Pressure point on the barrel from the forend- accuracy

[mac86951]

I'm just curious from an experimental point of view. You never know, it might work in these circumstances...
Anyways, I'll be trying it hopefully this saturday, weather permitting.
In the meantime I'll leave you all this video.
It kind of reminds me of my mindset in my early attempts at reloading
https://www.youtube.com/watch?v=qNGGlPIBZ6c

https://www.youtube.com/watch?v=qNGGlPIBZ6c

Enjoy

Good point we'll never know for sure if you don't try. Many breakthroughs have been made this way.

-Mac

 

[mac86951]

The idea of pressure points, How does temp and humidity effect pressures?


Ray

We'll focus on one: Temperature->

CTe (coefficient of thermal expansion). Everything expands and shrinks with temperature changes, applying a consistent pressure while controlling this is very tricky to accomplish. Hence free-floated barrels.

Next question is what is accuracy? 3-shots in one hole, 5-shots?, 20?
My experience with pressure on the barrel is in my weatherby vanguard S2 in 30-06.
Factory applied pressure on the barrel to make a manufacturable 3-shot group guarantee, and it'll 5-shot one-hole 180gr SMK at 100y, but guess where #6 goes... Hot days are more difficult.

Great hunting rifle, but not what I'd shoot F-class with, unless the target pullers take minutes.

Varible I'd change first is pressure location (length along the barrel), next variable would be force applied. I'd be willing to bet you'll be able to make a barrel tuner to find accurate nodes for most ammo types, but I'm interested mostly in the results on sequential 5 or 10 shot groups.

-Mac

 

[BigBlueandGoldie]

The idea of pressure points, How does temp and humidity effect pressures?


Ray

We'll focus on one: Temperature->

CTe (coefficient of thermal expansion). Everything expands and shrinks with temperature changes, applying a consistent pressure while controlling this is very tricky to accomplish. Hence free-floated barrels.

Next question is what is accuracy? 3-shots in one hole, 5-shots?, 20?
My experience with pressure on the barrel is in my weatherby vanguard S2 in 30-06.
Factory applied pressure on the barrel to make a manufacturable 3-shot group guarantee, and it'll 5-shot one-hole 180gr SMK at 100y, but guess where #6 goes... Hot days are more difficult.

Great hunting rifle, but not what I'd shoot F-class with, unless the target pullers take minutes.

Varible I'd change first is pressure location (length along the barrel), next variable would be force applied. I'd be willing to bet you'll be able to make a barrel tuner to find accurate nodes for most ammo types, but I'm interested mostly in the results on sequential 5 or 10 shot groups.

-Mac

This is my exact experience. 3 shot groups are great, but it needs to cool down after that. Fine for a hunting rifle, but not an option for a target rifle. Also, you need to be really consistent with your bipod pressure.

 

[rogn]

Ill hypothesize and say that in a controlled situation such as on a bench rifle or machine rest the barrel pressure can act as a "tuner" and as such be beneficial. On some of the lighter sporting rifles of years past used at less than 200 yds the pressure pad may have stabilized some of them to stay on minute of whitetail. My own experience with hunting rifles has been, that when crawling over rocks and stumps and trying to rest the rifle under those circumstances the uneven forend pressure will result in a unpredictably affected "0". Like the comment on sling pressure pulling the barrel pressure off. We seem to know that changing pressure on the fore end can push POI around.

 

[Macropod]

I have rifles that originally had a pad or such in the forearm and all of them ended up bedded and free floated. All shot better and maintained consistency and accuracy. It just adds a variable that is not needed. Some old military rifles are the exception though.

 

[Raythemanroe]

Pressure tuning, A cheap production gimmick.. How many guns on the lines are sporting pads..lol



Ray

 

[fgregorio]

This Saturday, I didn't manage to get the testing done, but next Saturday I will.
I think that before trying something like this, we need to consider a few points.

1- The forend of the stock must be VERY RIGID if you're shooting with a sling (me). If any bending happens on the forend, I guess that you'll be all over the plate.

2- My objective is, if this works, to actually have a delrin forend barrel clamp where the barrel is at neutral tension. Then I intend to vibration dampen the whole front section of the barrel and a bit of the rear section of the barrel with megasorb.

Keep in mind that my motivation here is not to use a cheap accuracy trick on a rig that is already very accurate. My motivation is to.... extend the accuracy nodes in size when doing a ladder test and to increase the accuracy nodes phases when tuning my barrel tuner.
So, to cut a long story short, to make the rifle a lot more tolerant to changes when she is fully tuned. That means loads, batches of powder, proj weight variance in the batch, temperature changes.


