why is one powder more accurate?

[super7]

This is kind of a dumb question but what makes one powder more accurate than another? For example, if you have bullets loaded with exactly the same components (and same oal etc) except brand of powder but those powders are loaded to the same velocity....then what makes one more accurate (group better) than the other. Lets assume that the bullet gets pushed up the barrel at the same speed. The rifling does its thing and the bullet leaves the barrel at the same speed and same amount of twisting etc. You would think that the POI would be the same and the deviation in POI would be pretty much the same from shot to shot....but its not (or may not be). The only thing I can think of is the gasses that follow the bullet for a short distance after it leaves the barrel. Is that what makes or breaks the accuracy/consistency? Do the gasses affect the flight of the bullet for a short while after the bullet leaves the barrel? Do they affect barrel harmonics as the bullet and gasses are travelling up the barrel? What other things come into play?

(as an aside, I saw a show on tv where someone had shot themselves in the abdomin with a rifle. The gasses from the shot entered the body cavity and caused the abdomin to expand. It caused so much pressure that the persons button came off their pants and their belt came apart)

 

[RePete]

It has to do with how the powder fills the case (load density), the burn rate for a particular bullet, how the pressure curve is (does it spike and then drop off or does it burn slower and build up pressure gradually).

That's just a few.

 

[super7]

It has to do with how the powder fills the case (load density), the burn rate for a particular bullet, how the pressure curve is (does it spike and then drop off or does it burn slower and build up pressure gradually).

That's just a few.

Does that stuff still come into play if the bullets are comming out of the pipe at the same velocity?

 

[colinjw]

Interior ballistics ( while in the barrel ) are very complicated and there are many variables. It would seem to make sense that 2 different powders that give the same velocity would be equal as far as accuracy. Sometimes but not always.
Different pressures, different burn rates etc.
It is why shooters who are very serious about accuracy will try numerous combinations in search of the best load.

Having said that, I work in accounting not physics & have little idea why things are different but I have seen it often enough to know it's true.

 

[Ice-Pick]

Other things

Other things to consider, you,ve seen dirty barrells from unburnt powder or barrells with the throat and rifling starting to be burnt away from over heating. Does this effect the bullet, obviously if the fire and the gases do this to the barrell then it would be safe to assume since they are pushing the bullet at the time that the back end , at least, is going to be effected by the same thing.
Now, if you have a cooler burning powder does that effect the temp. of the bullet while leaving the barrell and in flight , I would assume that the charge behind the bullet and the friction alone would make for a temp. on the bullet that would be very uncomforable to handle , but if you had a hotter powder, is that bullet like a hot coal flying through the air?
Does any of the black soot-ish type build up you find in some barrells stick to the bullet during flight? Would it effect it?
Bullet designs have different shapes to the back of the bullets so it is safe to assume that the guys designing the bullets believe that whats behind the bullet makes a difference, some tapered, some flat, some hollow-ish.
So I think to answer the origional question "I" would believe that the difference in the powder burn characteristics would make a difference on the effect had on the back of the bullet and change somewhat the characteristics of the bullets flight path. Assuming everything else is the same as mentioned. Barrell length, twist rate, bullet weight etc.

My $.02

 

[Demonical]

Why does a 165gr Nosler BT "spray paint" out of my .30-06 while the much cheaper Hornady 165gr BTSP is 1" MOA, with basically everything else exactly the same?

The Nosler BT has an excellent reputation for accuracy btw, just not in my gun.

Interior ballistics ( while in the barrel ) are very complicated and there are many variables. It would seem to make sense that 2 different powders that give the same velocity would be equal as far as accuracy. Sometimes but not always.
Different pressures, different burn rates etc.
It is why shooters who are very serious about accuracy will try numerous combinations in search of the best load.

Having said that, I work in accounting not physics & have little idea why things are different but I have seen it often enough to know it's true.

Pretty hard to say it any better than that!

 

[yodave]

The Nosler BT has an excellent reputation for accuracy btw, just not in my gun.

x2 on that, they were keyholing at 50 yards

I checked the actual diameter with the calipers and the were in spec. My gun just plain and simply does not like them, and that is like money in the bank.

 

[RePete]

Does that stuff still come into play if the bullets are comming out of the pipe at the same velocity?

Yes.

Demonical:

The jackets could be made of 2 different hardness jackets, and could be a source of your problem. Also a BT may be deforming during chambering.

I have a Rem Mod 4 in .308 that like the BT's.

 

[John Y Cannuck]

If you take it one step further, you can use two supposedly identical rifles, in the same cartridge, with everything equal including velocity except for the powder, and still have differences.
There are many many variables in handloading, and in rifle constriction, why the above happens is just speculation.

 

[thecollector]

Yes, lots of complex interrelated variables. The same bullet in the same rifle with the same muzzel velocity, but with a different powder will have a different pressure curve, imparting a different impulse to the bullet, wihich will produce a different barrel vibrational mode, altering the bullets point of impact on target.

 

[Johnn Peterson]

The list of contributing or governing factors is long & distinguished. IF you find some 'magic bullet' or formula, you'd be a rich man. That's one of the reasons for my tendency to follow the recommendations of some of the experts, past & present. Although I do 'try' some of the newer stuff, I still put a lot of credence in the efforts & results of the 'oldtimers'. 'They' put in the time & testing, no sence in me trying to 'reinvent the wheel'. So far so good.