Question about ft/lb's and grain weights

[WoodsmanJim 160]

I was wondering why do most people seem to state the ft/lbs of a gun with a particular grain weight of pellet? i.e. 11.7ft/lbs @ 13.5gr.

 

Having paid a little bit of attention in science class at school, I would've thought one figure was not reliant on the other as a finite statement. Here are my thoughts, could someone help point out where I may be wrong please as this is something I really want to make sure understand before I have my rifle tuned!

 

As I understand it there are 3 main measurements involved in ballistics testing:

 

-ft/lbs (power)

-fps (projectile speed)

-known weight of projectile (grain weight)

 

Any calculation for one always involves the other two. If you know two of the figures you can work out the other, and if you fix one and change another, the third is affected proportionally. If the fixed point is the ft/lbs (the magic figure for legality in the UK) then the grain weight is irrelevant surely? An 11.7ft/lb gun is an 11.7ft/lb gun whatever weight its projectile is, it is the fps that would change with a change in projectile weight. i.e. a heavier pellet would travel slower than a lighter given the same amount of power. The weight of pellet cannot change the power generated by the gun.

 

Perhaps the difference lies in how effective the pellet is at utilising the power on offer, a better designed pellet wastes less power and therefore travels faster than a badly designed pellet of equal weight. So here there could appear to be a variation in ft/lbs for various pellets, even though the gun makes the same power for each and every shot.

 

So this could be down to pellet skirt design, diameter, how well it fits your barrel etc etc etc...

 

So to me it makes more sense to state the apparent ft/lbs of a gun when measured with a particular brand and model of pellet as opposed to its grain weight?

 

I have been known to over think things, and I hope I've made a reasonable attempt at explaining my thoughts without being too long-winded!

 

your opinions gentlemen?

 

James

Edited November 23, 2012 by WoodsmanJim

[Roy Williamson 15]

No, no gun is a set power, depending on the weight of the pellet and its velocity this determines the ft/lb.

Here is a great site that explains it http://www.pyramydai...y_August_2003/5.

But basically, depending on the pellet weight and shape an airgun coulb be say 11.5ft/lb with one pellet and 12.1 with another.

 

....I Think

[barrywhite 282]

11.7 ftlbs 13.5 grains .I would guess that would be a .22 spring gun and giving the best possible performance .If a heavier pellet was use the gun would shot 10.5 ..11ftlbs .If it was a pcp gun shooting a 13.5 grain pellet at 11.7 ftlbs if a heavier pellet was used the gun may shoot at 12..12.5 ftlbs .So the pcp would say shooting 11.7 ftlbs 16 grain so a 13.5 grain pellet will shoot at 10.5 ..11 ftlbs......just a guide not just pellet weight affects power.

[WoodsmanJim 160]

Thanks chaps, although

 

Roy, I think your answer is pretty much what I was asking as the question, I guess what I'm looking for is an answer as to why? (if that makes sense?! )

Barry, I can't quite follow your answer.

 

It's worth me saying I understand how to calculate power from weights and velocity, that's not what I'm asking. What I'm asking is why do weight differences in pellets produce different measured power outputs? When according to the laws of physics, that factor alone shouldn't make a difference. (I don't think! )

 

Let me use an analogy from another area of interest I have; cars. (gun equivalents in brackets)

 

From what I can tell, folks are saying that how much stuff is in your car (projectile weight) and how fast your car is going (fps) affects how much power the engine can produce (ft/lb's). But this is not true, the engine produces a fixed amount of max power depending on whatever state of tune it's in. That figure is independent of how much weight it's trying to push along and how fast the car is travelling. So with a fixed amount of engine power (ft/lb's), a cars speed (fps) will depend on how much it weighs and how aerodynamic it is. Change weights or aerodynamics and you'll change the speed, but that doesn't affect the power output of the engine. change weight and tune up the engine (more power) and you'll change speed, but that doesn't change the aerodynamics, and so on and so on....

 

The same can be said of an air rifle, as it too is a mechanical process. The same spring, pushing the same piston, down the same bore, for the same length (swept volume), displacing that same volume of air, through the same sized port, into the same caliber barrel time after time will produce the same ft/lb's of pressure every time. obviously wear and tear become noticeable over time, but on any one day in the same conditions we can discount these factors. That's mechanics and physics 101.

 

So why should a different weight of pellet change the mechanical power outputted from the rifle? My suggestion is that is doesn't. It's more the design of the pellet that changes the other measurable variables and therefore the apparent power of the gun, not the actual power of the gun.

 

I'm not looking for an argument here, I'm of a scientific mind and genuinely find this stuff really fascinating, and I'm very happy to be corrected and educated. Just wondering if anyone has any scientific answers.

 

best regards to all,

 

James

Edited November 23, 2012 by WoodsmanJim

[Roy Williamson 15]

ahhhhhhhhhhhhhh get ya.

