How does chopping a barrel alter velocity nodes?

[steffen707]

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.

This brings up another question I had. How many of these nodes are there? You're talking about nodes that are only 20fps apart. I was figuring they would be more like 50-60fps apart.

 

[BoltActionBrotherhood]

This brings up another question I had. How many of these nodes are there? You're talking about nodes that are only 20fps apart. I was figuring they would be more like 50-60fps apart.

That was a typo 2830 and 2910ISH

 

[308ghost]

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?"

I chopped 6" off my 6.5 creedmoor and lost 100fps. The load did not change. As I did another charge test at 600 yards. This was a heavier barreled match and is not noticeably any more or less accurate. I do however have the ability to put a suppresser on and have it be a manageable length. I didnt test any other combinations in this set up. I also just chopped the 24" tikka t3x super lite barrel to 21" (just in front of the flutes) for the same purpose. Again I lost velocity. I seamed to lose less velocity with heavier bullets though. (Fluke? I'm m not sure.) I lost about 60-70 fps with my 168 ablr load. And with the 190 ablr I lost 40-50. With the Berger 185 classic hunter I lost even less! I had to increase my powder charge .1 to maintain the former velocity with those Bergers. (Again I had the can on.) 185 bergers classic hunter 3000 FPS is a good combo. My 190 ablrs are moving 2905 and I'm in the middle of a seating depth test with these ones. All three of these bullets are shooting 3 shot sub moa at 600 yards. I have photos of the 168's ABLR and 185 bergers sub 3" inches. My 190's are super close to that but I'm still tweaking depth. I have detailed notes on this stuff if you interested in more. I know there are a few variables like the first 150 rounds on the barrel and once vs twice fired brass, barrels speed up and settle in about there. Twice fired brass may be a bit faster then new? Hope this helps.

 

[imyourhuckleberry]

The chamber pressure remains the same. What changes is how long that pressure lasts in the chamber since the barrel is shorten so is the length of times the pressure in the chamber lasts, thus the loss in velocity. But depending on the barrel contour whether it is a heavy barrel or a sporting barrel used in hunting the whipping of the barrel is less accentuated now that the barrel is shortened. So depending when the bullet exited the barrel with the original length of the barrel the bullet now exits at a different location with a shorter barrel as it may not flex as much. Hypothetically speaking lets say bullets exit at the peak or top of the flex on both lengths of the barrel, the arc created by the harmonics will be smaller on the shorter barrel. Add to it the loss of velocity caused by the shortening of the barrel the impact will be lower even though all groupings were at the same location. So to answer your question yes you do have to get a new node for your shorter barrel as ballistics have changed.

 

[308ghost]

The camber pressure remains the same. What changes is how long that pressure lasts in the chamber since the barrel is shorten so is the length of times the pressure in the chamber lasts, thus the loss in velocity. But depending on the barrel contour whether it is a heavy barrel or a sporting barrel used in hunting the whipping of the barrel is less accentuated now that the barrel is shortened. So depending when the bullet exited the barrel with the original length of the barrel the bullet now exits at a different location with a shorter barrel as it may not flex as much. Hypothetically speaking lets say bullets exit at the peak or top of the flex on both lengths of the barrel, the arc created by the harmonics will be smaller on the shorter barrel. Add to it the loss of velocity caused by the shortening of the barrel the impact will be lower even though all groupings were at the same location. So to answer your question yes you do have to get a new node for your shorter barrel as ballistics have changed.

Great way of explaining it. Isn't that kinda like the OCW load development? Timing the bullets to exit the barrel, as the barrel is moving but when it slows down, stops and changes direction to move the other way?

 

[BoltActionBrotherhood]

I chopped 6" off my 6.5 creedmoor and lost 100fps. The load did not change. As I did another charge test at 600 yards. This was a heavier barreled match and is not noticeably any more or less accurate. I do however have the ability to put a suppresser on and have it be a manageable length. I didnt test any other combinations in this set up. I also just chopped the 24" tikka t3x super lite barrel to 21" (just in front of the flutes) for the same purpose. Again I lost velocity. I seamed to lose less velocity with heavier bullets though. (Fluke? I'm m not sure.) I lost about 60-70 fps with my 168 ablr load. And with the 190 ablr I lost 40-50. With the Berger 185 classic hunter I lost even less! I had to increase my powder charge .1 to maintain the former velocity with those Bergers. (Again I had the can on.) 185 bergers classic hunter 3000 FPS is a good combo. My 190 ablrs are moving 2905 and I'm in the middle of a seating depth test with these ones. All three of these bullets are shooting 3 shot sub moa at 600 yards. I have photos of the 168's ABLR and 185 bergers sub 3" inches. My 190's are super close to that but I'm still tweaking depth. I have detailed notes on this stuff if you interested in more. I know there are a few variables like the first 150 rounds on the barrel and once vs twice fired brass, barrels speed up and settle in about there. Twice fired brass may be a bit faster then new? Hope this helps.

What was the original length of your barrel for the 6.5cm?

