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If people time-travelled back to when Earth had a lot more oxygen in the atmosphere (35% instead of today's 21%) what would be the effect when they started shooting? Mainly small arms.

Would spent shell casings be setting things on fire all over the place?

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This article somewhat addresses this (it is a complicated read though)

http://www.fs.fed.us/ne/newtown_square/publications/other_publishers/OCR/ne_2004_wildman001.pdf

It talks about fire spread during the 35% O2 concentrations of the Paleozoic era. Their conclusions, if I interpret them correctly, is that flammability and fire spread would be somewhat increased but not dramatically so. Even today shell casings landing in very dry materials can start fires so I would imagine this would be slightly more likely in a higher oxygen concentration.

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I would be very surprised if there was any effect.

The bullets won't care, they aren't being propelled with atmospheric oxygen in the first place.

I do understand the threat of hot brass and the stuff that comes out the barrel (watch a gun in very slow motion--there's some fire along with the hot gasses) but if the environment is such that a minor ignition source can set it off I would think it would likely have burned from a lightning storm.

If things were tinder dry it would be more dangerous to use a gun then than in similar conditions now but you don't have that sort of situation in the first place.

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Interestingly, I haven't been able to find a real number for just how hot shell casings are when they get ejected. It's clear that they get hot enough to sting, especially in cleavage, but that's not really an adequate consideration. I can only speculate that, since ejected brass does not significantly oxidise, it is unlikely to be hot enough to set most matierals ablaze, even at high oxygen concentrations.

Certainly, there would seem to be considerable cooling during flight, since the walls of a cartridge are very thin and would cool rapidly.

Perhaps of more concern would be muzzle flash when firing from the prone position, although the flash duration is so short that I'm very dubious that solid materials would be affected.

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I would think the explosions would be a touch more efficient and explosive. Not by much, but enough to cause a poorly made weapon to shatter in the user's hands or blow apart.

Doubt that spent casings would ignite anything with more risk than they do now. More oxygen doesn't make materials more flammable 'without a equally higher ratio of fuel to burn or alteration of materials to drop their flashpoints enough'.

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    $\begingroup$ Actually, at least beyond a point, more oxygen does increase flammability. (Case in point: Apollo 1.) I doubt the things you would encounter in nature around that time would fall into that category, though; the first lightning strike would probably have done it already... $\endgroup$ – user Jun 8 '16 at 9:09
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    $\begingroup$ Edited my obvious mistake, O2 is just part of the explosive/flammable equation. but yes I doubt even at that level of O2, a fire arm would just spontaneously ignite it's surroundings. Flashpoints of materials would not drop considerably enough. $\endgroup$ – Dan V Jun 8 '16 at 9:26

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