Unfortunatly I only know what I have read, and that is pellet weight makes a huge difference to muzzel energy. Another can of worms is that the same rifle (pcp) in different calibres will give different amount of shots for the same charge of air. And the larger calibre like .22 shooting a heavier pellet will give more shots than a .177 shooting lighter pellets. You would have thought it would be the other way around.

ATB Roy

[Acuspell 329]

James,

you are thinking on paper, and air rifles don't read the rules!

 

the reason people state mass and power is because, as you say, the legal limit remains the same, but the results from different pellets differ in performance and efficiency in different barrels. NO TWO BARRELS are the same. You can have two 'identical' rifles off the production line and they can differ in pellet preference and power.

 

Take an air rifle. Put a particular pellet down it and the chrono reads the velocity, then because ti is a clever chrono it does the sum and reads the energy from that pellet (provided the weight of pellet was input correctly!). The next pellet style might be more efficient through that particular barrel and produce a compleely different power output - even if the pellets are the same weight. Change the weight of the pellet and the results can be skewed even more.

 

As a rule of thumb:

spring guns produce more power with lighter pellets

pcp rifles produce more power with heavier pellets.

 

My Rapid is a case in point. With Field Target Trophy at 11.5gns it drives them happily at 910fps.

JSB Exacts at 13.5gns you would think would be noticeably slower out of the spout and drop more over range. however, they only lose 10fps and over 35 yards the drop is only a pellet width. Due to the heavier weight going into the formula the energy figure is 3ft-lbs greater - nothing else changes with the rifle. It is just how my rifle happens to work with that pellet, and the FTT.

 

I have always regarded our 12ft-lbs limit in a more practical way and said it is a VELOCITY LIMIT. I said this for the same reasons as you state above. Given a particular pellet there is a limit to the velocity at which you can drive it to stay within the 12ft-lb maximum. As we tend to work with velocity rather than energy, it justseems to make sense to me to look at it this way.

[moxy 617]

So why should a different weight of pellet change the mechanical power outputted from the rifle? My suggestion is that is doesn't. It's more the design of the pellet that changes the other measurable variables and therefore the apparent power of the gun, not the actual power of the gun.

 

It won't and it can't. Or if it does. The variable is ever so slight.

 

Put simply. The internal mechanics and engineering of a rifle is machined steel. Which has very little tolerance in movement when shooting a far softer material such as lead.

An exception to this would be a fire arm. Which the tolerances are greater due to the expansion and contraction of the steel due to combustion.

This is then a totally different kettle of fish.

 

The output of an air rifle will remain a relative and consistant output of energy. It is the deformation of the projectile which will alter the energy output reading.

So in Laymens terms. The greater or lesser the projectile resistance upon exit will determine the energy output of the projectile. ie pellet weight. Softer or harder the material.

[moxy 617]

I was wondering why do most people seem to state the ft/lbs of a gun with a particular grain weight of pellet? i.e. 11.7ft/lbs @ 13.5gr.

 

James. In answer to your original question.

 

I think most folk state the above as they do is through habit, good practice or ignorance because they don't understand and it's something their mate says.

 

Or that's just the way it is.

 

However. You have just over complicated the whole issue over nothing.

 

As correct as you are with the physics of things and albeit it makes very little odds. You must understand, as clearly you do. That the pellet weight will contribute to the output velocity of an air rifle.

 

Why did somebody decide that shitters would be better in white. ( although aubergine is a real bad colour ).

 

Stop mulling over it all. Get your rifle tuned, find the pellets that it likes and keep you legal and get shooting.

 

Then get some posts put up on here of your forays and hunting trips and save us a little from your academic brilliance on a Friday afternoon. Lol don't you have a local pub or something.

 

Happy shooting. Atb

[WoodsmanJim 160]

Cool, thanks guys. I think basically you've confirmed what I was thinking; that's it's pellet design and behaviour that can skew the calculation somewhat, rather than actually changing the power output of a rifle (which remains a constant) as some people incorrectly think.

 

Moxy, I realise I over complicate things!! haha! :laugh: sorry, it's a habit I have in order to try and completely understand a subject, when as you say, it makes little odds when the powers-that-be have decided this is the way something is done, so that's just the way it is done. Although I'm not sure it makes little odds when it comes to "finding the pellets that keep it legal", if Mr Firearms Officer gets hold of it and finds a pellet that makes it not-legal, I'm in trouble! I guess that was kind of what kick-started the whole thought process in my head.

Funnily enough I've just got in from my local country pub, 2 pints of 'Ruckin Mole', very nice!

 

I appreciate the answers, I'll let it go now I've got my head round it. haha!

 

I've just bought a few bits to add to my rifle (torch etc) that I hope to do a kit review of soon so you can all the laugh at my dopey midlands accent! So my future posts might be a bit more interesting.

 

Cheer chaps,

 

James

[moxy 617]

Lol. It's not a duuudlaaay accent is it!!

 

In regards to your pellet choice and keeping it legal is as I'm sure a whole new long winded debate. Well, with you! Lol

 

Best of luck with whatever rifle you choose and I look forward to your reviews. Even if I can't understand the native tounge!!

 

Welcome to the forum. Atb