Home Forums Reloading Big Game Rifles Need some generalizations concerning burn rate, case size and bullet weight

Can someone school me in general terms about burn rate, case size and bullet weight. What I am looking for is this.

I often read or hear folks with a lot more knowledge than I have about reloading, make comments about a certain powder probably being better than another powder because of "that case size would do better with this powder".

Another statement might read "that bullet weight is better with this powder".

Or I might read, "velocity would be better with less pressure with such and such powder".

I guess what I am looking for is some knowledge to make informed decisions beyond cook book powder selections by using the manuals AND burn rate charts.

Go to the powders that fill the case to minimum 90% density up to compessed loads. Those tend to be more accurate and give the higher safe velocity. The range of powders will vary with each cartridge and different bullet weights.

bobski gave it to you straight. Look in your loading manuals for the round and bullet you want to use. Whatever powder will give you the best velocity and the greatest fill in the case will be the most efficient for that bullet/case/cartridge combination. In example; after a while you will just know that in a .300 Win Mag, with a medium to heavy bullet, that Re22 (or a powder with a similar burn rate) will be a good place to start load development. Each powder manufacturer will normally have a powder that competes directly with another companies powder - both will be at the right rate for a particular series of cartridges. Just read the manuals and compare - it will become evident to you with some study.

Yep, that is the kind of stuff I am looking for. Thanks for the info bobski and Odessa. I often read the manuals (even when not looking for specific loads), but always feel like I am missing something when it comes to powder selection. Pet Loads (Ken Waters) comes the closest to explaining the why and why not.

Bobski gave the straight dope. Most data is listed from fastest burning to slowest. And the corresponding velocity rises to a point then starts to fall back off, usually all these loads are close in pressure. There are some anomolies along the way where a faster powder yields better velocity at safe pressures, but the general trend is noticable in the data if you look at it, especially data that lists pressure for each load. The ones where the velocity peaks, and those loads just to either side of it are going to be the optimum burn rate for a given case/bullet wt. That, however doesn't mean that other's wont work, and especially if velocity is of no concern at all. However, I believe full cases give lower SD's, and that contributes to accuracy, especially over distances.

One caveat though...some powders have multiple personalities based on application as to case size, bullet wt, etc. "Burn rate" is relative, and data should be used, not just thinking it's the proper burn rate.

just my twocents on compressed loads. I avoid them. The real possibility of that powder column which has a bearing load on the base of the bullet which could cause the bullet to shift forward is potential bad news.

I do favor 90% or better case fill though. An article from back in the day detailed velocity/accuracy deviation in a .38 revolver cartridge as related to where the powder was in the case when fired. The powder charge was small allowing substantial air space in the case. The author would hold muzzle up then level the revolver and fire. Repeat for remaining 5 shots. Then do the same with muzzle down and raise to level and fire.

Have to surmise that a rifle cartridge with a fair amount of unfilled case allows the powder to shift at random thus potentially having a less consistent burn cartridge to cartridge.

If one looks, it becomes even more interesting. Take Vihta Vuori N135 for instance. It is excellent for 223 Rem.,middle level 30-06. Excellent for 8x57JS. Surprise. Superb for 9,3x62 Mauser, excellent for 45-70 in heavy loads. Increase the bore size and a slightly faster powder seems to really show its stuff. IMR 3031 does similar. Application seems to allow significant changes.

That's true of course. "Faster" or "slower" powders is relative to case capacity and bore size. The concept I oulined above holds pretty true with any case and bore size. Could be in one application R-15 is a very slow powder, and in another very fast. The 90% fill rule applies and works fairly consistently across the board.

Example: Same basic sized cases 30-06 and 35Whelen. Alliant lists a max load of 59.5 grs R-15 behind a 250 at 2550fps. Thats the "slowest powder" given for that application by Alliant, and nearly fills the case...you couldn't get enough of a slower Alliant powder than that in the case to get decent performance. OTOH, only 49grs of R-15 is max with a 200gr in the 30-06 at 2505fps. Leaving room for much more of a "slower powder" such as R-22 which is listed with a max charge of 58.4 at 2680fps, but at a very slightly lower pressure. Essentially a 180fps increase, at the same pressure. Same with the 35Whelen...a max charge of R-7 with a 250gr bullet gives gives 200fps less velocity, but at 2000 CUP higher pressure.

Burn rate is relative!

General rules of thumb:

The higher the instantaneous pressure in the cartridge, the faster the powder in it will burn, all other factors constant.

The larger the case, the slower the powder you want, all other factors equal.

The smaller the bore, the slower the powder you want, all other factors equal.

The heavier the bullet, the slower the powder you want, all other factors equal.

The powder you want is the one that produces the most efficient energy transfer from the chemical energy of the powder to kinetic energy of the bullet, while filling the case 90-105% full.

Usually, all other factors equal, choose the slower powder.

Sakoluvr,

Everybody is talking around the subject, and in most cases getting it partially right. 280REM is on the right track, and so is bobski.

What you have to look at are a couple of ratios. The first is the expansion ratio. This is the volume of the case as compared to the volume of the case plus the volume of the bore. A great big case and a skinny bore gives a small expansion ratio. A straight case gives a large expansion ratio. As you go toward a smaller expansion ratio, a slower powder is indicated.

The second ratio is the sectional density. This compares the bullet weight with the area of the base of the bullet. In other words, if you give the expanding gasses more area to push against, then the job to move a bullet becomes easier. If you increase the weight, for any particular bullet caliber, it becomes more difficult to push it down the bore. Thus, the higher the sectional density, the slower the powder should be.

Using these two principles, you arrive at the guidance given above that your powder choice should come close to filling the case, and can sometimes slightly compress the powder. Too much compression can lead to practical difficulties smashing the powder down in the case, and some powders resent being compressed, particularly some of the double based powders.

And I thought rocket science was a challenge.

Now that you point it out, this IS a lot like rocket science!

In another sense, it is a lot like a cylinder and piston in an engine. Minus a crankshaft and connecting rod, the basic physics of an internal combustion engine are much like the physics of bullets and propellant.

Sakoluvr-a little tip.When working up loads take five primed cases and fill them w/the 5 best(most velocity)powders listed in the manuals.I have 10 manuals.Slightly tap the cases on your loading bench after filling w/all five powders.The starting load for each is OK.Look inside the cases and see which powder fills the case the most-second-third etc.They should come close to the bottom of the neck or slightly up the neck.Fire these rounds over a chronograph and you'll know which powder will give you the best velocity per volume of that powder; knowing you can work up from there since you started low.It avoids a lot of testing and wasted bullets.There is nothing wrong w/a slightly compressed load and you will usually find your best accuracy and lowest shot to shot variation this way.It's a very easy way to find a good load for a particular rifle. powdr

I didn't really quite get exactly what powdr said about the filling the cases and looking in them.

However, I will add this to this thread. "Slower powders"...I'm talking the slow burners over all, not relative to case/bore size, meaning the slowest of the slow Rifle Powders such as H4831, R-22, R-25, H1000 etc...these powders really will only function best when near 100% capacity. They give their best when in large cases (7mm Mags of all sorts, 300 Mags of all sorts etc), or in Medium case/small to medium bore (basically the 30-06 based cases 30 cal and under ie 25-06, 270, 280, 30-06) with heavier bullets. Once you take the 30-06 case beyond .308 caliber, you powder choices will begin to get faster and faster.

As for compressed loads...they're fine. So compressed they're pushing bullets out is not, but if you have proper neck tension, then slightly compressing a load is fine.