Ideally you shouldn't see any implying that they actually are doing their job properly somewhere far away from your home so that IG can have a day off. After all, SMs are better off being used for combat infiltration and for boarding enemy ships. Except, once again, they prefer parading and hogging glory:D
Instead, the IG don't get days off because the Space Marines prefer parading and hogging glory. Also, with that bulky armor and their loud-ass guns and yelling, I wouldn't trust Space Marines to infiltrate a cub scout meeting, let alone combat infiltration against the enemy. Boarding enemy ships, yes, that's good, although in narrow ship corridors their armor becomes more of a hindrance instead of an advantage. And of course, in open battlefields, they're vulnerable as all hell from enemy snipers armed with anti-tank rounds. Forget other 40K factions, the modern-day marines can kill them in open battlegrounds using depleted uranium bullets or anti-tank rifles. Space Marines are walking tanks? Well, these guys chew tanks up for breakfast.
Ehm, allow me to stop you here.
"Also, with that bulky armor and their loud-ass guns and yelling, I wouldn't trust Space Marines to infiltrate a cub scout meeting, let alone combat infiltration against the enemy. "
Soviets were infiltrating with TANK DIVISIONS. Infiltration means only getting behind enemy line to attack their logistics and command without being bogged down fighting all enemy combatants. Space Marines are made for this, especially with their heavy drop focus.
"Boarding enemy ships, yes, that's good, although in narrow ship corridors their armor becomes more of a hindrance instead of an advantage."
And for everyone with less armor it becomes a deadly chokepoint devoid of breathable air.
"And of course, in open battlefields, they're vulnerable as all hell..."
Yes, tanks, SPGs, IFVs, prepared defensive positions wil chew them up too easily. Hence why infiltration comes in. Drop in, kill the target, place explosives on supplies, leave towards the dropship that will evacuate you.
"...from enemy snipers armed with anti-tank rounds."
Whom themselves are within the return fire from Space Marines. Or rather from preemptive fire as inhuman reflexes, centuries of training, enhanced senses and most advanced sensors Imperium has are not an empty sound. And bolter being a hybrid of ECM cannon and gyrojet would outrange even railguns and many types on vehicles armament.
Not to mention this isn't 1940, there is no such thing as antitank rifle anymore. Only antimateriel rifles. You can hope to damage a jeep or truck with them.
"Forget other 40K factions, the modern-day marines can kill them in open battlegrounds using depleted uranium bullets or anti-tank rifles."
Modern day crayon eaters are too used to having comfy conditions, air superiority and believing such bullshit as they'd have drones in a real war with another superpower that has comparable tech level. They've just lost tanks and command treats them just as infantry grunts for over a decade now. USMC is dead, they were turned into a glorified infantry that is playing a second fiddle to army.
Modern day class 6A/IV+ vest existing irl already stops armor piercing rifle rounds from pointblank. That's why 7.62x51 and 7.62x54R no longer cut it for snipers and marksmen. That's existing nowaday armor for regular soldeirs. What do we have in terms of antimateriel rifles? Nothing. Because .50 BMG is a completly impotent cost compromise in that regard. It doesn't provide accuracy, it lacks power. And USMC doesn't have 14.5x114 or 20x82-102mm rounds. You also should know that adding penetrating core(especially depleted uranium one) is basically saying "fuck off" to long range accuracy.
So nope, even modern day tanks and IFVs would pose a serious danger to Space Marines, but nowaday marines would do jack shit and just be gunned down without even getting proper info about targets.
Don't go overboard.
Last I checked, Tank divisions didn't yell FOR THE EMPEROR and waste their time with clumsy melee weapons. They drove in, fired a couple shells, and decimated enemy lines before they knew what hit them. Space Marines, on the other hand, call too much attention to themselves due to their fighting style (over-reliance on melee weapons and loud yelling) which tanks are nowhere near that unsubtle. You didn't hear German Panzer divisions yelling "HEIL HITLER!" as they punched through French and Russian lines. Also, most bolter depictions have them be medium to close-range weapons fired at medium to close-range targets. Whereas enemy snipers today can tag you from miles on end, before you can even see them.
So? Modern marines can dust off those 1940s anti-tank rifles and have the eggheads make modern modifications on them. They can kill Space Marines from MILES AWAY, far beyond their line of sight. And with Space Marines being more concerned with gutting enemy infantry with chainswords while yelling "FOR THE EMPEROR!" they'll make for easy targets.
What you need to understand is that modern warfare has more realistic solutions for war than say, Jedi Knights, UNSC Spartans, or Imperium Space Marines. The latter take the cake for the most unrealistic military force out there, to the point where they make the Jedi look practical since there's only one or two per unit. (And most of the time, those bozos stick to starfighter duty, where they really shine.) Dozens of large, armored dudes yelling atop their lungs, armed with medium-to-close range weapons and seeking out fights like medieval knights would get totally perforated by modern snipers using armor-piercing rounds aiming at them from miles away. As I said, all modern marines and snipers need to do is to dust off old WW2-era anti-tank rifles, have the eggheads give them a modern overhaul, and roll them out. Since Space Marines are more concerned with personal honor and up-close engagements, snipers can nail them from miles away.
