What does BC really do?


PDA






mgrych
November 18, 2008, 04:35 PM
i found a really good bullet for my .223 savage.. it really like this 68 gr. HPBT.. it has a BC of .355.... I just now found some berger 70 gr. VLD bullets with a BC of .437 ....Now does BC only effect the drag of the bullet? which would make its trajectory flatter or does it effect accuracy? Basically if i wanted to tighten up my group a little would the switch be worth the extra $12 per 100 bullets?

If you enjoyed reading about "What does BC really do?" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
R.W.Dale
November 18, 2008, 04:44 PM
BC can effect accuracy directly through being more aerodynamic thus wind and other atmospheric phenomena have to work harder to mess up your groups.

BUT

some bullet profiles without much bearing surface don't get along with all rifle barrels. An example of this is my 7.62x39 bench gun is more accurate with 125grn Speer TNT's than it is with 125grn Bergers

So in summary you'll just have to shoot some and see

woodfiend
November 18, 2008, 04:56 PM
My understanding is that the higher BC is, the more drag you have, the more drag you have, the steeper the trajectory and also poorer accuracy.

I'd have to agree with Krochus and say that, yes, you would have to shoot and see to be sure. My bet is that they would be more accurate, the .68 gr HPBT.

rcmodel
November 18, 2008, 05:13 PM
Wrong.

The higher the B.C. number, the less drag it has.

Also, steeper trajectory doesn't really affect accuracy either.

If you know the range, the trajectory, and the sight setting, bullets dropping from the sky will still land in the target.

Witness the 1,000 yard black powder matches!

rcmodel

Ben Shepherd
November 18, 2008, 05:27 PM
The higher the B.C number, the "flatter" the slug flies. Less drag, as noted above, so it loses velocity at a slower rate than a slug with a low B.C. number.

joneb
November 18, 2008, 05:56 PM
The higher the B.C. number, the less drag it has.

Less drag allows the bullet to retain it's energy over a greater distance.

Given all things being equal will the bullet with the higher B.C. stay in the air longer than a bullet with a lower B.C. where the barrel is parallel to the ground or will the flight time be the same and the bullet with the higher B.C. will only travel farther ?

rcmodel
November 18, 2008, 06:04 PM
Higher BC = shorter flight time = less time for gravity to do it's thing = flatter trajectory & also travel further.

SO I think the answer is yes, but I don't know if I understand the question! :scrutiny:

rcmodel

tribbles
November 18, 2008, 06:08 PM
Also keep in mind that unlike sectional density, BC isn't a static number - it's variable for each bullet and is dependent on velocity. Speer is the only manufacturer I've yet seen who lists different BC values for each of their bullets depending on velocity.

GP100man
November 18, 2008, 07:31 PM
then try using BC shooting uphill or downhill!!!!!!!

GP100man

1858
November 18, 2008, 08:44 PM
Given all things being equal will the bullet with the higher B.C. stay in the air longer than a bullet with a lower B.C. where the barrel is parallel to the ground or will the flight time be the same and the bullet with the higher B.C. will only travel farther ?

If both projectiles have the same initial kinetic energy (1/2*m*v^2) but one has a higher BC, then the bullet with the higher BC would have traveled further (since it experienced less drag) but both projectiles should hit the dirt at approximately the same time since they're both experiencing the same acceleration downward due to gravity (m*g). I say approximately because if the two projectiles have different BCs (for the direction of flight) they'd most likely have different BC's for other "views" too.

Speer is the only manufacturer I've yet seen who lists different BC values for each of their bullets depending on velocity.

Sierra does too.


then try using BC shooting uphill or downhill!!!!!!!

Bullet drop increases when shooting uphill compared to downhill right? Shooting uphill the y component of the drag is in the same direction as g whereas shooting downhill, the y component of the drag is in the opposite direction to g.