 

[Doom2]

You can search for Chris Long and OBT to read his paper. He actually worked with Dan Newberry of OCW fame. Not the easiest read.

I've been following this subject for the last 11 years or so and believe there are numerous methods but it seems they all end up with similar loads. Lately I seen a proponant with a slightly different approach. I'm beginning to think that it has merit. His approach is to pick a charge that gives the velocity you want then vary the seating depth to tighten the group. I haven't tried it.

I personally use Dan's OCW method because it was the easiest method to employ on my range. Don't have to map shots at 300yds or set up a chronograph.

 

[BoltActionBrotherhood]

You can search for Chris Long and OBT to read his paper. He actually worked with Dan Newberry of OCW fame. Not the easiest read.

I've been following this subject for the last 11 years or so and believe there are numerous methods but it seems they all end up with similar loads. Lately I seen a proponant with a slightly different approach. I'm beginning to think that it has merit. His approach is to pick a charge that gives the velocity you want then vary the seating depth to tighten the group. I haven't tried it.

I personally use Dan's OCW method because it was the easiest method to employ on my range. Don't have to map shots at 300yds or set up a chronograph.

This is how I load, find my velocity and then play with seating depths. You will only see large velocity spikes in seating depth changes if you start getting close to the lands. I also find if you are within .005 of the lands and into jam is when velocity starts to change. If you are farther off you can do this method.

 

[Stammster]

The bigger question here is why you would want a short barrel 7 RemMag.

 

[nocannons]

I chopped 6" off my 6.5 creedmoor and lost 100fps. The load did not change. As I did another charge test at 600 yards. This was a heavier barreled match and is not noticeably any more or less accurate. I do however have the ability to put a suppresser on and have it be a manageable length. I didnt test any other combinations in this set up. I also just chopped the 24" tikka t3x super lite barrel to 21" (just in front of the flutes) for the same purpose. Again I lost velocity. I seamed to lose less velocity with heavier bullets though. (Fluke? I'm m not sure.) I lost about 60-70 fps with my 168 ablr load. And with the 190 ablr I lost 40-50. With the Berger 185 classic hunter I lost even less! I had to increase my powder charge .1 to maintain the former velocity with those Bergers. (Again I had the can on.) 185 bergers classic hunter 3000 FPS is a good combo. My 190 ablrs are moving 2905 and I'm in the middle of a seating depth test with these ones. All three of these bullets are shooting 3 shot sub moa at 600 yards. I have photos of the 168's ABLR and 185 bergers sub 3" inches. My 190's are super close to that but I'm still tweaking depth. I have detailed notes on this stuff if you interested in more. I know there are a few variables like the first 150 rounds on the barrel and once vs twice fired brass, barrels speed up and settle in about there. Twice fired brass may be a bit faster then new? Hope this helps.

Cut 6 in. off my 26 in. hvy. varmint barrel 243 to make it manageable in deer blind. Have not checked speed but accuracy with load i worked up did not change. still under 1 in, @ 100. Hate to use term good enough but thats what fits. Where i deer hunt ( Missouri woods) never shoot that far. If i need to reach out and touch something i have others that are more than capable.

 

[Tripodmvr7]

OBT was mentioned and that is all about barrel time. Accurate nodes occur at intervals of 100 - 200fps depending on barrel length. The shorter the barrel the closer the nodes become.

The 223 is a good candidate to look at. With a 24 inch bolt rifle the highest safe node is close to 3250fps with our propellants and a 55gr Vmax. Pressure calculates at 50K psi in QL. Reducing the barrel to 22 inch the charge needs to be upped and node speed is again 3250 - 60fps. Pressure now increases to 55K psi. For a 20 inch barrel the speed is still close to 3250fps and pressure now at 60K psi.

Shorter barrels are more rigid than long ones and that helps reducing the effect of barrel harmonics. Thicker tubes have the same effect.

Nodes and harmonics are two different animals and occur simultaneously. Node position we can calculate whereas harmonics can not be pinpointed easily. Nodes are just a name conjured up to explain the position of the sound pressure pulse that effects the diameter and shape of the barrel when oscillating from the action to the muzzle. Accuracy nodes are longest and easier to use when the wave is at the knox of the barrel and has the least influence on the muzzle shape and diameter.

Shorter half nodes are found central between the major nodes, but temperature changes can reduce or speed up the bullet and take it out of the band of the node. Thus the shorter the bandwidth is, the easier temps will influence groups.

 

[Rflshootr]

Chopping a barrel won't change chamber pressure, but it will increase the muzzle pressure

Care to explain?

 

[imyourhuckleberry]

Isn't that kinda like the OCW load development?

Exactly! With the optimal charge weight all one is doing is finding the load that allows your bullet type to exit the barrel at same level of frequency or vibration of the barrel consistently. Change the charge by one grain and everything can be thrown askew. Same thing with changing barrel length what ever load you had may now alter the course where the bullet exits during vibration of the barrel.

 

[308ghost]

What was the original length of your barrel for the 6.5cm?

Sorry it was 26" benchmark. We chopped 6" off and finished it at 20"