7 PRC Powder burn rate vs velocity? I don't understand.

[jcoop]

I've recently noticed something that I do not totally understand when it comes to burn rate and velocity.

During the powder drought I started searching for an alternative to Retumbo for my 300 prc, 338 lapua and my 6.5 prc. I ended up trying N570 and each of my rifles performed better with N570 than they did with Retumbo. All had greater velocity, lower es, and the same or better groups overall. I really like having one powder for several of my rifles, so its been great.

I see most say that N570 is too slow for the 6.5 prc with the 143gr. but I was able to get more velocity in my 20" barrel with N570 than I was with H4831 and Retumbo. And with the N570 I ran out of room in the case before I ever found pressure. I'm getting about 2950 fps with 143 eldx and a 20"barrel with excellent accuracy.

Recently I have been working up a load for my titanium action OMR 7prc rifle with a 22" barrel, 180gr ELDMs, and Hornady brass.

I loaded up a pressure test from 65-71 grains of Retumbo. I started seeing slight pressure at 70gr, and at 70.5 gr I had a heavy bolt lift and ejector marks. In my experience this is Typical with Retumbo to see pressure come on fast. At 70 gr of Retumbo I was getting a disappointing 2742 fps.

At the same time I also loaded up the same bullet and seating depth with N570 In charge weights of 67-73gr. I never had a heavy bolt lift but I was showing ejector marks on the brass above 72gr. My velocities were considerably faster. For now It looks like I will be settling in on a load at around 70 gr. of N570 at and average of 2925fps where my SD was around 8fps. (DO YOUR OWN TESTING THIS LOAD MIGHT NOT BE SAFE IN YOUR RIFLE)


This is the part I do not understand about burn rates.

If Retumbo is a faster burn rate than N570, then why are the velocities so much slower with the same charge weight? Especially considering this is in a shorter 22" barrel.

Retumbo 70 gr. = 2742

N570 70gr. = 2925

And why am I able to load more N570 and not find pressure if the velocity is that much higher?

This is almost the exact same experience I had building a load with the 6.5PRC with N570, and Retumbo and H4831SC in a 20" barrel, so it does not seem to be a fluke.

I understand there are nuances to this but overall with N570 and Retumbo there is a huge difference between what I am hearing from people about N570 being a slow burning powder and what I have been experiencing. I feel like N570 is slow at building pressure but fast at pushing bullets.

Can anyone offer some insight?

see N570 and retumbo in this Powder burn rate chart.

 

[Jud96]

N570 is a very high energy double base powder. Retumbo is a single base powder and isn't considered "high energy". RL26 is another high energy powder that delivers more speed than others in the same burn rate.

 

[jcoop]

Thanks for that Jud.
What does the term High energy mean other than it makes the bullets go faster? How can a powder burn slower and make more velocity at lower pressure? To me it seems like this is the perfect powder for the 4 rifles I have used it in. What are the disadvantages to this?

Why would so many people say that N570 is too slow for say 6.5prc with 143's? Are they simply basing this off of burn rate charts and not actual testing?
Thanks Again.

 

[Jud96]

I'm not a powder expert, but high energy powders tend to have more nitroglycerine in them that increases their "energy" and they also tend to burn at higher temperatures. The downside is often less barrel life because of the higher temps and more energy content.

I can't say for certain why people said N570 is too slow. Maybe they tested it, maybe they didn't. If it works for you then roll with it

 

[SheepShapeAK]

I will have to try some N570 once I get my 7PRC barrel spun up. Tagging in on this thread for sure.

N570 has been very accurate in my 300 PRC and 338NM.

I do know guys are running RL16 and N555 for their 7 - 6.5 PRC's (7PRCW). Both of those are much faster on the burn rate chart compared to retumbo and N570. They're pushing the 180 grain Berger hybrids right around 2930-2950fps.

 

[Jud96]

I will have to try some N570 once I get my 7PRC barrel spun up. Tagging in on this thread for sure.

N570 has been very accurate in my 300 PRC and 338NM.

I do know guys are running RL16 and N555 for their 7 - 6.5 PRC's (7PRCW). Both of those are much faster on the burn rate chart compared to retumbo and N570. They're pushing the 180 grain Berger hybrids right around 2930-2950fps.

