short barrel's vs long barrel's - help me understand reloading rules

[Mr. Friendly]

I suck at science and am not very mechanical, so I hope someone here has the patience to explain to me the factors at play when you're loading ammo for shorter barrel's vs longer barrel's.

I would have thought that shorter barrel's would release the pressures much more quickly than a longer, which would in effect reduce recoil. but from what I've seen in posts, it's the exact opposite.

and regarding powders, I've seen comments on using the slower burning powders on shorter barrel's, not faster, which is what I would have thought you'd want/need, to get as much acceleration before barrel runs out.

if this is right, I hope someone can share cliff notes as to why that is. if I'm still off the radar with my interpretation of what I've ready, I humbly present myself for re-edjumication.

if there needs to be cartridge specific details, 7mm-08, 6.5 Grendel & .358 Winchester are the ammo in question.

thank you!!!

 

[Eagleye]

Generally speaking, the actual recoil generated is determined by a number of factors: weight of rifle, weight and velocity of the bullet, weight of powder in the charge, etc.
"Felt" recoil is also affected by stock design, recoil pad type, somewhat less by barrel length, and the position of your body when you fire the rifle.

As far as powder selection goes, the powder that gives the highest velocity in a long barrel, will usually give the highest velocity in a shorter barrel as well.
For example, I use Norma MRP a lot in the 30-06 with 180 and 200 grain bullets.

It gives top velocities in 26, 24, 22, and 20" barrels. Velocity diminishes as barrels become shorter, but it still gives me the best velocities at sane pressures in all barrel lengths
I have used it in. Using a faster powder in a shorter barrel usually finds you running into pressure issues before maximum attainable velocities are reached.
Remember, "peak" pressures are reached far before the end of even an 18" barrel D.

 

[BCBRAD]

For a 7mm-08 and a 150gn NAB to be on a node you will need ~41.6gn of Varget in a 20" bbl 2682 ft/s.

For a 24" bbl on a node (nodes are different for each barrel length) 39.5 gn Varget for 2660 ft/s.

For each change in barrel length expect 25-30 ft/s change, this alters where the node is.

Any compatible powder will work similarly for any barrel length that you may use.

 

[Ganderite]

As stated above, the powder that gets the best velocity, gets the best velocity. Barrel length (18" to 32") does not change powder choice.

I have a number of rifles with both long and short barrels. I am more concerned with accuracy than I am velocity, because for practical purposes, the short barrel will get about the same velocity, regardless of the powder I use. I have not found that a deer or moose can tell the difference of 100 fps one way or the other.

So each rifle gets a work up to find the powder/charge that gets the best accuracy.

But there is one other consideration. A 20" or less barrel can have a terrible bark that is painful on the ears. For a hunting rifle (no muffs) I want to use a "quiet" powder. I might have to try 2 or 3 to find the one that does not bark.

 

[BCBRAD]

In the 7mm-08 example the difference in exit pressure is 2000 psi between a 20 to 24 inch bbl, or an increase with the shorter barrel of 27%......this equates to more 'bark' for the short barrel.

 

[rick357]

You work up a load / bullet combination for your rifle. You don't make up a bullet / load combination and then start cutting off your barrel an inch at a time to see if it works better. . .

 

[bigedp51]

I have a Winchester 94 Trapper model with a 16 1/2 inch barrel and a Savage Hog Hunter 308 with a 20 inch barrel.

The biggest thing to contend with these short barrels is muzzle blast and recoil. And people complaining sitting at the next shooting bench getting more blast and noise.

If you look in the Lyman reloading manual you can look at the rifle section and then the shorter barrel Contender pistol section for comparisons.

After I read your post I looked up the Contender data for the .308 with a 15 inch barrel, and Varget was the most accurate powder with a 150 grain bullet.

You can also shoot reduced loads for longer shooting sessions if the recoil bothers you. And you can do this with H4895 or Trail Boss.

308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″)
https://rifleshooter.com/2014/12/308-winchester-7-62x51mm-nato-barrel-length-versus-velocity-28-to-16-5/

Loads to make the rifles recoil pad softer.
I ruptured a disk in my neck and had two disks fused together and recoil can irritate my neck, and reduced loads will make the shooting sessions longer and more enjoyable.

 

[Mr. Friendly]

I do understand that weight affects recoil, but if you were to use a shorter but thicker contour barrel and keep approximately the same weight, that shouldn't be an issue, correct?

also, would not using a faster powder that would burn out before exiting a longer barrel not be beneficial on a shorter barrel, where it doesn't run out?

 

[rral22]

I do understand that weight affects recoil, but if you were to use a shorter but thicker contour barrel and keep approximately the same weight, that shouldn't be an issue, correct?

also, would not using a faster powder that would burn out before exiting a longer barrel not be beneficial on a shorter barrel, where it doesn't run out?

First part is true. As to the second part, I will repeat and reinforce those who already said that the powder that produces the highest velocities in a long barrel will almost always produce the highest velocity in a shorter barrel. What other "beneficial" things are you looking to find?

 

[Mr. Friendly]

simple explanation of why it's like that...not just being told it's like that.

 

[jethunter]

I do understand that weight affects recoil, but if you were to use a shorter but thicker contour barrel and keep approximately the same weight, that shouldn't be an issue, correct?

Yes. But a couple ounces more or less won't make enough difference to matter.

also, would not using a faster powder that would burn out before exiting a longer barrel not be beneficial on a shorter barrel, where it doesn't run out?

