powder variations?


October 28, 2009, 10:39 AM
can someone explain to me how changing powders can change the group size of any given gun? i can understand different velocities affecting group sizes, but how would the powder itself make a difference. in my thinking, 2100 fps should be 2100 fps whether it comes from imr3031, or varget, or reloader 7 or anything else. but it certainly does not work that way, and for the life of me, i can not understand why.

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October 28, 2009, 10:45 AM
I think it has something to do with the pressures that each powder produces. Each would be able to push a bullet at, say, 2100 fps; each would do it at differing pressures. I think that pressure affects barrel vibration, and that varying pressures give varying harmonic vibrations in a given barrel.

October 28, 2009, 01:08 PM
ojibweindian has it.
Each powder may peak at different points of the barrel, yet yield similar velocity.
This allows differing forces to act on the bullet as it exits the barrel.


October 28, 2009, 01:28 PM
Also,the brass expands on firing differently depending on the pressure curve. A load in 243win, 68gr bullet, IMR 4350 in a light starting load may not expand the neck, but a faster powder will.

Steve C
October 28, 2009, 03:07 PM
Different powders have different burn rates and characteristics. Simply looking at average velocity being the same as mentioned different powders will produce different peak pressures. How consistently a powder burns is dependent on the pressure it generates which is determined by a number of factors such as charge weight, case capicity, primer and bullet weight. Each powder has an optimum range of pressure where it burns with most uniformity and produces consistent shot to shot pressure and velocity. Out of that pressure range the results become more erratic which leads to less accuracy as the variation in shot to shot velocity becomes greater.

October 28, 2009, 04:10 PM
Put 1 charge of powder on cement & light it - not very impressive. At least not compared to the same charge in a properly loaded round.

The powder will react different if burned under heat and pressure. So what is happening is the variables are changing during the whole time the cartridge is fired. It is not linear or constant. It is a curve and constantly changing.

Look at some info on barrel harmonics, OCW loads, Audette's Ladder method and some other related topics.

October 28, 2009, 04:28 PM
I have seen post where guys think barrel harmonics is BS. I have also seen guys that claim if there rifle shoots well with powder 'X' @ 2750 FPS they can change to powder 'Y' @ 2750 FPS and be very close to the accuracy of the other powder. I don't have a chrono so I can't even try that.

I am curious as to some more specifics on what you have found to be your experience. I'm not trying to "Call you out" on this, just honestly curious.

One funny thing I had happen was a local guy seemed to get bent when my best load of Varget in .308 was a tenth of a grain off of his pet load. Never mind I used a different 168 BTHP and different rifle than he did.

October 28, 2009, 07:41 PM
I'm a firm believer in barrel harmonics (and the sims barrel damper).
I load and switch based on listed velocities in the reloading manual instead of via chrono(meaning, if 41.2 grains of IMR4320 gives me X velocity, I'll switch to however many grains of varget give me that same velocity in the manual).
Each gun will have a different "pet load", use what works best in yours. Variations in barrel twist, barrel length, freebore, and more all effect what load shoots best in your gun.
Different powders have different burn rates and each will have its "peak range", just like Steve C said. Each gun's favorite powder will vary, you just have to experiment until you find something you (and more importantly, your gun) are happy with.

October 28, 2009, 10:19 PM
The same thing is true in a pistol, and I don't believe harmonics comes into play.

A fast burn powder Vitn320 pushing a .45 at 800fps will give different patterns than
Vitn 340/350 (slowerburn) at 800fps. n320 =Bang n350=Push

I don't exactly know why, but what changes?:

How fast the bullet engages the rifling-jumps head space? (possibly minor/different distortions in the jacket?)
Powder burn rate vs Rate of acceleration in the rifling early and late?
And unless we're dealing with "The Man of Steel": recoil differences

October 28, 2009, 10:34 PM
Same barrel, same bullet, same case, same primer, same velocity but different powder ... right? If the velocity of each bullet is the same the instant it exits the barrel, the only thing that will affect group size is where the barrel is pointing ... kind of obvious right (ignore recoil differences which DO affect group size too). If you assume that the rifle is in a vice or some other rigid support to remove human error and to maintain a consistent recoil reaction, the only explanation has to be barrel harmonics. The vibration frequency of the barrel may be very different with a different powder and therefore the barrel could be pointing at a different place on the target.

For rifles and pistols, anything that changes where the barrel is pointing will obviously affect POI and group size. This includes harmonics and recoil for rifles which have a lower vibration frequency and higher amplitude compared to pistols which have a higher vibration frequency but smaller amplitude due to their reduced mass and barrel length. In other words, pistols are more prone to recoil induced "errors" rather than harmonic induced "errors".


November 1, 2009, 06:42 PM
Well, i believe in barrel vibration harmonics. More so on some rifles than others. A short bull barrel, is not going to be affected nearly as bad as a 26" standard profile "snake whip" magnum rifle barrel. And as for something like a 30-30 with a barrel band somewhere in the middle and almost at the end of the barrel, i dont have a clue what harmonics would do with them. I do know that the amount of rounds in a tube magazine will affect the p.o.i. I have checked this for myself, and found it to be true. At least with my marlin 30-30. Which is the rifle i am talking about. I used to load it with reloader 7. I switched to imr 3031, and had a heck of a time. Finally found a load that would work, but is 400 fps slower. Well, i bought another bottle of reloader 7, but for grins, i tried a couple of different loads with reloader 15, and a couple with varget. I did not spend a lot of time trying to develop loads with the last two, they just didnt work as good (with the little bit of loading i did with either of them) the reloader 7 has been the best i have used so far, and that is what i will probably stick with. I have a couple of other rifles that i need to get a really good load worked up for. So this question seemed like it was begging to be asked. As i really do not want to work up 300 loads to find out which is best. That is horribly time consuming, and expensive. I am going to see if any of my buddies has a copy of "pet loads" , and see if i can borrow it. And just fine tune one of those loads. If nobody has a copy, i will check our librbary, and last resort, i will just have to buy one. But i would still like to know "why". I do get what you guys are saying about the pressure curves being different. It is to bad there is not a mathematical equasion (that did not need a phd in triganometry and calculous) to perform that would tell you what powder, at what velocity would be the best bet for a particular barrel length and size. If einstein was a balistician, we would have that formula! Lol!

November 2, 2009, 07:16 AM
Some powders just work better for certain calibers. IMR 4895 for 30-30 and 308 win. The 243win is IMR 4350.

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