One thing to note, the faster powders being used in the 7-6.5 PRCW and velocities in that neighborhood are with 30-32" barrels. Drop that back to 22-26" barrels used in hunting rifles, you'll be in the 2750-2850 range more than likely. F-Class shooters will pick the most accurate and consistent powder for the cartridge even if something else would give more speed.

 

[SheepShapeAK]

One thing to note, the faster powders being used in the 7-6.5 PRCW and velocities in that neighborhood are with 30-32" barrels. Drop that back to 22-26" barrels used in hunting rifles, you'll be in the 2750-2850 range more than likely. F-Class shooters will pick the most accurate and consistent powder for the cartridge even if something else would give more speed.

Which makes me wonder if any of them have even tried powders on the slower end.

When my stock arrives I'll have to give it a try. My barrel is a 33" K and P with 1:8.7 twist.

 

[jcoop]

Just FYI
when I cut my 6.5 prc barrel from 24" to 20", I lost 188 fps with my N570 load, but I lost less than 100 fps with my Retumbo load. This kind of made sense to me with the N570 being slower burning and needing a longer barrel, but the N570 was still faster which didn't totally make sense.
I suspect N570 would really shine in the 7-6.5prc with a long barrel.

 

[Tiny Tim]

Jud96 is correct. While N570 has a "slower burn rate", it is a double base powder. As has been pointed out, barrel life usually suffers and to the extreme under high rates of fire. It's often claimed to produce 100-200 fps gains over other powders. While I have no personal experience, many claim 40-60% of barrel life compared to othe powders like H1000 and Retumbo.

In my experience, H4831/SC has never been known for exceptional velocity, but often produces exceptional precision.

Your case fill being at maximum, possibly even compressed, while not exceeding pressure, the case fill likely explains your low SD and ES.

 

[MagnumManiac]

2 things you need to understand regarding burn rates (actual) and burn rate charts. Firstly, burn rates are NOT CONSTANT, they will flip on each other and behave completely differently depending on case size and bore size.
Secondly, no burn rate chart is accurate, there is no standard for each powder to be compared against. Company 'A' lists IMR 4320 as RQ100 (Relative Quickness) in their Calorimeter bomb. Company 'B' lists H4198 as RQ 100 in their Calorimeter bomb and this is posted. No comparison between these 2 powders has EVER been made, only the RQ 100 number is used to determine WHERE it should be placed in the list. A higher number than RQ100 is faster burning, and a lower number is slower burning within EACH COMPANY. No standards are used to determine these numbers…
To answer your question, double base powders per kernel produce roughly 20-30% more gas, which is what pushes the bullet down the barrel which we all refer to pressure, but it is the curve of that pressure, that stays higher for longer that creates the extra velocity.

Cheers.

 

[Earnhardt]

2 things you need to understand regarding burn rates (actual) and burn rate charts. Firstly, burn rates are NOT CONSTANT, they will flip on each other and behave completely differently depending on case size and bore size.
Secondly, no burn rate chart is accurate, there is no standard for each powder to be compared against. Company 'A' lists IMR 4320 as RQ100 (Relative Quickness) in their Calorimeter bomb. Company 'B' lists H4198 as RQ 100 in their Calorimeter bomb and this is posted. No comparison between these 2 powders has EVER been made, only the RQ 100 number is used to determine WHERE it should be placed in the list. A higher number than RQ100 is faster burning, and a lower number is slower burning within EACH COMPANY. No standards are used to determine these numbers…
To answer your question, double base powders per kernel produce roughly 20-30% more gas, which is what pushes the bullet down the barrel which we all refer to pressure, but it is the curve of that pressure, that stays higher for longer that creates the extra velocity.

Cheers.

Great post man!

 

[carpassasin]

This is exactly why Enjoy this site. Great information!

 

[bultinkle]

In my experience the powders that produce the best velocity in long barrels produce the best velocity in short barrels too I'm sure there is a point when it's short enough to not be the case but for rifle barrels and not pistol barrels eh I've not found it.

 

[cajun]

It's what the racing folks call area under the hp curve. The faster burning powder reaches peak pressure sooner but falls off quickly. The slower powder pushes the bullet longer.