
You can’t jump, climb, or run. Look for other ways to reach high places. Break crates for items. Enemies respawn in certain areas, so be careful going back. Look for a book to pull on bookshelves to find secret passages.
SMGs benefit greatly from using burst or single shot modes as opposed to full auto. Additionally, if you aren’t using the scope, the accuracy of the Sporting Rifle is pretty bad. The scope is ideal when used, but it’s zoomed in.
I believe that priming the action is only necessary if a magazine is completely empty; I know of no other circumstances in which it is necessary.
Although guns don’t unload on their own, they do if you swap out the weapon in that slot; perhaps that’s what was going on?
There are just two total weapon slots because heavy and light weapons occupy the same slot.
Only levels 1-3 have a Holmes puzzle; levels 4 does not.