Thoughts?

 

[fgregorio]

I finally managed to try out this pressure point on the forend with a delrin block. 7 kg upward pressure. I got the barrel to working temperature without it and fired a 5 shot reference group. Then installed the pressure point and fired another 5 shots.
Due to strong winds, I focused on my elevation spread. The horizontal spread was the same for both groups.
Range: Anzac, Malabar, NSW Australia
Discipline: Palma/ Fullbore
Caliber: .308
Projectiles: Optimus HBC HBN coated, 155 gr weight batched and pointed- G7 0.252
Date 29th of November, 2015
Time 11am
Temp: 24 C
Weather: Sunny
Distance:600mts
Elevation: 30mts from sea level.
Wind: 16 mph from 3 o'clock
Windage: 11 moa

First group (free floating)
0.75 Moa total elevation spread
0.25 moa av between shots

Second group (pressure point)
0.25 moa total spread
0.1 moa spread av between shots

Feel of the rifle:
As soon as I fired with the pressure point, I felt that the rifle went out of tune- I could feel it in the recoil.
The sound of the shot (ring) went up several notes in pitch with a ring akin to a smallbore free rifle.
The zero at 600 metres went up 0.75 moa after applying the upward pressure block.

Next: the rifle will see installed a forend delrin barrel clamp with zero vertical pressure. This clamp is part of the forend of the stock and is screwed into it with 4 bolts. In its design, you can dial any vertical pressure- positive or negative.

Testing will resume in the next few weeks.

 

[fgregorio]

On another note, in the afternoon of that day, I competed with unpointed Optimus HBC projectiles and my 11.120 rd barrel.
First score 50.7V 10 shots in the morning (before the tests to get the barrel to working temperature).
Second score (this one for the interclub competition) 70.8V (14 shots) for a finish well inside the top 10 of the range out of a field of prob 170, highest score in the range was 70.12V by Ricky Ashton Ballistitian of the Australian National Palma Team.

The low shot on the plot was my one and only sighter.

Distance 600 metres
5 ring= 2moa
V ring=1moa

 

[Macropod]

Are you happy with the Optimus HBC projectiles? I have done limited testing with the HBC's at 900m and they held outstanding vertical. I shoot F class standard (an Australian unique class limited to 308 and 223 cal using 155gn and 80gn projectiles respectively for those in the US).

 

[fgregorio]

Very happy indeed.
From early last year I found that the production quality improved through the roof in terms of meplat consistency etc.
I found them, after measuring and weighing batch after batch of each brand, to be superior in uniformity to the Berger 155.5 and the Berger 155 Hybrids both in weight and bearing surface length uniformity.
If you want to really see them go, try this:

1-Point a batch of 500 proj with a meplat pointing Whidden die using sewing machine oil at the tip.

2-Douse the projectiles using spray-on automotive contact cleaner in a bowl and dry them on an old tshirt. This removes all swaging oils and debris- look at the liquid and residue left in the bowl and you'll understand my point.

3-Batch by weight 0.1gr increment batches and storing in a tradies or a fishermans sorting case. No need to batch by bearing surface: the variance is below 1 thou altogether I found then.

4- After loading the ammo, sort the bullets with a bullet comparitor in order to shoot the longest bullet first( less jump/ more pressure/ cold barrel), all the way down to the shortest bullet last( more jump/ less pressure/ hot barrel), .

 

[Macropod]

Will do. I have an older batch that was a little ordinary. They still managed a 5 shot group of 1/6moa (yes, that is correct) at 100m using BM8208 (XBR8208) in load development.

Well worth the effort of getting them to work.

 

[Raythemanroe]

All these exceptional groups are of course if you do your part correct...lol




Ray

 

[fgregorio]

Yep, you need to be able to shoot the guts out of the rifle

 

[Macropod]

All these exceptional groups are of course if you do your part correct...lol





Ray

"If I do my part" has become a catch cry.

 

[fgregorio]

The forend clamp is getting made from today

 

[fgregorio]

I'm running some calcs using Daniel Lilja's article: http://riflebarrels.com/the-ideal-barrel-for-a-308-tactical-rifle/

Knowing that the rifle after installing the clamp has a 20 inch length of the barrel free floating instead of 30 inches, I can calculate the relative muzzle deflection using his formula:
"The basic formula for calculation of muzzle deflection is:

D = (W*l^3)/3*E*Ix

Where: D is the deflection at the muzzle in inches, W is the force or load applied at the muzzle in pounds, l is the free length of the barrel in inches (not including threads), E is the modulus of elasticity or Young’s modulus for the barrel material, and Ix is the moment of inertia for the barrel."

