18 vs 20"

[LRRPF52]

If you study the history of aviation, breaking the sound barrier was a big deal and a major challenge from an engineering standpoint, because the control surfaces of the aircraft behaved differently than in subsonic flight, and stability became a serious issue.

Bullets are spinning thousands of RPM, but stability becomes an issue as well for some reason. One theory is that the Mach wave of pressure built up on the face of the projectile or airfoil changes shape, and can even exhibit resonant disturbances that create strange buffeting patterns that wreak havoc on airfoils, whereas bullets begin to wobble out of their optimum stability spin behavior.



In long-range shooting, we can clearly see the effects of it when an accurate load at 1000yds opens up all over the place at 1400-1600yds. I think there is merit to spinning the bullet faster so that once it begins passage of the transonic barrier, it is more stable than it would be from a looser twist, and Todd Hodnett's work with shorter barrels and tighter twists seems to confirm this hypothesis.

[VASCAR2]

Does anyone know if the rotational speed of the bullet decreases as the velocity decreases. I know a faster twist spins a bullet faster than a slower twist but didn't know if the rotational speed of a bullet would be much different at 300 yards compared to 600 and 900 yards. I know I've thrown in a click or two of windage to account for spin drift but I didn't know if there was a formula for how to account for the spin drift at greater range?

[LRRPF52]

In practical terms, we don't really mess with spin drift because wind is much more of a factor. 1000yd competitors get sighters, and the first shooters in a practical match are the guinea pigs. There are programs that have spin drift data available, but I stopped using spin drift factors years and years ago because the feature is almost useless to me. The reading error for wind is so much larger than spin drift, that spin drift becomes small noise in the solution, and ignored after first round.

[BjornF16]

If you study the history of aviation, breaking the sound barrier was a big deal and a major challenge from an engineering standpoint, because the control surfaces of the aircraft behaved differently than in subsonic flight, and stability became a serious issue.

Semantics...LOL. It wasn't that the control surface was behaving differently, it was the tail control surface (rear tab) was being blanked out by the transonic shock wave. The solution was to move the tail control surfaces up/forward and out of the blanked region (where they behaved normally).

At higher than Mach 1, surface control laws would be reversed...but that wasn't the challenge in "breaking the sound barrier"

[bigbear_98]

So does my understanding have merit or am I in left field?

[Michael]

Can you guys post your average velocities for 18 and 20" barrels with 123 gr. Amax or 123 scenars?

I see such a wide range of velocities and barrel lengths its really throwing me off. I sometimes see 22"+ lengths have velocities you would expect from a 18 so I'm a little confused on whch length to get....I'll be shooting out to 800yards on a regular basis..occasionally 1000.(But i got a bolt gun for that!) thanks!

cst - For Factory A-Max out of an AA 20" (or is it 19.5?), see below

10 Shots
Av 2484
ES 22
SD 6.4

Elevation =2200ASL

Hope this helps.

[calshipbuilder]

between 2450 and 2500 depending on the chrono from my 18" LW barrel. 1050 Yards is no problem, had keyhole impacts at 1260. 2500ft, 28.5 inhg

I wanted a 16" for my lilja barrel, but settled for a 20"

[montana]

Semantics...LOL. It wasn't that the control surface was behaving differently, it was the tail control surface (rear tab) was being blanked out by the transonic shock wave. The solution was to move the tail control surfaces up/forward and out of the blanked region (where they behaved normally).

At higher than Mach 1, surface control laws would be reversed...but that wasn't the challenge in "breaking the sound barrier"

I thought it was the coke bottle fuselage that was the break through for enabling fighter planes to break the sound barrier or maybe I'm going off into left field.

[BjornF16]

I thought it was the coke bottle fuselage that was the break through for enabling fighter planes to break the sound barrier or maybe I'm going off into left field.

Left field...LOL

The "coke bottle" design (e.g. the T-38 fuselage) makes the aircraft more aerodynamic (i.e. less drag).

One of the reasons the tail of the Bell X-1 was getting "blanked out" was the straight wing design. Notice that today, modern fighter aircraft (and even some of the trash/bomb haulers) have swept wings. That delays the onset of shock wave at the leading edge of the wing (i.e. allows aircraft to travel at a higher speed before onset of the shock wave and the corresponding increase in drag) [shock wave is relative to "normal" of the wing].

