FAC Air rifles?

[trixx]

I believe that the rifle does have an effect.

There are 3 separate things to look at.

Internal ballistics - what happens to the pellet before it leaves the barrel. Basically, it is accelerated, and subjected also to deformation and friction in some degree.

External ballistics - what happens between leaving the muzzle and hitting the target. Like Mojo says, basically that's physics - a projectile of a certain mass and velocity, with a particular ballistic coefficient (measure of aerodynamic efficiency) subjected to forces from gravity and air pressure.

Terminal ballistics - what happens when the projectile strikes the target.

The external ballistics bit is what's modelled in that software, and it can be done irrespective of calibre of rifle or projectile. The rifle is merely an agent in causing the projectile to leave the barrel at a particular velocity, and once the pellet has done that, the rifle can obviously have no further effect.

That's a bit of a simplified explanation, but I'm sure you get the drift.

[spamel]

I expected that different barrels may have a different effect on different pellets! It does look like a very helpful progam though, I've had a quick play, the visual thing is quite good, with the bunny on the golf course. Just havin your scope slightly canted can affect the POI quite dramatically!

[trixx]

I expected that different barrels may have a different effect on different pellets!

They certainly will do, but ultimately that just affects the ballistic coefficient of the pellet in that particular pellet/barrel combination. If you're seriously studying a particular combo, you need to chronograph the pellet at various ranges and work out its particular ballistic coefficient.

Having said all that, over the sort of ranges we're talking about - 30 yards-ish - I doubt if it makes any significant difference.

[Andy]

how much difference does the spinning really make with the wasp shaped pellets anyway? Doesn't most of the stability come from having drag at teh tail end of the pellet?

[Cones]

No, the spinning effect is what makes it stable. Normal bullets don't have a cone shape.

FAC pellets look more like traditional bullets.

Mark

[mirage]

The chrono measures the speed just after the pellet has left the barrel and does not measure down range performance.

Once the pellet has left the barrel, it is subject to basic laws of physics that would have been instantly comprehensible to Isaac Newton in the 17th century.

How, or from which make of rifle, the projectile achieved its velocity, rotation and trajectory are irrelevant.

mirage

[Andy]

FAC pellets look more like traditional bullets.
Mark

Ahh I see

[Basemetal]

Even with the waisted pellets, the directional stability comes from the spin. The waist just keeps the attitude apprroximately right.Try a smoothbore .177 if you've any doubt

If you watch a reflective pellet you can see the trajectory isn't quite a simple parabola, but has a precessional element (or several but that's getting just too nit-picky) from the interaction of gravity with the gyroscopic spin of the pellet.

I see the start of a slow right handed screw as I watch pellets fly. Zeroing, of course, means it all comes right at the range you zero for.

BTW I was wondering if the pitch (fractions of turn per inch) of the rifling is constant for most air rifle barrels (regardless of length)?

[Andy]

I was wondering that (partly why I was wondering how much the spin affects a slow pellet)

Is there a standard for proper guns. I seem to have a figure of one turn in twenty inches in my ind but I'm not sure where that has come from (another figure floating about is 1 in 72inches but I think that's the rate for a baker rifle from 1810 or so)

[Basemetal]

I was wondering that (partly why I was wondering how much the spin affects a slow pellet)

Bear in mind that even if it's one turn in 72" (6') then a 600 fps pellet is spinning at 100 rps =6000 rpm.

[trixx]

Is there a standard for proper rifles?

No, the optimum rate of twist is dependent on the characteristics of the individual bullet and its velocity.

[Basemetal]

No, the optimum rate of twist is dependent on the characteristics of the individual bullet and its velocity.

That answers Andy's "proper rifle" question...and here's a link citing typical pitches.

How to build Rifle Barrels

Now what about air guns?

[Woodsman Sam]

cz style rimfire every time mate

[mirage]

I was wondering if the pitch of the rifling is constant for most air rifle barrels?

The rifling twist of most air rifle barrels is about one turn in 12" to one turn in 18" (I believe).

The twist rate is constant, i.e. does not increase from, say 1:18 to 1:12, in any given barrel.

mirage

[Basemetal]

Thanks M,
According to this article, from the link I posted above, "gain twist" has at least existed in th epast. Makes sense if you think of accelerating the bullet but sounds awfully complicated to calculate and machine.

Accuracy depends a great deal on rate of twist, which must be tailored to the bullet. Fast-pitch or sharp-twist rifling is required for long bullets at modest velocities. A short bullet driven fast requires less spin. There's some latitude: A 1:10 pitch (one rotation in 10 inches of travel) works for a variety of bullets in the .30-06. Barrels for .223s used in long-range shooting have a very fast pitch–1:9 to as sharp as 1:7–to stabilize 65- to 80-grain bullets. Gain twist, or an increase in the pitch of the rifling toward the muzzle, used to be favored by some riflemen. Lilja once employed gain twist but found after many trials that it failed to deliver measurably better accuracy than barrels with a uniform twist rate.

Rifling type and bore finish have a lot to do with accuracy. Generally, barrels with shallow grooves shoot tighter than those with deep grooves because they distort the bullet less. But shallow-groove barrels don't last as long. Land size is a trade-off, too. Narrow lands distort the bullet less and create less drag but also burn away faster. Reducing the number of lands, a maker can keep them wide while minimizing distortion and drag. Land configuration has long been debated, with no definitive answers: Witness the two-groove Springfield barrel and Marlin's 16-groove Micro-Groove rifling.

A barrel's groove circle must be concentric with the bore and land corners free of irregularities. Some uniform roughness in land and groove surfaces may actually prove beneficial because a very smooth surface increases friction that can pull jacket material from bullets as effectively as the tool marks in a rough bore. Conventional wisdom is that a uniform surface ripple of 10 to 20 micro-inches delivers the best accuracy. Careful lapping of the bore can deliver a properly smooth bore. Kenny Jarrett's super-accurate barrels are hand-lapped–a process that can take hours. But other makers say the process isn't necessary, given a careful rifling job.

Bore uniformity after rifling can be measured with an air gauge, a probe that is moved through the barrel with constant air pressure recording variations from specified dimensions. Shilen's air gauges are sensitive to 50 millionths of an inch. John Krieger recommends that every barrel be trimmed at least an inch at the muzzle when fitted to the rifle because the tooling used in bore finishing can leave a slight flare at the ends. Krieger barrels are lapped to just under 16 microinches in the direction of bullet travel. They're held to a tolerance of .0005 of an inch over nominal groove and bore dimensions, but the dimensions are uniform to within .0001 of an inch. Pac-Nor (button-rifling) and H-S Precision (cut-rifling) specify tolerances of .0003 of an inch for the bore diameter. Pac-Nor limits variation in groove diameter to .0001 of an inch.

MidwayUSA
5875 Van Horn Tavern Rd.
Columbia, MO 65203
(800)-243-3220
www.midwayusa.com

You might also consider choking a barrel if you're shooting lead bullets, as in a rimfire rifle or a competition airgun. Parallel bores are considered top choice if you shoot jacketed bullets.