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barrel length vs. powder usage

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Unread 12-15-2013, 10:07 PM
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Re: barrel length vs. powder usage

Originally Posted by lloydsmale View Post
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.
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Unread 12-16-2013, 03:36 AM
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Re: barrel length vs. powder usage


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!!!

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Unread 12-21-2013, 01:15 PM
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Re: barrel length vs. powder usage

I would say someone did their homework! That was a good answer!
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Unread 12-21-2013, 01:38 PM
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Re: barrel length vs. powder usage

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%.
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Unread 12-21-2013, 01:42 PM
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Re: barrel length vs. powder usage

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.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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Unread 12-21-2013, 07:10 PM
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Re: barrel length vs. powder usage

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.
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Unread 12-21-2013, 08:23 PM
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Re: barrel length vs. powder usage

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.
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