[BjornF16]

So does my understanding have merit or am I in left field?

bb_98,

BC is speed dependent as you indicate, but there are changes in BC even at supersonic speeds. Litz's numbers likely includes speeds well beyond the Grendel mv, which might negate his published average BC (i.e. his published BCs are the average of multiple BCs in particular speed ranges). I have his book and modify some BCs based upon mv I have with Grendel ammo (e.g. 129 SST, I eliminated the 3000 fps BC and averaged the 1500, 2000, and 2500 fps BC).

Litz's speed ranges are (fps): 3000, 2500, 2000, 1500

He doesn't bother calculating BC into his average BC at speeds less than 1500 fps since they are not practical speeds (i.e. there is a dramatic drop in BC as the bullet approaches Mach 1 and below).

ETA:
Litz 129 SST G1 = .483 (3000 fps = .520; 2500 fps = .499; 2000 fps = .481; 1500 fps = .430)

I use G1 = .470

[JASmith]

...and then Herters brought out a coke-bottle shaped bullet.

Of course he didn't realize that the coke bottle came from the "area rule" applied to aircraft design of the era. BTW -- as I understand it, "area rule" refers to keeping the combined cross-section of the wing and fuselage more or less constant in the aircraft mid-section.

[JASmith]

So if 1100 FPS is the theoretical super/subsonic line of demarcation for given conditions and range, and your round out of a 20" barrel at that line is 1110 FPS (supersonic), or is 1090 FPS (subsonic) from 18" barrel, is the 20 FPS really that big of a difference? Is the target receiving the impact going to give a damn about the difference?

If supersonic kills, then I am perplexed as to why my subsonic 300 AAC Blackout has little problem in dropping game of almost any size at 200 yards while traveling at less than 900 FPS...

Am I missing something? Is it the transition between the two that makes the difference?

It isn't the velocity, it is the disruption of vital systems that brings the animal down. It is difficult to break enough bone to reliably anchor the animal unless one uses very large bullets. Hits on the central nervous system are really iffy and should not be counted on. That leaves arterial, hear, and venous rupture. The lungs are there too but it is difficult to cause enough direct lung damage to kill quickly.

The most reliable way to bring an animal down quickly is then to dramatically drop the blood pressure in the brain, which generally causes fainting within about ten seconds. There is a relationship between animal weight and permanent wound channel needed to have good chances for the cutting of enough major arteries or the heart to cause this to happen. Combining this relationship with empirical penetration and expansion of hunting bullets then lets one assign bullet weights to animal weights. We then get a chart that looks like this:



The method and background is described in this paper: Ideal Bullet Weight

I also assume you are designed to expand at Blackout velocities, which should make the chart valid for your bullet weights too.

[BjornF16]

...and then Herters brought out a coke-bottle shaped bullet.

Of course he didn't realize that the coke bottle came from the "area rule" applied to aircraft design of the era. BTW -- as I understand it, "area rule" refers to keeping the combined cross-section of the wing and fuselage more or less constant in the aircraft mid-section.

I'm not familiar with the "area rule"...T-38 was a 1950's design and a good example of the "coke bottle" fuselage. Modern fighter aircraft don't exhibit that same design. With the advent of variable wing geometry, a lot of the 1950's design went by the wayside.

The T-38 was a "cable and pulley" flight control system. The few times I was supersonic in the T-38, I didn't have to reverse my flight control inputs between subsonic and supersonic.

[JASmith]

Bjorn -- tried to PM to you but your mailbox is chock full!

[LRRPF52]

The topic of Area ruling (coke bottle fuselage shape) makes me think of the F-102 and F-106 legacy, where the F-102 fell way short of the speed expectations due to transonic aerodynamic drag. They took the F-102 design, applied the area ruling features to the fuselage, and created the F-106, which had the wasp shaped fuselage, and a maximum speed of Mach 2.3, whereas the F-102 could barely make Mach 1.2.

Area ruling had been discovered and applied by German aerospace scientists in the 1940's, and most of their work was transferred to the US in the post-war era.