system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] What's the smallest 20th century weapon that could kill a Space Marine?

Greetings;

I am posting this query to the public communications forum in hope of an answer.

I am a Techpriest serving with a Space Marine chapter, The Immortal Hearts, en route to suppress an orc Waugh in a neighbouring sector, but this is irrelevant information. 

There is an irregular occurance whenever we enter the Immaterium; a fluctuation occurs in the Gellar Field and a random Space Marine vanishes into a void. Gathered data was input to a Logic Engine and it is estimated that the marines are sent to the year 2000, give or take a decade.

They are sent back without any weapons, armor, equipment or any other such gear. It is my duty to educate my companions on the dangers of such an era. I can only assume that since history does not mention any space marines coming from the future and revolutionizing society that they were quickly dispatched and made relatively little impact on history.

What should I teach them to look out for? Are their current ranged weapons a threat? Would handheld explosives like dynamite or grenades injure an unarmored Space Marine? What caliber weapons would be sufficient to harm them?

I assume their nuclear devices would suffice, as would multi ton bombs, but the more comprehensive a list I can compile the better.

-Techpriest Maldorum

~Sent from terminal Delta Primarus on the Emperor's Fury at 1436 hours

EDIT: A follow up question. Battle brothers have recently been vanishing along with their weapons and armor. How will they fare? What can they withstand, and what will they fall to? I have advised all Marines to keep a heavy load of munition and a melee weapon on them at all times, to aid them in case they vanish. How much will this change?
gpt
Without the blessed plate of the Mark VIII Powered Armor the brothers of the Adeptus Astartes would find themselves susceptible to damage from nearly all 20th century ballistic weapons.

To further refine your query, while even a hand held pistol of large caliber (such as of the archaic .45 caliber utilized on some of the less advanced agri-worlds) could theoretically end the life of a Marine, and some Marines in full plate do sometimes fall to relatively primitive weapons, the usage of such a weapon would require far too many rounds to be effective.  Especially when one considers the speed, strength, and tactical prowess of the average Battle-Brother.

That said, a high volume of fire from 1 to 3 Assault Rifles (comparable to a standard stubber used by Hiver gangs on some of the insufficiently pacified hive worlds) would be sufficient with flanking maneuvers.  Or a single .50 caliber sniper round delivered to the skull, a tech that is still in use today.