@Background Pony #F777  
Spike is still a young dragon at that point. Plus he was taking notes all day for TWILIGHT. Older dragons definitely wouldn’t be bothered by that. Given how big we’ve seen the oldest dragons get and how strong they are, if the dragons ever truly got united they’d lay waste to Equestria.

FLAMMENWERFER

I just love this scene.

@Commutare  
I figured I’d do the hard math and figure out what actually is going on in that pint sized monster.

@Daneasaur  
I like how you went to science this…

SAY HELLO TO MY LITTLE FRIEND.

Mhmmm mm mhm!  
_{[That’s my cat!]}
 

@Background Pony Number 17  
Spike also lit the guardsman on fire who grabbed him when Twilight was captured. He obviously doesn’t usually use his fire breath on others, though he gave RD a rather harsh warning about ticking him off in Over a Barrel when she woke him up to pop the last few bits of popcorn by nearly taking her head off with the fire stream he made.
 
But beyond that, no, he never intentionally used his fire ON anyone until this movie and it has lethal force behind it.

@Background Pony #8D18  

 
I’m pretty sure that Twilight has drilled into Spike’s head that he’s not allowed to use his fire-breath without permission, which is why we get all those “zippo Spike” scenes. In the movie, Capper asks Spike, “Say, aren’t you a fire-breathing dragon?” It’s not meant as a question. Right there, Spike realizes he’s just been given permission to use his fire-breath, which is why he gets such a broad smirk on his face.
 
Another subtle point: We’ve never seen Spike use his fire-breath on people before, only on inanimate objects. Note that it’s Capper who picks Spike up and lets him cut loose as a flame thrower, NOT one of the ponies. Ponies are too “nice” to do something unsavory as this; but Capper, being the “rogue with a heart of gold” character, is still enough of a “bad guy” to get away with using a flame thrower on his enemies.

@Background Pony #3715  

 
I just saw the movie. While Spike’s grin isn’t as evil as I figured it’d be (more of a smirk) he had zero qualms about lighting everyone on fire who was in their way, and I’d firmly say that guardsman was dead.
 
Spike’s role was somewhat better that I anticipated, which is good as I usually set my standards low and expect the writers to basically forget what his character is about.
 
I’m fond of the idea of Spike being willing to do the dirty work and deal with the stigma that follows it to keep said stigma off his friends.

@Background Pony #3715  
The hand cramp lasted until there was a cutaway and wasn’t mentioned again.
 
As for magic effect, we have seen that levitation and twilights teleport work on him, but those are “outside” effects. It’s possible that, like the Dread Malwurth, their hides will shrug off magic assaults.
 
This would explain why Twilight and Rarity (and RD) fled when confronted by Garble and the other adolescent dragons instead of fighting since combat magic is near null when used against them.
 
Most Dragon conflicts were resolved with “flee in the opposite direction”.

@DashVader  

And Rarity:  
 
and who else.

@Background Pony #3715  
Now that you mention it, I can’t think of many characters who’ve used magic on Spike, and the almost all of the few I can are either explicitly stated to be far more powerful than normal (Twilight, and while I’m not entirely certain about them, I think Celestia, Cadence, Discord, and Starlight) or had their magic artificially boosted at the time (Trixie with the Alicorn Amulet). While that doesn’t necessarily mean he’s magic-resistant, it also doesn’t indicate that he’s not- just that these characters are powerful enough to overcome what, if any, resistance he has.

@Daneasaur  
I wouldn’t say “indestructible/invincible”. Spike still can get hand cramps, indigestion from overeating ice cream, Dragon Sneeze flower turns him into a mess, he’s not particularly immune to magic. Not to mention there are magical artifacts/plants that could disable even Discord, so…

@Yoga is boring  
Dragons in equestria are truly a terrifying creature.
 
Other than comedic effect, we’ve never seen any of them be injured by anything less than attacks from another dragon. The gauntlet of fire was nothing more than an obstacle course and other than the possibility of Ember drowning, none of the dangers were actually dangerous to the dragons. Volcanic explosions? The wimps turned around. Massive shards of ruby slamming into them? The wall gave way before the dragon did and he was up and ready to fight two other dragons in no time.
 
Look at all the abuse Spike has suffered and document one case where he had an injury that lasted for more than one cutaway; it’s impossible since he doesn’t get injured.
 
The closest we can see is when Spikezilla’s head spines were cut off by the wonderbolts, though those scales seem to be as flexible as hair.
 
Honestly, had he n0t loved Rarity as much as he does, Spikezilla may have been one of the most dangerous events to happen to ponyville due to how indestructible dragons are in this series.
 

I fear no pony…  
But that cat…  
_{Shuffles closer}  
It scares me.

@Daneasaur  
Jeez, no wonder he gets nerfed so much, Spike is OP as hell.

Celestia is going to get some weird things in the mail

“Dracarys”

@Blissey1  
It’s one of the most common metals to use for old locks.
 
Remember, Iron age.
 

@Background Pony #38BC
 
Pasting something I documented over a course of a couple hours:
 
Spike’s jaw pressure can casually shatter items rating 9 on the Moh’s scale. For reference, that is the rating of Ruby and Sapphire. Diamond is 10.
 
The pressure needed to crack either ruby or sapphire is roughly around 4,351 PSI. That’s just to CRACK the gemstone, that’s not what is required to break it, much less grind it into a fine powder.
 
For comparison, a 12 foot long Nile Crocodile’s max bite force is clocked in at 5,000 PSI At this pressure, broken teeth and jaw damage is highly likely. Spike is less than two feet tall and exhibits close to that bite force CASUALLY with no damage to his teeth or jaws.
 
Spike’s fire also is in excess of temperatures of 2,800 degrees F/1,538 degrees C, strong enough to melt iron in less than a second.
 
Basically, what I’m saying is, I wouldn’t want Spike to have a reason to be mad at me.

@Background Pony #38BC  
wait how do we know the lock is made of wrought iron

 
 
RIP

@PonyPon  
Usually by showcasing a “burnt” soldier running away in another scene.
 
Lacking that, I will say that, yes, Spike has done the unthinkable and actually has a kill count now.
 
Somewhat fitting; they want to use Spike for stuff “they don’t want the main characters associated with”. Guess vengeful retaliation with lethal force is one of them.

@Daneasaur  
He’s a generic soldier, how would you know if he did?