[quote=@Beach Burrito]\

[hider=Some points]*Is it actually rifled? As in does the inside of the barrel contain helical grooves which impart spin to a projectile around its long axis? Spin that serves to gyroscopically stabilize the projectile, improving its aerodynamic stability and accuracy?
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Yus. yus it is.

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*You note that the springs they're using are superior to what we have today--that's all well and good but you still have them wound by cranks. 
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[s]They also have superior crank technology[/s] They're designed so that the user doesn't have to bring a lot of force into it's use, but yeah, it's a definite drawback. You have to have some guns of your own to be able to use the rifle.

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**The above makes the Aladeous more of a primitive grenade launcher than analogue to conventional firearms
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Note I did say experimental weapon. The Aladeous is the most promising weapon design the Emperor had at the time, so he gave it to the Expedition. As the Imperial Boffin corps (ooh, new group name) continues to tinker with how to kill people, they'll figure it out.

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-Why hasn't anyone thought of using fire crystal to explosively propel regular bullets--it would be more effective. The design would even be similar to what we use now.
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Mainly because large firecrystal production is still in it's infancy. These people are figuring out how best to use this stuff. (I considered doing it this way, but this way it gives me more room to grow the tech further along.)
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-A sling or staff sling could likely toss these 'ball grenades' further and more accurately than a turn-spring mechanism.
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This is going into the codex: they have these, but the Emperor in his technophile glory decided that he wanted to use the Aladeous instead. They will be going with you on the expedition. They're mostly used for blowing the shit out of fortresses and stuff, they hold much more volatile firecrystals. 

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Carrying around a quiver/quarrel of contact explosives just seems nuts when everyone is shooting at you with the same sort of ammunition. 
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They're not. Well, not originally. The Empire mainly came into being because his troops would shoot someone, and that person would explode, and when they shot back, the Emperor's soldiers didn't explode. That's cause firecrystals weren't really that widespread, and outside of a few essays talking about it's potential, most people didn't use firecrystals for anything other than starting a fire. The Emperor took over because he used exploding bolts when his enemy's didn't. During the Southern Rebellion, there were ludicrous gibs to be sure. Most people can't get firecrytals that volatile, so it's not an issue. The Southerners, however, who do have access to highly volatile firecrystals... Don't really seem to care that much about the potential of being exploded by being hit first. Their goal is to shoot the other guy before he can even draw his crossbow.