Maybe correct if you're comparing extremes, for example comparing a 12" bbl with a 24" bbl. But for regular hunting-rifle length rifle barrels if the powder is suitable for the cartridge you can assume the burn is complete before the bullet exits. The expanding gas are still pushing the bullet after the powder is burned which is why the longer barrel produces higher fps.

 

[BCBRAD]

I do understand that weight affects recoil, but if you were to use a shorter but thicker contour barrel and keep approximately the same weight, that shouldn't be an issue, correct?

also, would not using a faster powder that would burn out before exiting a longer barrel not be beneficial on a shorter barrel, where it doesn't run out?

In the 7mm-08 example 95% of the powder is consumed at ~13" down the barrel, this is for Varget. Muzzle pressure ~9500 psi.

If you used H4350, which is too slow for the application , 95% burn is at 19+ inches down the barrel. Muzzle pressure is 10,226 psi in a 20" bbl.

You will not run out of barrel using compatible powders......with ultra slow powders for application you may.

Also, powders that burn 95% of charge in the first 8 or so inches promote throat erosion more than say one that is 15"+ down the barrel.

 

[Mr. Friendly]

thank you Brad n Jet...that helps me understand. much appreciated.

 

[Andy]

CGN has come a long way in the past 10 years or so with regard to accepting the fact that rather than needing to use a fast powder for short barrels and needing a very long barrel to get a "complete burn" of slower powders, it's simple: the load (x grs of a given powder) that gives the highest MV with a long barrel will produce the highest MV in a short barrel.

Regardless of what "Quickload" suggests, all of the powder that will burn at a given pressure ("full burn"), is burnt in a mere few milliseconds, which translates into at most about 4" from the boltface. In some small cartridges (e.g. 9mm Luger), Pmax can occur before the bullet has even fully left the cartridge. You can pinpoint where "full burn" occurs, because it coincides with "Max Pressure" (Pmax) on a graph. Pressure declines consistently from this point as the bullet travels down the barrel and increases the volume behind it. The bullet will continue to accelerate while in the barrel until the residual pressure is overcome by the friction of the bullet. This continued acceleration is not because the powder is "still burning", it's because there's still sufficient residual pressure to accelerate it. The "report" (noise coming from the barrel) is a product of the volume of gas and its pressure, so if you want to reduce it, (aside from noise suppressors) reducing muzzle pressure is the most effective path. Using a longer barrel works as well, because for a given load, a longer barrel will have a lower muzzle pressure than a shorter barrel, and moving it farther from your ear reduces the "apparent report".

 

[MRP]

At some point of barrel length, bore diameter and pouder capacity plus volume of expansion reaches its max. Berrel friction starts slowing it down. Even .444, 45-70 gain little and depending on pouder lose after 24 to 26" pistol cartridges like .44 max out at about 18 to 20".

 

[BCBRAD]

CGN has come a long way in the past 10 years or so with regard to accepting the fact that rather than needing to use a fast powder for short barrels and needing a very long barrel to get a "complete burn" of slower powders, it's simple: the load (x grs of a given powder) that gives the highest MV with a long barrel will produce the highest MV in a short barrel.

Regardless of what "Quickload" suggests, all of the powder that will burn at a given pressure ("full burn"), is burnt in a mere few milliseconds, which translates into at most about 4" from the boltface. In some small cartridges (e.g. 9mm Luger), Pmax can occur before the bullet has even fully left the cartridge. You can pinpoint where "full burn" occurs, because it coincides with "Max Pressure" (Pmax) on a graph. Pressure declines consistently from this point as the bullet travels down the barrel and increases the volume behind it. The bullet will continue to accelerate while in the barrel until the residual pressure is overcome by the friction of the bullet. This continued acceleration is not because the powder is "still burning", it's because there's still sufficient residual pressure to accelerate it. The "report" (noise coming from the barrel) is a product of the volume of gas and its pressure, so if you want to reduce it, (aside from noise suppressors) reducing muzzle pressure is the most effective path. Using a longer barrel works as well, because for a given load, a longer barrel will have a lower muzzle pressure than a shorter barrel, and moving it farther from your ear reduces the "apparent report".

On the Quickload pressure curve there is also a Z1 line, this line will be to the left of Pmax line if the powder is to fast and on the right if the powder is too slow for the components used.

The Z1 line is there to illustrate were in the process that the powders deterrent chemicals are consumed. Ideally this should occur at Pmax but some powder/bullet/cartridge combinations wont do that.

The pressure push after the Z1 line is considered the 'fizzle' stage of the powder burn and that fizzle may produce inconsistent results.

With QuickLoad a pressure curve that has the Z1 line overlayed on the Pmax line and on a vibration node (Ms) will be as good as it gets so long as the mechanics of the platform and the shooter are up to it.

 

[Longwalker]

I have several rifles that have both short and standard barrel length in the same chambering. If accuracy is equal, and velocity is acceptable, and therefore the only other consideration is muzzle blast or apparent "noise" I tend to use somewhat faster powders in my 20" barrelled rifles vs the others at 24" length. As an example, my 24" 30-06 gets good velocity and accuracy with H or IMR 4831 powders, but for my 20" barrel 30-06 I may use 4350 for the same 180 gr. bullet. In my .308 rifles, moderate burning rate RL15 or Varget works well with 165's in longer barrels, faster IMR 3031 in shorter. But if the faster powders did not produce adequate velocity or accuracy in the shorter barrels, i'd hunt with the slower powder and not worry about noise, since I always wear hearing protection, even while hunting. Note I said "adequate" - only you can determine what fits your needs, but I don't always go for maximum.