How does chopping a barrel alter velocity nodes?

steffen707

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I know chopping the barrel will reduce muzzle velocity, but does that shift the nodes, require you to find completely new nodes, or does the same grains of powder yield another node, but just at that lower velocity?

My guess is that you need to re-establish a new node because of barrel harmonics changing.

One other question, lets say my 24" bbl 7mm rem mag shoots this 168gr ABLR really well with 62gr of 4000-MR. If i chop 6" off the barrel, the velocity will lower, but will the chamber pressure also decrease?

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Reloader222

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It could be up to 30-40 fps per inch. Difficult to say - it may vary caliber to caliber and bullet weight to bullet weight. My advise is not to go below 22". Otherwise is is perhaps just good to buy another rifle.
 

steffen707

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It could be up to 30-40 fps per inch. Difficult to say - it may vary caliber to caliber and bullet weight to bullet weight. My advise is not to go below 22". Otherwise is is perhaps just good to buy another rifle.
I know it will reduce velocity, but I was wondering about the other points in my post.

"I know chopping the barrel will reduce muzzle velocity, but does that shift the nodes, require you to find completely new nodes, or does the same grains of powder yield another node, but just at that lower velocity?

My guess is that you need to re-establish a new node because of barrel harmonics changing.

One other question, lets say my 24" bbl 7mm rem mag shoots this 168gr ABLR really well with 62gr of 4000-MR. If i chop 6" off the barrel, the velocity will lower, but will the chamber pressure also decrease?"
 

BoltActionBrotherhood

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I'm going to find out soon tomorrow. Two identical gun, one with a 26-inch barrel, and the other with a 16-inch barrel. 6.5 Creedmoor.
I'm estimating a 200 fps velocity lose. Which I load my round on the hotter site so I am not too worried about it, I will be basically be getting close to factory ammo speeds with a normal barrel. This particular load I was getting 3100 fps with my 26 inches, I assume I'll still be at 2900 with my 16 inch. (Barnes 127gr)
I'll check my quick load today and see if that estament matches quickload.
 

Doom2

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Changing barrel length will not affect maximum pressure. Changing the barrel length does affect harmonics. The degree to which varies with the amount removed and the profile. The degree to which this affects the accuracy of a given load varies. It should not affect the extreme spread of a load, assuming the crowns are consistent in both cases.

It seems that most people find similar nodes both in 223 and 308, regardless of barrel length or profile, in different manufacturer’s rifles with different stocks. This tends to indicate that barrel length is not a significant factor in determining accurate loads.

If you consider the standard for factory ammunition in 308, Fed Gold Medal Match, 168 gr, this round shoots accurately in almost every length barrel. In almost every gun!

I would load from a half grain lower to a half grain higher than your present load. You should not need to perform a complete workup.

That said, in the case of rifles using slow powders, the reduction in barrel length can have some major velocity impacts on velocity as these powders maintain higher pressures further down the barrel. In the case of the 7mm Rem, I would not think an 18” barrel would be desirable.
 
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rammac

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The node theory is based on the resonant frequency of the barrel, changing the barrel length changes the resonant frequency and it's harmonics. The longer the barrel the longer the wavelength of the resonant frequency and it's harmonics. That would mean that a load with a particular powder charge in one barrel would need to have a less powder in a longer barrel to match the same accuracy node.
 

BoltActionBrotherhood

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Changing barrel length will not affect pressure. Changing the barrel length does affect harmonics. The degree to which varies with the amount removed and the profile. The degree to which this affects the accuracy of a given load varies. It should not affect the extreme spread of a load, assuming the crowns are consistent in both cases.

It seems that most people find similar nodes both in 223 and 308, regardless of barrel length or profile, in different manufacturer’s rifles with different stocks. This tends to indicate that barrel length is not a significant factor in determining accurate loads.

If you consider the standard for factory ammunition in 308, Fed Gold Medal Match, 168 gr, this round shoots accurately in almost every length barrel. In almost every gun!

I would load from a half grain lower to a half grain higher than your present load. You should not need to perform a complete workup.

That said, in the case of rifles using slow powders, the reduction in barrel length can have some major velocity impacts on velocity as these powders maintain higher pressures further down the barrel. In the case of the 7mm Rem, I would not think an 18” barrel would be desirable.
I would have to imagine barrel length change will affect all nodes. Groups and velocity nodes. I find velocity nodes are not determined by a powder charge per say but by the velocities they produce. Like with a 6.5 cm I know now matter what powder I use, I always see a velocity node at around 2930 fps, and then one at 2910 fps etc.
 

cohunt

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Technically the "barrel pressure" is not changed as the pressure at that specific barrel length is unchanged--- but the actual "uncorking" muzzle pressure increases for the "end" of the muzzle when you shorten it--- this really is only needed if you run suppressors to make sure it can handle the increased pressures from a shorter barrel/muzzle blast.
 

Doom2

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Technically the "barrel pressure" is not changed as the pressure at that specific barrel length is unchanged--- but the actual "uncorking" muzzle pressure increases for the "end" of the muzzle when you shorten it--- this really is only needed if you run suppressors to make sure it can handle the increased pressures from a shorter barrel/muzzle blast.
I corrected my post to “maximum pressure”.
 

steffen707

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That would mean that a load with a particular powder charge in one barrel would need to have a less powder in a longer barrel to match the same accuracy node.
Or with a particular charge in one barrel, one would need to have MORE powder in a longer barrel to match the same accuracy node.

So hopefully on my particular 24" 7mm my highest velocity node is actually underpowered, so that I can use more powder without blowing up the gun, and get back to a higher node. The opposite of course would be that I'd have to go down in grains to find the next lowest one.

I think at the end of the day I need to do testing with the 24" and at 20" and at 18" and see what works.
 

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