:)

joneb
November 19, 2008, 12:40 AM
both projectiles should hit the dirt at approximately the same time since they're both experiencing the same acceleration downward due to gravity
I agree.
My understanding is, both should "hit the dirt" at the same time, Regardless of forward velocity. So if one bullet were fired from a rifle at 3000fps and the other was simply dropped from the same elevation at the same time then both will impact the ground at the same time, only the bullet fired from the rifle will have traveled a greater distance.

mgrych
November 19, 2008, 09:03 AM
damn... i like the discussion.. just to clear the question up for some, i'm trying to figure out if anyone has been in my shoes where they upgraded to a slightly heavier bullet with a much better BC, and of course the results... if accuracy was improved

I dont know if this is good or not but i can cover a 5 shot group of my current pet load with a nickel @100, thats just with a bipod no sandbags or vices... i just wanna know if you'll think i would see an improvement on that

DickM
November 19, 2008, 09:43 AM
Bullet drop increases when shooting uphill compared to downhill right? Shooting uphill the y component of the drag is in the same direction as g whereas shooting downhill, the y component of the drag is in the opposite direction to g.

Bullet drop in relation to line of sight, i.e. for the purpose of "correcting" aim for a steep uphill or downhill shot, is always less for angled shots, regardless of whether the angle is up or down. The reason is that the drop is determined by the horizontal distance from the muzzle to target, which is always less when the shot is angled (the horizontal distance is given by the "true" distance times the cosine of the angle). The point of aim for an angled shot, therefore, will always be below the point of aim for a horizontal shot of the same distance.

Envisaged
November 19, 2008, 12:00 PM
Try the Berger's - they are nice. I was fortunate enough to get a tour of the factory - very impressive tolerances kept there.

rcmodel
November 19, 2008, 12:04 PM
i just wanna know if you'll think i would see an improvement on thatBC has absolutely nothing to do with accuracy.

It affects trajectory at long range, and makes for a flatter shooting & harder hitting load is all.

It has no effect whatsoever on group size at 100 yards.

Buying a higher quality bullet with a higher BC may give smaller groups, and probably will in fact.

But it is not due to a higher BC number.
It's due to a higher quality bullet.

rcmodel

Walkalong
November 19, 2008, 12:11 PM
It does not affect accuracy, as rcmodel has stated. The benefit is if it stays in the air less time, it is less affected by it on the way to the target and that makes it a bit easier to shoot good groups. Less time in the elements is always good for "accuracy". That does not make it a more accurate bullet, but it may make it easier to shoot it accurately.

Better bullets will outshoot lessor bullets that have a better BC than the better bullets. (Say that 3 times fast)

everallm
November 19, 2008, 05:31 PM
Caveat on the accuracy comments.

VLD and high BC rounds are also less liable to wind drift and on variable wind conditions that will increase reliable grouping and accuracy.

To be really truthful, you probably have to be in the 0.1% crew and shooting 500 metres+ for it to be a real bonus.......

rcmodel
November 19, 2008, 05:41 PM
The OP is only shooting groups at 100 yards.

I still contend BC shouldn't be much of a factor at 100 yards.

rcmodel

mgrych
November 19, 2008, 06:32 PM
what about the 300 yd range which is what i'm really aiming for..

Walkalong
November 19, 2008, 06:41 PM
The OP is only shooting groups at 100 yards.Agh, I missed that. Yep, it's not making a difference at 100 yards, or even 200 yards, or we would not shoot flat based bullets in Benchrest.

300 yards? Maybe...but high BC bullets are usually used at 600 and beyond to the best of my knowledge.

USSR
November 19, 2008, 07:34 PM
...high BC bullets are usually used at 600 and beyond to the best of my knowledge.

+1. And it's not necessarily because they shoot flatter at long range, but rather that they are effected by wind less.

Don

mgrych
November 19, 2008, 09:02 PM
good points by everyone, i don't think i'll reach out much further than maybe 400 or so... well, at least not right now..... Whats a good powder for a 70gr.. 4895 works great with 68's.. i read somewhere that the 69 gr bullet is where 4895 leaves off and varget takes over for the best powder.. any truth to that?

rcmodel
November 20, 2008, 12:17 PM
No, probably not.

The new Lyman #49 manual lists most accurate loads in each bullet weight.
For the .223:
50 grain = AA-2015
55 grain = Varget
60 grain = AA-2015
63 grain = AA-2230
69 grain = Varget
75 grain = Varget
77 grain = RX15
80 grain = H-4895

There doesn't appear to be a "leave off" point, and if there is, it's the other way around with 80 grain bullets.

rcmodel

If you enjoyed reading about "What does BC really do?" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!