The reason why science fiction series bother with things such as Jedi, Spartans, and Space Marines, is because they look cool, not because they're realistic. In reality, tanks trying to infiltrate behind enemy lines nowadays would get blasted by hit and run attacks by insurgents armed with MANPADS and mines. If you don't know what a MANPAD is, it's an insurgent's best friend, especially when the enemy shows up in tanks or planes. Which is why walking tanks would also not be as effective, as WW2-era anti-tank rifles can put them down before they even figured out where the enemy is.
And of course, visual evidence from games and movies show Space Marines to be far slower and more fragile than the books themselves state. Sources such as the Ultramarines film and the Space Marine videogame show Space Marines as slow, walking mounds of flesh and ceramic armor that easily fall to a few .75 caliber bullets. Which would make easy targets for say, modern snipers using WW2-era anti-tank rifles that have been retrofitted with more modern technology, aiming at the Space Marines from miles away. And if you say the books are more canon, well, I've got bad news for you:
"With Warhammer and Warhammer 40,000, the notion of canon is a fallacy. [...] Warhammer and Warhammer 40,000 exist as tens of thousands of overlapping realities in the imaginations of games developers, writers, readers and gamers. None of those interpretations is wrong."
Gav Thorpe, Lead Designer, Games Workshop
"It all stems from the assumption that there's a binding contract between author and reader to adhere to some nonexistent subjective construct or 'true' representation of the setting. There is no such contract, and no such objective truth."
Andy Hoare, Game Designer GW
"There is no canon. There are several hundred creators all adding to the melting pot of the IP."
Aaron Dembski-Bowden, co-author Horus Heresy series
"Keep in mind Warhammer and Warhammer 40,000 are worlds where half truths, lies, propaganda, politics, legends and myths exist. The absolute truth which is implied when you talk about "canonical background" will never be known because of this. Everything we know about these worlds is from the viewpoints of people in them which are as a result incomplete and even sometimes incorrect. The truth is mutable, debatable and lost as the victors write the history…
Here's our standard line: Yes it's all official, but remember that we're reporting back from a time where stories aren't always true, or at least 100% accurate. if it has the 40K logo on it, it exists in the 40K universe. Or it was a legend that may well have happened. Or a rumour that may or may not have any truth behind it. Let's put it another way: anything with a 40K logo on it is as official as any Codex... and at least as crammed full of rumours, distorted legends and half-truths.
I think the real problem for me, and I speak for no other, is that the topic as a "big question" doesn't matter. It's all as true as everything else, and all just as false/half-remembered/sort-of-true. The answer you are seeking is "Yes and no" or perhaps "Sometimes". And for me, that's the end of it. Now, ask us some specifics, eg can Black Templars spit acid and we can answer that one, and many others. But again note that answer may well be "sometimes" or "it varies" or "depends". But is it all true? Yes and no. Even though some of it is plainly contradictory? Yes and no. Do we deliberately contradict, retell with differences? Yes we do. Is the newer the stuff the truer it is? Yes and no. In some cases is it true that the older stuff is the truest? Yes and no. Maybe and sometimes. Depends and it varies.
It's a decaying universe without GPS and galaxy-wide communication, where precious facts are clung to long after they have been changed out of all recognition. Read A Canticle for Liebowitz by Walter M Miller, about monks toiling to hold onto facts in the aftermath of a nuclear war; that nails it for me. Sorry, too much splurge here. Not meant to sound stroppy. To attempt answer the initial question: What is GW's definition of canon? Perhaps we don't have one. Sometimes and maybe. Or perhaps we do and I'm not telling you."
Marc Gascoigne, former head and chief editor of the Black Library
So yeah, all that tripe about the Space Marines' reflexes could be nothing more but propaganda. At least with Jedi, they're weaker in the films, but they do show a lot of power in other media such as cartoons and video games, in both the old Legends era stuff and the new Disney canon.
The marines I've met in real life would put serious doubt to those lofty assertions of yours. And of course, other fighting forces around the world like say, the Israeli army, can totally show you what a real fighting force looks like. As someone who has played video games primarily focused on Space Marines, I can tell you now with certainty that if what the Space Marine game and the Kill Team game portray is what the Space Marines are capable of, there's more than enough modern and even older tech that can take them down.
THAT is why I won't trust Space Marines for infiltration. Even modern day tanks are having problems with that, what with insurgents laying mines and hitting them with rocket and grenade launchers with hit and run attacks. Why would I entrust a bunch of loud yahoos who fight like a mix of Arnold Schwarzenegger from Commando and a Medieval knight with the same mission? They're less subtle than tanks, have lighter armor than tanks, and their fighting style leaves a lot to be desired in terms of modern warfare.