Rubber BBs

[TomFahey]

So, I was thinking of a physics question that one of my tutors asked me a while back, and it got me thinking - is plastic the best material for an airsoft BB?


The question basically related to a gun, loaded with two different bullets, being fired at a tree, which, when hit, would bounce backwards. One bullet was rubber and the other made of steel, but were the same weight and size (impossible, I know, but that's not the point of the question), and were fired at the same velocity, so that they had the same kinetic energy. The question was, which would make the tree move more, steel or rubber - the answer was in fact the rubber bullet, which you might not expect, but the answer becomes quite clear when you think about the conservation of momentum in a collision (or at least, in an elastic collision).

So, that got me thinking about conventional, real ammunition, and by extension our plastic BBs. Full metal Jacket (FMJ) bullets are tipped at the end, for greater penetration, but are often found to actually be less lethal then the alternative jacketed hollow points (jhp). This is because the penetrating properties of the FMJ round will actually cause it to go straight through a body, taking the majority of its kinetic energy with it, whereas a jacketed hollow point round - which is designed to break up and expand upon impact - will release all of its kinetic energy in the target, causing massive damage and trauma to the unlucky victim.

Now, if one were to extend this line of thinking to the humble airsoft BB, we can draw some interesting parallels. The plastic BB - which is rigid and designed to fragment on impact - is best compared to the JHP round, which suggests that, after shattering, it would transfer the maximum amount of its energy to the target, and thus do the most "damage".
Enter the rubber bb. A rubber BB (when compared to a plastic BB that shatters on impact) would have a shorter collision time and a total change in momentum that is double the amount of a plastic BB, resulting in a stronger force upon impact, and thus a much stronger pressure on the skin. However, the rubber bb would still retain the majority of its kinetic energy and so cause less "damage". Given that nerve receptors in the skin are quite sensitive to pressure as opposed to changes in energy (think - a slow moving pin is much more painful than a fast moving object hundres of times heavier), this suggests that the rubber BB would hurt more (good so that people take their hits more often), but in reality be safer (paradoxical, I know). Perhaps even better, as the majority of the kinetic energy is being retained by the projectile, a heavier weight BB can be used, thus increasing the range and accuracy of the weapon.




So, what do you guys think? Just thinking aloud here...

[rjb]

Re: Rubber BBs

Interesting idea... I suppose in theory it would be possible to produce rubber BB's of different hardness levels for different scenarios which is an interesting possibility. Producing BB's to hurt more is perhaps a little sadistic... I know when I'm hit and I really don't want it to hurt more than it already does!

I suspect that in reality it would be near impossible to produce rubber BB's cheaply and with the consistency necessary for "accurate" fire. Changing levels of humidity may cause issues with swelling and/or may cause the development of a thin layer of moisture on the BB which would play havoc with the hop. The other issue perhaps is the issue of material density. I suspect that the rubber used would mean a heavier BB...

My 2p

[seansamurai1]

Re: Rubber BBs

Plus the BB would deform and distort on its initial acceleration and travel down the barrel. This would also mean inconsistency when shooting.

[No Probs]

How do rubber bb's compare to plastic bb's in terms of; aerodynamics?, being accelerated from a rif?. I know nothing about the subject but the OP has an interesting point of view

Sent from a galaxy far, far away...

[dannyeprice23]

Rubber is a heavier material for a kick off. Also silly as each time that rubber beeb hits the inside of the barrel on the way out, it would completely destroy the effect of the backspin as the resistance from rubber coming into contact with the inner of the barrel would not be far off the resistance bestowed upon the beeb from the hop rubber.

[Tariq182]

Re: Rubber BBs

Also consider friction between the rubber hop and the rubber bb.
You might just find the bbs getting stuck.

[RedHawk]

Re: Rubber BBs

Plus you also get rubber of different densities and hardness, such as Latex compared to vulcanised rubber. How does this effect thing which other posters above have asked - aerodynamics, energy transfer, effect of hop-up, rubber hop etc.

[No Probs]

I feel that these issues were addressed prior to the birth of our beloved airsoft, yet always interesting to know how.

Sent from a galaxy far, far away...

[danhc]

Re: Rubber BBs

You will also find that the air resistance is greater due to rubber having a higher friction coefficient also you will probably experience strange hop effects from the BB interacting with the barrel

[TomFahey]

Re: Rubber BBs

Isn't a heavier BB better, because it means more retained kinetic energy, and therefore better accuracy - thus why snipers use heavy weight BBs? Yes, it also means a lower FPS, but, as I speculated in the first post; in a collision where a projectile bounces off a target, the majority of the energy is retained by the projectile - and so with rubber rounds it should be safer to use a higher muzzle velocity (although I know this has legal implications).

There is smooth rubber, no? It'd be interesting to look at the chemical properties of rubber vs plastic. It seems that most rubber (58% apparently) nowadays is synthetic and so the previously mentioned natural rubber, with unwanted characteristics such as a high friction co-efficient, is actually relatively uncommon. Given that most of the synthetic rubber is carbon based, you'd imagine that would mean a relatively wide range of synthetic products - I've found references to urethane and silicon based rubber, which is meant to be smooth. Are there any chemists/material scientists around to weigh in?

Also, given that rubber bullets are already very much a thing (they're used in many different forms, including a slug-like "baton" round, as well as pellets in shotguns), you might expect that there'd at least be some forms of rubber with appropriate aerodynamic qualities.

[sparrowhawk]

Re: Rubber BBs

id like bbs made like minstrals, chocolate with a hard shell

[Half Trigger]

id like bbs made like minstrals, chocolate with a hard shell

I think the number of people getting their teeth knocked out would increase dramatically if this happened.

[Fensome]

Re: Rubber BBs

Cool idea, and any paintballers among us will know that a paintball that bounces off hurts much more than ones that explodes. Generally though i find bbs do just bonce off and ive never had one shatter on me. Really id like to see some slow mo footage of how much a bb compressed during contact with someones skin. Saying that it would probably need to compress at a higher rate than the skin its hitting otherwise it wouldnt have that elastic collision that you're talking about where all the energy is conserved, This would mean you need to make it out of something really really soft or a fluid in some form of bag (sounding far too much like paintballing to me). As for the bb deforming when first accelerated and the hop rubber being more grippy im sure these could be overcome with some of Japans engineering brilliance and deforming would be less of an issue if you could make the bbs much heavier.

[Coz]

Re: Rubber BBs

I think the number of people getting their teeth knocked out would increase dramatically if this happened.

They certainly would if they tried eating my Minstrels

Mmm Minstrels

[Half Trigger]

They certainly would if they tried eating my Minstrels

Mmm Minstrels

I know I'd lose a few teeth if you starting shooting chocolate at me!