Solid metal crossbow-bolts

[Ottriman]

Greetings everyone, so I am a GM of a game where all sorts of things get thrown together into a nightmare realm. One of these is a vampire with super-strength who currently needs a good ranged weapon.

Now with access to some TL 8-10 materials on hand our craftsman character is interested in giving her the hardest hitting crossbow possible. We did some mental math and found out that this crossbow would have a (Kyos) ST of 26!

We are scaling up damage and hp logarithmically by the way, not just lift. The base thrust we got was 6d, and that was before the damage modifier of the weapon itself!

Our only problem is that this manner of absurd draw weight (40 times the draw weight of a ST 10 crossbow) wouldn't lend itself to conventional crossbow bolts.

Thus there are two main questions.

1. Would a normal crossbow bolt even be able to survive being launched from this monstrosity?
2. If a crossbow bolt were made out a dense and strong metal like steel or bronze all the way through, how would that modify the weapons stats?

Question number two is of course most pertinent if the answer to the first one is "no".

Rules variants we use that might mater: KYOS (incl damage and hp), aggressive conversion of adds to dice (anything larger than +2 gets converted to a dice or dice-1 as makes sense until there's just a leftover of +2 or less).

Rules variations we certainly will not be using: Deadly Spring pyramid rules, Harsh realism rules for archery and the likes from Low Tech.

Thank you very much, answers would be appreciated.

[Fred Brackin]

Let's begin by bypassing Gurps-isms as much s possible for a while.

With 6D of ST damage and a +4 Add for the crossbow you're looking at 7D or about the same as a 7.62mm NATO bullet. Yet the maximum veleocity you can get out of a crossbow is in the vicinity of 1/10 the velocity of a 7.62mm. Call it 280 feet per second rather than 2800.

To do as muhc damage at 1/10 the speed you will need 100x the mass (you're making up for the squaring of velocity in the KE equation). Call it going from a 150 grain bullet to a 15000 grain crossbow bolt. The standard Gurps bolt is 0.1 lbs or 700 grains.

So you want aamterial that's more than 20x as dense, Fortunately a wooden shafted crossbow bolt could be moslty ash at a density at a dnsity as low as 0.5.

So maybe mostly steel with a tungsten tip?

[Ottriman]

Originally Posted by Fred Brackin

Let's begin by bypassing Gurps-isms as much s possible for a while.

With 6D of ST damage and a +4 Add for the crossbow you're looking at 7D or about the same as a 7.62mm NATO bullet. Yet the maximum veleocity you can get out of a crossbow is in the vicinity of 1/10 the velocity of a 7.62mm. Call it 280 feet per second rather than 2800.

To do as muhc damage at 1/10 the speed you will need 100x the mass (you're making up for the squaring of velocity in the KE equation). Call it going from a 150 grain bullet to a 15000 grain crossbow bolt. The standard Gurps bolt is 0.1 lbs or 700 grains.

So you want aamterial that's more than 20x as dense, Fortunately a wooden shafted crossbow bolt could be moslty ash at a density at a dnsity as low as 0.5.

So maybe mostly steel with a tungsten tip?

The velocity of the bolt can be higher than is realistic for historical crossbows, not necessarily supersonic but definitely higher. We have UT materials to work with, so a bowstring that can move faster isn't out of the question.

I am mostly just worried about the bolt shattering when fired if it has a wooden shaft.

[Fred Brackin]

Originally Posted by Ottriman

The velocity of the bolt can be higher than is realistic for historical crossbows, not necessarily supersonic but definitely higher. We have UT materials to work with, so a bowstring that can move faster isn't out of the question.

I am mostly just worried about the bolt shattering when fired if it has a wooden shaft.

The velocity probably can't be that much higher. I've heard of 400 fps for surgical rubber tubing powered contraptions but that might be about it.

A wooden-shafted bolt would not carry the full amount of energy the bow can produce and it probably wouldn't survive an attempted firing at full strength either. The breaking of arrows with very strong bows is a well-known phenomenon.

If it helps you the desired weight for a modern hunting arrow is between 10 and 15 grains per pound of pull the bow has. I don't know what the values for crossbows are but they are probably similar.

[Ottriman]

I see, understood. I think I now have a good enough idea of how this would work to generate halfway-realistic stats for the bow.

Bolts would be 10 times more massive (solid metal) and this bow couldn't fire normal bolts. It's range would probably be ~2 times that of a normal ST 10 crossbow but no more than that as velocity can only go so high on a crossbow platform.

Thank you for your answers.

[johndallman]

Many modern crossbow bolts seem to be all metal, apart from the fins. They seem to usually be aluminium, rather than steel, but that's a way for you to get the weight up. Indeed, tungsten might be suitable.

[Apollonian]

You could also just have the thing fire bullets instead of bolts.

[Anthony]

Modern metal arrows and bolts are generally hollow rather than solid, but that's mostly an issue of weight. The main reason to not use super heavy arrows is that there's no terribly good reason to do so for a reasonable weight bow; the efficiency of a bow in large part is determined by the ratio of the effective mass of the moving parts of the bow (which includes a fudge factor for how much it actually moves and is thus only a few percent of the total weight) to the mass of the arrow, and there just isn't that much to be gained from heavier arrows; if a bow has an efficiency of 2/3 at a given arrow weight, increasing arrow weight by a factor of 10 only increases total arrow energy by 42% while reducing arrow speed by 62%.

Of course, super strength might mean super heavy bows, but there's just not a lot of reason to bother. A 2,000 lb draw weight bow with a draw length of 30" has a total stored energy of 5,000 ft-lb and might put 3,500 ft-lb into the bullet. A typical .50BMG bullet has a total energy of a bit over 13,000 ft-lb, and the .50 caliber rifle will weigh less than the crossbow.

[Fred Brackin]

Originally Posted by Apollonian

You could also just have the thing fire bullets instead of bolts.

You would not get anything like the velocity of a bullet from a gun from anything at all like a crossbow. Never mind how you'd spin the bullet to stabilize it.

Heavier projectiles for heavier pull is the norm. An extra-powerful 100-lb bow does not propel arrows of the same weight as a 40-lb bow just 1.58(square root of 2.5) as fast. Instead good practice is to use arrows 2.5x as heavy.

Even gunpowder won't swap reduced mass for increased velocity quite that freely.

[Icelander]

Real heavy draw crossbows used all-metal bolts weighing 0.5+ lbs., so I don't see any problems in theory. Realistically, any high ST bow or crossbow needs to be using heavier projectiles to get a higher damage, as increasing velocity runs into efficiency problems and diminishing returns.

Also note that crossbows with 40 times the draw weight of bows that could be spanned by hand existed historically. Of course, the poor efficiency of medieval crossbows means that even 2,000 lbs. siege crossbows weren't that much more effective than 50 lbs. hunting bows. Certainly nowhere near 6d imp and anything that got that kind of Dmg would have to be feither shooting an extremely heavy bolt or fundamentally different from historical crossbow designs. Most likely both.