is changing powder for more mv, less pressure correct?

[rockwind1]

i've been reading on a forum i belong to that specializes in the 6.5 saum gap. some folks report a switch from H1000 to RL26 and getting a higher muzzle velocity with out any signs of pressure (as opposed to getting signs of pressure at same velocity with H1000). was just wondering if this seems reasonable to the highly experienced folks on here. i mean, to me, if you are using the same brass and just switching powder, it's the case that shows pressure and if you are pushing a higher velocity you must generate higher pressure so,,,,but I have been wrong before,,,,, ( once I think,,,, back in the '70's)

 

[dkhunt14]

I think it has to do with the pressure curve. If it burns with pressure spread over a longer period through the barrel. Like RL33. Matt

 

[Bart B.]

Oft times, going with a slower powder loaded to the same pressure as a faster one will produce higher velocities. But often at a loss of accuracy.

There's a small range of powder burn rates that produce very repeatable pressure curve shapes for a given bullet weight. When faster or slower powders are used, pressure curve shapes start varying. That tends to vary bullet times in the barrel as well as increasing muzzle velocity spread; both impair accuracy.

So, like the DuPont powder rep said at the end of our conversation on this matter: Do you want all your bullets to win the race to the target? Or land on it a tiny bit later in a much smaller area? Seldom do you have it both ways.

 

[SteveOak]

Higher velocity is the avowed goal of RL26. ....Reloader 26 is great ballistic efficiency, high bulk density so you can get more of the slow powder into the case to harness the energy....

In general, there is a powder/load for every cartridge and bullet combination that will produce the highest velocity at the same chamber pressure as other powders. It will not necessarily be the most accurate load. In fact it rarely is but that is a different question.

As Matt said, it has to do with the pressure curve, or more precisely, the area under that curve. Bart's explanation is very good also and speaks to other elements of internal ballistics.

This is why you can typically get higher velocity with Vihta Vouri N5XX powders. They are formulated to produce a combustion pulse that stays closer to the maximum pressure for a longer period of time so even though the maximum pressure is not higher, the area under the curve is greater resulting in more velocity before you reach pressure limits. The burn rate table I have does not list RL26 so I don't know exactly where it is at but I would expect that you can get even higher MV with VV N570.

If recoil impulse is of interest to you, using a VV N5XX powder and in general, any slower powder, will produce a more tolerable recoil as the recoil impulse is spread out over a longer period of time. More of a push than a strike. That pesky equal and opposite reaction thing you know. In general, a slower burning powder will result in a less peaky recoil impulse.

 

[Laurie]

The big difference with and gain from some Swiss Nitrochemie produced propellants used by Alliant (Re17/26/33/50) is a mix of the infused nitroglycerin high-energy process (as per Viht N500 series - in fact Nitrochemie invented the technology and sold it to Vihtavuori a long time ago) and the much improved 'EI' deterrents impregnation system which is unique to Nitrochemie. This is much improved as in compared to plain surface coatings used by everybody else. It extends the initial high-pressure part of the pressure / burn time curve and as Matt and Steve say above increases its area which gives MVs a considerable hike, but within SAAMI / CIP allowed peak pressure levels.

Here is a comment from a January 2015 'Bulletin' piece on Re16 and Re26:

Speed and More Speed with RL 26
Think of Reloder 26 as a high-velocity powder for big cartridges. Furrier explains: “Reloder 26 is produced with Nitrochemie’s latest generation EI® process technology. This is the same impregnation coating process used to produce Reloder 17, Reloder 33, and Reloder 50 for us, and it is fantastic. The “so what” on Reloder 26 is great ballistic efficiency, high bulk density so you can get more of the slow powder into the case to harness the energy, and decent, predictable extreme temp response. Reloder 26 is not as flat at temps as the TZ or Australian materials, but it is very manageable, usually in the 0.5 fps/°F range (depending on the application). Just as important, the pressure increases at hot are very manageable. We are using quite a bit of this powder in our Federal factory ammo due to the fantastic ballistics and accuracy.

There are no free lunches in internal ballistics however. The combination of the propellant's specific energy / flame temperature (in this case high) allied to duration of the peak pressure curve determine the energy input into the barrel so extra heat is being applied to the area compared to a conventional powder. Also, the bullet is moving out of the case and down the barrel whilst this is happening, therefore a longer section of the chamber end of the barrel is subjected to this peak heat / pressure combination. As the pressure / heat / duration combination is a primary determinant of barrel wear, barrel life is severely curtailed if all of the available performance is actually used. This is OK for the sporting rifle shooter who only fires a handful of rounds a year, but is something the high volume range or gong shooter has to take into account.

Re26 is an identical powder to Re17 from the same source using the same technology and ingredients, except that it is slower burning. Re17 (Reload Swiss RS60 in Europe) was hailed for the near miraculous MV increases it gave in those cartridges it was particularly well suited to when it first appeared - Tubb 6XC and the 284 Win in heavy bullet long-throat match form being two such. The downside was that if all the potential extra velocity was loaded to, barrel life was nearly halved. Re26 will do the same thing with a new set of cartridges. There is nothing at all wrong in this as long as would-be users understand that they may buy the desired performance at a price.