Question for Bryan Litz

[jlmurphy]

I shoot 6mm and 6.5mm heavy bolt guns and to make life interesting, Black Powder Cartridge Rifle. I have read your Applied Ballistics book, and for the most it deals with high B.C. jacketed bullets at high velocity. In the other world of BPCR, the velocities are transonic and wind deflection from drag is counter intuitive and slower is sometimes better, but drag is still drag. I am no expert. My question is about the B.C. of large caliber cast bullets. Many of the designs are offered in similar profile for each caliber ( .38, .40, .45, .50), increasing in weight for each caliber. My question is would the B.C. be the same for each caliber of the same profile?
John

 

[JPeelen]

It does not take Bryan Litz to answer this.
When you increase caliber, bullet weight raises faster than bullet cross section. Both combine into sectional density.
So for a given bullet shape, sectional density of a .50 caliber bullet is larger than that of a .38 or .45 caliber bullet. A larger caliber bullet always has a larger BC than the same bullet shape in a smaller caliber.

 

[Grimstod]

Was it Newton that said "it becomes a question of mass"

 

[Laurie]

It does not take Bryan Litz to answer this.
When you increase caliber, bullet weight raises faster than bullet cross section. Both combine into sectional density.
So for a given bullet shape, sectional density of a .50 caliber bullet is larger than that of a .38 or .45 caliber bullet. A larger caliber bullet always has a larger BC than the same bullet shape in a smaller caliber.

Yes, absolutely.

However, working partly counter to that, I imagine that 'families' of large calibre BPCR type bullets retain very similar if not identical shapes despite their different diameters. Where the shape and nose-form radius remain fixed, and especially if as calibre increases so does meplat diameter, then the form-factor value will also increase (ie become poorer as the form factor measures drag) making the design less efficient.

Overall though the large increase in SD will still see BC rise substantially. I'm looking forward to what Bryan L says though about this given these are mainly subsonic in flight and if this factor injects any differences from what we'd see with supersonic streamlined jacketed designs.

Turning the question back to John, the OP, does any BPCR calibre show any marked superiority over others at longer distances on the range in variable winds?

 

[jlmurphy]

The effect of increased velocity at the transonic zone is exactly opposite of that experienced at multiple Mach velocities. The deflection from wind at 1400 fps is much greater than it would be at 900 fps. It is because the drag increases greatly in the transonic zone and that increases the lag time of the bullet, which is the major component of wind deflection. It is counter intuitive, but slower can be better in the wind. The B.C. of the bullet still matters, but stability is more important. The velocities also mean that lock time is much greater, recoil greater, consistent follow through really critical. All in all, BPCR is much more challenging than High Power. To top it all off, the entire discipline worships the Old Dead Guys from 1890's and considers anything technical as heresy. Most shooters push heavy .45 cal 550 gr cast bullets as fast as possible and live with the consequences. I ought to mention that lead bullets can deform upon firing, which can change it's profile.

 

[Mozella]

....... snip.........The deflection from wind at 1400 fps is much greater than it would be at 900 fps. It is because the drag increases greatly in the transonic zone and that increases the lag time of the bullet, which is the major component of wind deflection. It is counter intuitive, .............. snip...........

This statement is indeed counter intuitive. I know almost nothing about this part of the sport and I've given it zero thought......... until now. I'm not a ballistics engineer, but know a little bit about physics and I shoot F/TR and BR matches using what you might call conventional equipment. As a result, I've got several ballistics calculators handy and I use them from time-to-time.

I found specifications for .458 caliber bullet weighing 500 grains with a B.C. of .300 for comparison. Then I plugged in some numbers appropriate for slow, large, heavy bullets.

Using a 90 degree crosswind and eliminating any spin drift calculation, a muzzle velocity of 900 fps results in a wind deflection at 500 yards of 30.7", a velocity of 714 fps at the target, and a flight time of 1.882 seconds. That doesn't strike me as surprising or unreasonable.

At a MV of 1400 fps, the wind deflection is predicted to be 45.8" (more), velocity 898 fps (higher), and the flight time is 1.401 seconds (shorter time). This does surprise me.

It is difficult for me to imagine how a bullet starting out one and a half times faster will take longer to get to the target. Sure enough, it doesn't. The faster bullet has, as most people would guess, a shorter flight time. So that part of your explanation doesn't match with my ballistics program.

However, as you correctly say, the windage is significantly more with the faster MV. In that respect, your statement and the ballistics program agree; but why?

I, for one, would think that a faster bullet with a shorter flight time should result in a smaller windage correction than that required for a slow bullet. To my surprise, the calculated windage is essentially 50% more for the faster round. That doesn't make sense for a bullet with half again as much MV, at least it doesn't make sense to me.

How fast should I launch this big chunk of lead to match the 10mph full wind deflection for a round producing a 900 fps muzzle velocity at 500 yards? According to the calculator I used, the answer is 2765 fps, pretty darned fast. At 500 yds the velocity is 1467 and the flight time is .747 seconds, only 40% of the time the slower bullet takes. Yet the wind deflection at 900 fps and 2765 fps is identical

How can that be? I admit to knowing nearly nothing about large caliber slow bullets, but if someone zeroed his rifle for a 10 mph cross wind using a 900 fps load and then said to me. "Watch me hit the X with this 2765 fps load without changing my scope, I would laugh.

I'm still on my first cup of morning coffee as I type this; consequently, I could have completely botched up the calculated data, but I don't think so. Can someone explain these non intuitive numbers?

I'm aware that as a well-supersonic bullet slows through the transoceanic range, drag gradually increases and then rapidly drops when the bullet goes sub-sonic. But that fact is not an explanation for these examples. Comments?

 

[Laurie]

The deflection from wind at 1400 fps is much greater than it would be at 900 fps. It is because the drag increases greatly in the transonic zone and that increases the lag time of the bullet, which is the major component of wind deflection. It is counter intuitive, but slower can be better in the wind

This applies to .22LR RNLR bullets too where HV rounds suffer more wind drift than match subsonic types. I've seen the 'proof' published occasionally over the years, but have now forgotten it. Something to do with drift affected by the amount of speed lost and comparing flight time in air v in a vacuum.

 

[XTR]

You want to have some real fun, look at the come up from 100 yards to 1000 yards with a 535 or a 550

(Depending on the bullet design between 175 and 225 MOA) Most are launched at somewhere in the 1300 to 1450FPS range, so they transition very early in the flight to the berm. That is why that rear sight looks like it's about a foot long.

And yes, the worship of the ODGs is just plain frighting in this crowd.

 

[jlmurphy]

Laurie is correct. The amount of wind deflection is a function of the relative time difference between a flight in a vacuum and the time of flight in the atmosphere. I have tested this with the same bullet loaded to 1100 and then to 1300, shot alternately in a full value crosswind at 500 meters, the faster bullet drifted more. The problem with shooting slow is that at extended ranges the vertical component becomes critical. Although believe it or not, black powder loads commonly have single digit E.S. and S.D. of 2 or 3 fps.

 

[XTR]

Doesn't Raton have a minimum MV requirement of something like 1350 or so to compete at 1000?

 

[jlmurphy]

XTR,
The velocity restriction was designed to prevent high arcing slower bullets from hitting the crew in the target pits. In Silhouette competition you must knock the animals off the rail to score, the heavy steel plate Rams at 500 meters are the most difficult to knock down.