Why?? BC Question

[joed]

This is something that has bothered me for a long time. Actually ever
since my first reloading manual in the late 70's.

I've noticed when a new caliber comes out the bullets have high BC's. At
the time of my first manual the newest was the 7mm Rem Mag. BC's
were higher for the newer bullets then any other caliber. I wondered why
but never looked for a reason.

Today I was looking at BC's for the .204, Sierra lists a 40 gr bullet with a
BC of .287, pretty high for such a small cal. But look at .224, the
absolute best I see is .272 for a 55 gr.

It's bothering me that newer seems better. Are they making newer
bullets for newer calibers more pointed? Or is it my imagination?

 

[EddieNFL]

But look at .224, the absolute best I see is .272 for a 55 gr.

Keep looking. The Sierra 80SMK is .4XX (as I recall).

 

[JimKirk]

B.C. for a bullet is kinda like MPG and horsepower for a car.... some inflate the numbers to sell... for both bullets and cars.

Any bullet that has a heavier than "normal" bullet weight for the caliber ... will have higher BC than the same shape bullet of lighter weight.

That .22 80 gr. HPBT Match Sierra has a .420 BC

The .22 90 gr. HPBT Match has a BC of .504!

The .243 107 gr. HPBTMK has a BC of .527!

Jimmy K

 

[rcmodel]

BC is at best an optimistic WAG by the bullet manufacture anyway.
Some are probably more truthful then others, but all of them want you to buy thier brand of bullets because they have a high BC.

Then, if you shoot the same BC bullets in two different rifle barrels, and measure the actual long range drop on targets, then reverse engineer the BC, they will come different.

BC also changes as the velocity changes over the course of a long range shot.

What starts out at .400 BC at the muzzle may be .350 BC by the time it gets to 100 yards, then as the bullet settles down and flys right, it may be back up to 375 or perhaps even 400 again at long range.

At best, use published BC as a rough guide, nothing more.
Cause it won't turn out to be exactly what they say it is in your rifle anyway.

rc

 

[EddieNFL]

BC is at best an optimistic WAG by the bullet manufacture anyway.

I disagree that it is a WAG (maybe a SWAG). I do agree many are inflated. but some are really close.

Some years ago I was headed to a mid-range match with a new rifle, new load (Lapuas). I had zeroed at 200, but no farther. I plugged the info into a ballistic calculator and printed the results. Next day during sighters I cranked the knob up for 600 and added one MOA for the reasons you mentioned. Wound up coming back down one MOA to center up. I have repeated that same scenario a couple other times.

Also, the conditions we shoot in are probably seldom the same as the computer model used to extrapolate BCs.

My nickel's worth...valued at +/- 5 cents.

 

[Hondo 60]

OK, I'll bite... what is BC?
(probably something obvious that's just not coming to me at the moment)

 

[WNTFW]

Ballistic Coefficient

 

[GaryL]

I've always wondered about the impact of the crimp on the BC of a bullet. I pick up a nice shiny smooth Nosler BT with a BC of .5xx, and proceed to put a crimp into it after seating it. It can't be quite the same.

 

[JimKirk]

Air is a fluid just like water ... does water run over a smooth stone easier than it runs over a rippled one?

Jimmy K

 

[918v]

Generally speaking, long for caliber bullets have a higher BC than short bullets. A 40gr .204 is the equivalent of a 69gr .224" bullet. Similarly, a 160gr .284" bullet will be the equivalent of a 220gr .30 cal.

 

[cheygriz]

Lotsa folks, over the last 50 or so years want to hunt big game, but they're too busy (or lazy) to learn how to stalk game animals to within reasonable range.

IMHO 300 yards or less. Preferably less.

So they buy a Remchester "belchfire" magnum, put an astronomical scope on it, and try to leaern how to snipe big game at ridiculous ranges like 500-600 yards or more.

The bullet manufacturers are very happy to sell them 250 grain .30 caliber bullets for their belchfires, and advertise that you can drop that big buck over in the next county from your car or pickup.

There's a legitimate place for these bullets, among military snipers, experimenters, and long range target competitors. But the rest of us are very well served with old stand-by bullets.