That means that "For a round beam, like a barrel, its rigidity increases with the 4th power of its diameter but decreases with the 3rd power of its length."

So the muzzle deflection on a 30 inch barrel secure clamped leaving 20 inches out is according to the article, and extrapolating the last 2 inches: 0.000787" instead of 0.003223. So it is 4.1 times stiffer.

Also, it retains the velocity of a 30" barrel (mine is at 2976 fps av) with the accuracy of a 20 inch barrel. If it would be 20" the equivalent velocity would be 2780 fps, according to : http://rifleshooter.com/2014/12/308-winchester-7-62x51mm-nato-barrel-length-versus-velocity-28-to-16-5/

http://truthaboutguns-zippykid.netdna-ssl.com/wp-content/uploads/2013/09/13.png


Looking at this graph, keep in mind that less is more= less radial moa, smaller group. Also, look at the shorter ranges where all of the projectiles are travelling at very similar retained velocities. No point looking at the 900 y range. I'd think 300 yards reflects the differences the best in terms of accuracy related to the amplitude of the muzzle deflection (projectile has gone to sleep, precession is minimized because the aerodinamics have taken over the effects of the internal ballistics and the pich up and gyroscopic/ spin drift effects haven't really kicked in).


Lets look at barrel vibration:
The 3rd harmonic (donut shaped expansion shock that travels forwards and backwards the barrel from chamber to muzzle and back) is related to the speed of sound of the metal that the barrel is made of and the chamber to muzzle length of said barrel. That will not change because the barrel is still 30 inches long.

2nd and 1st harmonics are related to the ratio of length vs width of the free floating section of the barrel i.e.- it's rigidity.
So for equivalent energy, Frequency=1/wavelength. Meaning, the barrel will vibrate at 4 times the previous frequency with a quarter of the amplitude. So each stroke of vibration (and that goes to muzzle deflection as well) is only 1/4 of the amplitude of previous set up of this rig.

My guess?

The maths might be right because the barrel was ringing at a MUCH higher pitch when it was being shot. AND the rifle went out of tune (ammo loads).
If the maths stack up in the real world it means that we will have larger nodes, and the rifle becomes more tolerant to powder and proj changes.
Keep in mind that the stock forend is way more rigid than the barrel itself so torsion and pitch momentums are on a different scale and could be negligible for the scale that we are working in.

Now I'm really curious about this...

A BIG Thank You to Lilja Barrels, rifleshooter.com and to Truth About Guns for making this precious information available publicly. You guys Rock

 

[fgregorio]

My forend barrel clamp isn't ready yet for testing but check this out:

Just found this interesting website online. Well worth the read:
http://www.accu-strut.com/index.html


"One interesting comment we have heard is how the Accu-strut significantly reduces the ammo-sensitivity of the Mini. A good thing if you like to use different types of ammunition, or experiment with different bullet weights and powder loads."
http://www.accu-strut.com/pages/accurizing.htm

 

[fgregorio]

Hi mate, I did a lot of testing and fine tuning. The conclusion is that it really works provided that some conditions are met:
1- the stock must have an extremely rigid forend (no fibreglass). Preferably a metal cantilever for best results and it must not be of a material that absorbs moisture (no wood).
2- it needs a clamp above the pressure point.
3- the best clamp material to be delrin because it expands less than other materials with temperature.
4- both the pressure point underneath the barrel and the clamp above the barrel must be vibration dampened (I put a 1mm sheet of rubber both on top and underneath and fastened the whole thing together).
5- the pressure point under the barrel works best with an upward pressure of more than 6 pounds and less than 10 pounds at the point where the pressure point is.

 

[fgregorio]

What you can expect (these are the findings from my experience):
1- the velocity of a 30" barrel with the precison of a 17" barrel (4.3x more rigid).
2- same zeros with iron sights. Those zeroes might change if using glass.
3- extreme tolerance to variations in ammunition. Much wider accuracy nodes.
4- same muzzle velocity as with no clamp.
5- higher pitch noise when firing a shot.
6- barrel tuner: same distance between notches per range (ex: no clamp 300y- notch 3 in tuner, 600y notch 5 in tuner; with clamp 300y notch 4, 600y notch 6).
7- same pressure as before in ammo. I found the rifle to have noticeably less recoil.
8- shoots 4 times tighter groups. I have target screenshots to prove it. It is now the central feature of my system.