Air Rifle Ballistics..............

Discussion in 'Shooting, Hunting and Fishing' started by alhucoll, Jan 18, 2010.

Welcome to the Army Rumour Service, ARRSE

The UK's largest and busiest UNofficial military website.

The heart of the site is the forum area, including:

  1. Yep, I know I'll get ripped for this - air rifle shooting is the most practical option for keeping up a skill I learned whilst in blue (deep blue not crab fat blue).
    Just after some thoughts on what I "think" I may be seeing:

    Gents,

    Hope you can help me on this.

    Just switched from a HW100 carbine .22 to a HW100 FSB full length in .22 - using the same stock and scope, so only the metal work has changed.
    The carbine for food, loved the H&N FTT's and didn't miss a beat - accurate and consistent.
    The FSB using the same pellet had too many flyers for my liking (all the H&N's were sized but not weighed).

    I got a tin of the RWS Superfield, and straight from the tin the consistency and accuracy was excellent - even better once I'd sized the pellets.

    To myself, (RWS pellets made by JSB on a separate die) are 1 grain heavier and are softer than the H&N's - my theory goes like this:

    1. The heavier weight aids in a more stable flight? Less flip when leaving the barrel possibly.
    2. The softer make-up allows the rifling to engage better, thus enabling a more stable flight?
    3. Any other ideas?

    I know that each barrel has its own preferred flavour, but surely there is physics involved? Or is it just a case of accepting what works regardless of the reasons - personally I like to try and understand some of the reasons.....................
    The muzzle velocity is a lowly 572 fps on average – over 60 shots it varied by 9 fps.
    I was looking at "flip" as a possible - I understand "flip" as being induced by the air immediately behind the pellet when it's released from the barrel.

    The blast of air immediately behind the skirt catching up with it? If the only air is behind it, theoretically the pellet is beyond influence.

    A tighter fit of the pellet to the bore should have less turbulent air passing by as it exits from the barrel and thus influencing it's flight less when compared to a looser fitting pellet as more air has passed by causing a more turbulent atmosphere around it as it leaves the bore?

    Would the lower air turbulence on exit of the barrel have that greater effect?

    Just theorizing, but beginning to lean towards the "softer" pellet base.
    Any ideas or advice on the above Gents?
     
  2. Just to say thanks for any replies - couldn't edit to save my Ipod!
     
  3. Air rifle ballistics is dependant on weight of pellet. Surprisingly a rifle with a muzzle velocity of 570 fps may exceed the legal limit when firing certain pellets. I use RWS Superdome and Hollow point. There is an excellent ballistice program on the net called chairgun, you put in weight of pellet and a few other factors and it will give you the muzzle velocity. If I find it I will post the link
     
  4. Cheers Taffnp,

    You're on about Chairgun - a pretty good programme that reasonably well matches my physically achieved ranges and aim points.
    Differing weights of pellets can have an effect on the 12 ft/lb rule - be sure I'm well within that limit using the Bisley Magnums as a high maximum (RWS @ 15.9gr = 11.5 ft/lbs) - as there is no HO set testing procedure mostly everyone who is responsible (or even aware) hopefully keeps within the upper limit sensibly.
    Just bugging me about the weight/softness relationship, it makes sense theoretically....................................

    Al
    ex Killick Stoker (I knew the bottom of your glass)
     
  5. that's what you call a double post! Timewise.
     
  6. Even notionally identical airguns will have small differences in tolerances which can make them perform differently with the same pellets. There is no substitute for just testing to see what works best in each rifle.

    Flip is not what you describe. In any rifle, the shock of firing causes the barrel to vibrate like a tuning fork. If the pellet exits the barrel on an upward motion, this is 'flip' or more accurately 'positive compensation'. If it is going down it is 'negative compensation'. In general, the best accuracy will come when the pellet exits at a 'nodal point' when it is neither moving up or down. Centerfire shooters can do this by adjusting the size of the powder charge. Some benchrest guns also have a 'tuner', which is an adjustable weight on the end of the barrel. In an airgun, the best way is to change pellet weights until you get the least barrel movement at pellet exit.

    The air does pass the pellet when it uncorks from the barrel. The best way of ensuring that this has no effect is by ensuring that the crown of the the barrel is perfectly square to the rifling. Some target airguns also have a 'still air brake' which is designed to strip away the turbulent air from the pellet.
     
  7. Things are getting clearer..................
    Thanks GN.
    So I'm looking at barrel harmonics and the only way is experimenting with differing weights to find the best compromise?
    That's going to be quite a few hours of quality time - just like a woman, just as you think you've figured it out..................................... :eek:
    At least I've learned a bit - so thank you.

    Well our Champ in HFT is a Sapper - good guy as well! 60/60 both days!

    So, we are looking at harmonics, and that will be pure trial to find the consistent best grouping shot - aim points can be learned.
     
  8. Biped

    Biped LE Book Reviewer

    Barrel bore diameters, even for same-brand rifles can vary a little. For this reason, you may wish to experiement with pellets that are offered in multiple calibres, such as the JSB brand (arguably the best pellet in the world for field target, used in the olympics and by FT shooters all over), and which comes in 5.51, 5.52 or 5.53 variants for '.22', and 4.51, 4.52 and 4.53 for the '177' calibre. Generally speaking, in Falcons, Steyr 110 LG and other rifles using the Lothar Walther barrel, the 5.52 and 4.52 pellets seem to be the most consistant.

    The answer for you would be to try different diameters in one of the top brands and see if that fixes the problem, and if it doesn't get it checked out mechanically, including scope mount and check as well as whether the action is fitted in the stock correctly.

    Edited to add: Barrel harmonics my arrse. Might be the case with live ammo, but it is insignificant with lower power air rifles. Match barrels tend to be a lot thicker so 'flex' simply isn't a big enough factor. You will find target shooters putting weights around the rifle (including the barrel) to get a better balance and to counter 'flip', as well as using muzzle-flip compensators to deal with that aspect. Getting the crown checked is also a good idea because that does have an effect.

    I believe also that if 'flip' is a constant, so will your grouping be, and the way to achieve that constant is to use a very consistent (in weight and shape) pellet like the JSB, and to weigh each pellet you wish to fire in batches so that you get even better consistancy.

    JSB will allow you to visit their factory (requires flight tickets) and do your own batch-testing too, if you want to go the extra mile.

    Another thing you can do is get the rifle 'blue-printed' by a reputable firm to further reduce inconsistancies.