Guidance on Powder Burn Rates

[OMB]

As a reloader, I've used the relative burn rate charts along with the usual cast of reloading manuals to guide my powder selection and starting charge weights. In using the burn rate charts we’re told that the associated numbers are only ordinal ranks with no implicit value, other than to indicate 110 is slower than 100.

For example, take these three old reliables, starting from fastest to slowest:

Varget 108

IMR 4350 121

Reloader 17 125

I used QuickLoad to generate pressure-time and %Combustion-time plot for these three powders (attached). I picked charge weights for all three powders to achieve and estimated 50,000 psi peak chamber pressure in a .30-06, Lapua brass, 167gr Lapua Scenar, 24” barrel. What does the plot reveal?

• Pressure
• All three achieve peak pressure about the same time.
  
  
• After achieving peak pressure, RL-17 retains higher pressures longer than either IMR 4350 or Varget.
  
  
• Combustion
• Varget’s combustion curve is fastest up to about 85%, at which point RL-17 catches up. The final 15% for Varget and RL-17 are similar.
  
  
• IMR 4350 mirrors RL-17 up to about 65%, after which IMR 4350’s last 35% of combustion is decidedly lower than Varget and RL-17.
  
  
• Varget and RL-17 achieve nearly complete combustion before the bullet exists the muzzle; IMR 4350 does not.
  

In light of this plot, I’m not sure how to interpret relative burn rates. Is Varget a lot faster than RL-17 or just a little? Where does the middle-sitting IMR 4350 really fit. Much slower, a little slower, what? In these times of scare (nonexistent) reloading supplies, having a better feeling for the nature of the burn rate jumps across powders would be helpful. My stock is limited and very stressed.

Help!

 

[shoot4fun]

A discussion with one of the top techs at one powder company was enlightening for me about just how worthless the burn rate chart may be.
Your data from QL really does validate everything he told me.
"Why would a powder company, such as Alliant, offer so many powders in the same area of the chart?"
His answer was that even though they were in close proximity on the chart they were very different in how they generate pressure. In the case of RL16 v. RL17 I use both quite a bit. There are some very stark differences in how those two powders work. I get more velocity from RL17 but way better performance in the area of accuracy and temperature sensitivity from RL16.

 

[Mulligan]

I know nothing about QL, but I have to admit, this is really interesting.
CW

 

[dellet]

Take all three loads and compare them in the recoil table.You likely/ find some noticeable differences.

Powder charge weight, the pressure slope both rising and falling as well as the duration/amount of pressure to the muzzle will change felt recoil.

If it is measurable, you can probably feel it. If all else is equal, accuracy, velocity, it might come down to the type of recoil impulse you handle better. A 50 pound punch, or a 50 pound shove.

 

[358WCF]

"In light of this plot, I’m not sure how to interpret relative burn rates."

Seems to me you answered your question in your 1st paragraph.

"In using the burn rate charts we’re told that the associated numbers are only ordinal ranks with no implicit value, other than to indicate 110 is slower than 100."

Burn rate charts are very general information. Nothing more. For example I've seen rather large lot to lot variation in IMR4350 over the last 40 years. It may have moved 5 or more points on the burn rate chart, or enough to swap spots with some lots of IMR4831. The 2 lots of RL17 I've used recently were different enough to make an unsafe load if the order of use was reversed without a reduction. The 3 powders you mention also bulk differently. There is no absolute linear answer to choosing a particular powder for load development... only suggestions of what might work from published manuals or people who've actually done the testing. But then we come back to lot # variation.

Also QL, while interesting & a decent tool, is not foolproof. I saw a QL posted somewhere for 7x57/160s using RL26. The results looked good so I set out to try it. Six steps down from the QL max was overflowing from the case. Oh well. Back to the proverbial drawing board.

 

[6ShotsOr5?]

The burn rate table is attempting to condense several variables into a simple story when it’s really not simple at all. Variables include the kinetic reaction rate of the powder, the specific surface area (area per mass) which is mostly determined by the kernel size, and whether the specific surface area of the powder is increasing, remaining almost constant, or decreasing as the powder burns. Extruded kernels with special geometries like tunnels through them can have more surface area as they burn so they speed up - some of the Vihta powders are made like that. So if you are trying to put the powders in order on a burn rate chart, which factor do you use to rank them: kinetic rate, kernel size (specific surface area), initial specific area, or something else?

 

[OMB]

Take all three loads and compare them in the recoil table.You likely/ find some noticeable differences.

Powder charge weight, the pressure slope both rising and falling as well as the duration/amount of pressure to the muzzle will change felt recoil.

If it is measurable, you can probably feel it. If all else is equal, accuracy, velocity, it might come down to the type of recoil impulse you handle better. A 50 pound punch, or a 50 pound shove.

Re-crunched the numbers

Remember, these are all at roughly 50,000 psi. Kinetic energy and momentum track with MV, as you'd expect. The duration does not follow that sequence. Not sure how to interpret less recoil with shorter duration over higher recoil over shorter duration.

Powder	MV fps	Kinetic Energy ft-lb	Momentum ft-lb/sec	Duration msec
Varget	2,703	12.4	94.9	1.97
IMR 4350	2,715	13.3	98.5	2.08
RL-17	2,840	14.3	102.0	2.01

 

[OMB]

The burn rate table is attempting to condense several variables into a simple story when it’s really not simple at all. Variables include the kinetic reaction rate of the powder, the specific surface area (area per mass) which is mostly determined by the kernel size, and whether the specific surface area of the powder is increasing, remaining almost constant, or decreasing as the powder burns. Extruded kernels with special geometries like tunnels through them can have more surface area as they burn so they speed up - some of the Vihta powders are made like that. So if you are trying to put the powders in order on a burn rate chart, which factor do you use to rank them: kinetic rate, kernel size (specific surface area), initial specific area, or something else?

Here's what I was hoping to be more efficient about. I loaded up IMR 4895 test rounds and my standard deviations across the load groups were horrible. I decided to try a slower powder and limit as best I can the number powders I have to test. I have a few "slower" powders on hand, but with the component situation today, which powders make logical sense to try? Probably a fool's errand?

 

[K22]

I think most of the work has already been done for you - just consult reliable published reloading data especially powders that they* found produced the most accurate load.

*Lyman, Sierra, and Nosler has narrowed it down fairly well in my opinion.

 

[GLShooter]

I wish more reloaders would grasp the burn rate fable as has been explained here. I've spent years 1:1 explaining that burn rate charts should be labeled "HERE BE DRAGONS!!"

Greg