barrel length vs. powder usage

[brentc]

magnummainiac is right in that barrel lenght means litte to nothing when it comes to picking a powder. What works in a 7mag using a 28 in barrel is going to work in a 20 inch barrel. Most of the powder is burned before the bullet clears the chamber. Heavier bullets can take advantage of slower powders because it takes more to get them moving so it gives a slow powder a nano second more to burn. QUOTE]


OK, so anybody, help me wrap my head around this, cause I am struggling....... I saw a guy load a couple ( three if memoery is correct ) shells at max load, shoot through a chrony, then added 1 grain of powder and gained 50 fps, then added another grain and gained another 50 fps. Then did it again and gained only 15 fps. ( Not very intelligent , I know ) Why did the last round not gain the additional 50 fps like the others unless there was unburnt powder no longer pushing the projectile?
No, I don't remember the rifle, or the load. It was some time ago.

I load my cartridges down if needed for accuracy so this is not about gaining speed. It is about using what is at my disposal.

Seems the general consensus agrees a 24 inch barrel will do. Thanks all!

24" will do, but you'll get more from your loads with a 26" or even 28", especially if you want it for long range. The 168 Berger/Retumbo combination is a proven performer for long range accuracy and energy delivery.

 

[Kennibear]

lloydsmale

I am surprised no one has tackled your question on velocity increase vs. powder increase.

In a nut shell:

The chemistry of smokeless powder is such that the burning rate of the powder (speed) does not increase with the pressure in a straight line. That is if you graph pressure and quantity of powder going up and bullet velocity increase as moving to the right on a graph the line is not straight at a fixed angle. It forms a parabolic curve so at low pressure and speed the increase seems steady and linear. At some pressure it turns up very quickly and pressure climbs but velocity stays the same or increases very little.

Most powders have a critical pressure where they "run away" (sort of). That is the pressure rise heads vertical. Shotgun/ pistol powders around 18kpsi - 35kpsi. Older IMR types in the middle speed range around 45kpsi - 55kpsi. Newer technology powders run up to 65kpsi safely. But at 65kpsi powder chemistry reaches its current limit and changes over from burning to exploding.

There are exceptions to the pressure at which powder rate increases, some middle burning rate powders (H322, Varget, Benchmark, 2460 etc.) run great at 65kpsi for instance. But at some pressure point your slow burning powder speeds up a bunch and the velocity increase falls into the toilet or even completely off the map. That is, ballistics techs have recorded DROPPING velocity with increased powder and pressure.

This is why I believe your first purchase when starting to reload should be a chronograph. When you add powder and velocity increases drop back you are done. That load has peaked.

To put my $0.02 in the barrel length question, longer is better. Put the longest tube on you can live with. Like chicken soup: it may not help but couldn't hurt!!!

KB

 

[Rupe]

I would say someone did their homework! That was a good answer!
Rupe

 

[MontanaRifleman]

When I started loading for the 7 RM in 88, IMR 4831 gave me the best velocity results @ ~ 3000 fps with a 160 gr bullet out of a 24" barrel. Since then, Retumbo and RL17 were developed and to date they have given me my best velocities with 3100 and 3150 fps respectively. I considered RL17 to be too fast as it only filled the case ~ 85%. Retumbo filled the case ~ 99%.

 

[Michael Eichele]

I'm not sure I buy into the powder burning completely before the bullet leaves the case mouth. I've actually have had unburned powder stuck in cardboard 15' infront of my muzzle. I needed either a faster powder or a longer barrel. I went with a faster powder.

For a given powder, barrel length may affect velocity very little but it will allow you to use a slower powder. That said, there is a balance here and I agree that bullet weight should weigh in on your powder burn rate choice more than length but length does matter.

One thing I notice with longer barrels is reduced muzzle blast which gives me less of a headache.

 

[Mikecr]

If there is an implication somewhere that barrel length doesn't change %powder burned -it's wrong.
That heavier recoil with badly designed cartridges(like a 30-06) is the result of unburned powder added to bullet mass. Some is not burning in the chamber, and some not in the bore at all.
That fireball at the muzzle is powder burning off outside the bore, and you can bet this excess muzzle pressure which is slapping the back of boat tail bullets, isn't 'good for' accuracy.

It doesn't make sense to me that people would want to burn a lot of barrel killing powder, from large cartridges, in barrels too short to take advantage.
This to me makes about as much sense as using light for cal bullets in large cartridges.

 

[Pdvdh]

If you want to use the Retumbo you have at your disposal, I think you'd be better off with 160 to 175 grain bullets. I use it with 160 and 168 grain bullets with good accuracy and velocity.

It may not be the best powder for 140 gr bullets with a 24" 7mm RM barrel, but it will still spit them out the muzzle. Try it out and see what kind of accuracy you get. Then let us know.

 

[Kennibear]

Michael Eichele

I too have done the same thing, unburnt or partially burnt powder scattered on the ground or stuck into the (very close) target. Even in pistols. I think that is a very clear sign the powder burn rate is way slow. I cannot see how a load can be accurate with powder pouring out the barrel unused.

MontanaRifleman

Conventional wisdom says if RL17 @ 85% loading density is getting 3150fps and Retumbo @ 99% load density is only 3100fps, then RL17 is too fast and Retumbo is too slow. This assumes that RL17 is @ Max pressure and Retumbo is not. You did not mention your apparent pressure levels in these two loads so I assumed.
Therefore something in between RL17 on the fast side and Retumbo on the slow is the "optimum" powder.

From Hodgdon's 2013 Loading Manual with 145 listed powders, RL17 is @ #115 and Retumbo is @#138 giving a total of 22 powders between those two. There is clearly other choices. First choice in my book is RL19 because RL17 did so well. But closer to Retumbo are three powders that I favor personally; IMR 7828, RL25 and H1000 - which holds great favor with a majority of subscribers to this website. That being said I would pursue H1000 to achieve pressure "Maximum" and I'll bet the top velocity and excellent accuracy are sure to follow.

In the early '70s when I started reloading, chronographs were thousands of dollars and you got one shot through each pair of tin foil screens. The velocity was a combination of lights that illuminated as you rotated a switch. In other words we could only guess. So the rule of thumb was, "Use the slowest powder that filled the case and still developed maximum pressure and that would be the highest velocity." When I bought my first Shooting Chrony in the '80s I discovered we were either lucky or right.

Since I'm not that good at the Craps Table in Elko.....

KB

 

[MontanaRifleman]

KB,

I agree the RL17 load is too fast as I like 90% capacity or better as a general rule of thumb. However the Retumbo load was what I considered a max working load at 72 gr and at ~ 99% capacity which is an ideal load in my book. I did run some at 73 gr but thought it was a little too close to the edge for me based on signs.

I did not get a chrony until about 95 when I got a Prochrono and that is when I first knew my actual (or close) velocities. Until then I just used measured drops to 300 yds and guesstimated velocities. No range finders back then and not much shooting past 300.

 

[Greyfox]

Retumbo has in recent years become my go to powder for my heavy bullet, large capacity LR hunting loads in 6.5x284 through 338 Lapua. While other powders work, I find that I get high velocity, low ES, low pressure, filled cases, and excellent temperature stability. I see no problem with burn rate effects using my 24-26" barrels. I believe Hodgdon did change it about the time they started labeling their powders as "extreme", claiming broad temperature stability. My loads at that time had to be reduced 2-3% by weight to develop the same velocity. They did something to change the formulation. It' seems to pretty consistent since then.