Hand-spanned crossbows take about 5 to 6 seconds to reload and fire.
I was thiniking of between 3-8 seconds (depending on skill level) to reload the powerful stirrup loading crossbow (which could probably exceed even the longbow for penetration and range). I could imagine myself fumbling for about 20 seconds whereas someone who knew the weapon might be able to reaload in 3-4 seconds at a pinch. The repeating crossbow of Chinese antiquity was even faster with top-mounted gravity-fed cartridge and rapid lever action. It was probably not particularly powerful - but speed and accuracy or speed and massed fire can easily negate the need for raw power to find the chinks in the enemy's armour. I'd suggest that the addition of crossbows to the game should include a crossbow skill with a perks tree of its own. Anyway, those stirrupped crossbows could probably take 300 to 400 pounds of tension without becoming unusable to most of the population because they are drawn with the legs (very strong) instead of with the arms (very weak by comparison with the legs).
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Also, someone mentioned having no arc. I think this needs to be strictly optional. All projectiles follow an arc. Have people never seen a PSO-1 scope - which is designed so that even a child using the rifle for the first time could anticipate and compensate for the arc from the bullet...? The real player skill in operating ranged weapons only comes in when the target is at a range well beyond the direct shot and an elevation of 10-30 degrees above the target is necessary to make the range. Oblivion, though far from "perfect", still wasn't too bad when it came to the ballistics. One of my favourite shots was the headshot at a range requiring a 15-25 degree elevation above the target (with anything less sending the arrow to ground short of the target). Admittedly, this is not something any sensible person would ever do in a real life situation under fire (except by serendipity), but it is quite the ego-stroking fantasy!
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The problem in this game is that the arc is in the wrong direction. Skyrim bowshots tend to curve up and over the point of aim instead of simply falling below the aim point with time. I could understand if magic projectiles from spells had this reverse arc, but bows which we already know follow the laws of ballistics? If someone could correct this problem, bows would become playable and would no longer totally smash the game's immersion. If not corrected, crossbows will be unplayable because they will be subject to the same bug. Another Skyrim bug that will affect a crossbow mod is that where firing an arrow or bolt past the edge of an obstruction, the projectile hits the obstruction as if it were fired from your feet - or bounces off the air in the vicinity of the obstruction as if the air were solid. Fallout and Fallout: New Vegas have the same bug - so maybe the method of fixing the bug is already known...?
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Yet another crossbow-impacting bug is that projectiles that have not returned to ground when reaching a certain range simply disappear. This is simply too unrealistic (like carniverous horses, a spring carnival of flowers above the snowline, taking a dip in water in snow country without taking damage, taking a dip in lava, or nibbling on monkshood without dying from poisoning within 30 minutes). The kind of thing where familiar behaviour and facts are contradicted in-game is very much an immersion breaker. One of the realities of projectiles is falling fire. When directed at an enemy out of sight (via a spotter), it is called indirect fire. If I recall rightly, longbowman would be massed and used precisely this way. The Chinese also had a high-powered crossbow with a range of 700m back in the 7th century - this is not a direct fire range. Even with modern firearms, anything over 500m is very much a hit or miss affair and relies on massed fire for intermediate range and indirect fire for long range. Whenever you see some homocidal ignoramus firing an AK-47 in the air, all those bullets still exist and are still just as dangerous when they come back down to earth. The same goes for arrows. At a sufficient height, you need not use a bow to shoot them down on a target at a lower level (almost directly below such as at the base of a cliff) - just toss a few arrows over the edge and let nature and good fletching take its course. The old arrow-drop or bolt drop might also make a good feature for a crossbow upgrade to the ranged weapons model in the game.
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Just some thoughts....

