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PostPosted: Thu Dec 25, 2014 7:46 pm 
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Woah, those are some serious rifles you're modifying there Jon!! If I ever get a $4k rifle I doubt I'd have the courage to take a drill to it!


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PostPosted: Thu Dec 25, 2014 7:55 pm 
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Location: England.
If beefing up any barrel the sleeve needs to be a good fit and bonded on the entire length.
It will still flex at weakest point being the breach.

Just so happen to find some 18mm o/d aluminium with 15.1mm bore for 15mm barrels done for a totally different exercise on a certain production rifle that's problematic.


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PostPosted: Thu Dec 25, 2014 8:26 pm 
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Location: Victoria, BC and Clarkston, WA
EverHopeful wrote:
I'd be interested in trying the carbon fiber approach. What inner diameter do you use and how do you glue it? I can see 11 and 12mm ID on ebay at fairly reasonable prices. Not sure if the 11 mm could be fitted over a 7/16 barrel, but 12 sounds quite loose?


11mm will be a tight fit so I will probably make a reamer up from and old 7/16" reamer and weld a piece of drill rod on it to make it a metre plus long. then a little bit of two part epoxy to hold it, forever, in place.

Good thought Kim on using a threaded receiver and barrel if you are using Belleville washers under tension.

I was thinking of a tensioned barrel was a WWII Machine gun that had three external rods and a plate at the barrel and receiver end to hold the barrel in tension. That may turn into a tuning nightmare, constantly trying to tweak the three rods!

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Pardini K10
Brocock Concept .22
3 Custom Crosman 2260
Benjamin Discovery .22 w/Joe Hickey stock!
Crosman 150 pistol .22
4 Crosman MK 1 pistol .22
Mrodair CP-1M .22 and .177


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PostPosted: Thu Dec 25, 2014 10:32 pm 
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Big problem with three rods and a machine gun barrel that gets so hot is the barrel growing and slacking off the adjustment to near zero.... That's one advantage to the Bellevilles, a small length change from a difference in expansion between the barrel and the shroud won't make as much difference in tension as a simple torqueing of the muzzle "nut" between two solid chunks of metal....

Bob

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Airsonal;
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Proud Member of the 2000+fps Club!


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PostPosted: Tue Jan 27, 2015 5:13 pm 
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My Belleville washers arrived today, and I already had the shroud ready.... Here is a photo of what it looks like on my Disco Double....

Image

The barrel is a 7/16" OD Crosman, threaded 7/16-20 NF, and the shroud is 7/8" x 0.035" 6061-T6 Aluminum with the appropriate plug at the muzzle to hold it concentric with the barrel but slide on it.... The Bellevilles I used are 0.045" thick, I am using three in series, and it takes 495 lbs. to collapse them flat.... I marked the point of the nut that was at the top when the nut just touches them, and I can only tighten the nut one flat with my fingers.... With a wrench it's not a problem, of course, and I have at least a turn and a half of useable travel available.... The nut is tightened 1/2 turn in the photo, so the tension is about 150 lbs....

I plan to tighten it a flat at a time and see what happens....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Tue Jan 27, 2015 5:37 pm 
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Location: England.
Bob weigh the nut.

Slightest movement on the nut due to 20TPI thread will see a lot of change for little movement, that's why I run 0.75mm pitch or 1mm.
If you can control systematically assuming nut weighs 12 to 18g, will see 4 distinct group patterns your only looking at revolving the nut 1/2 of a rev. So 180 degrees divide by 4 = 45 degrees for each test shot pattern.
May find it shoots low BDC don't worry or adjust nut so that same applies at TopDeadCentre about 1/2 a rev.

Look forward to the results then we can decipher.
Notice whilst shooing how the firing cycle feels, let me know.


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PostPosted: Tue Jan 27, 2015 9:58 pm 
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Here are some initial test results with two pellets, JSB 15.9 gr. Exacts and JSB 18.1gr. Heavies.... With my Disco Double I have the ability to change the velocity over a wide range very easily by varying the amount the retractable bolt pulls back to clear the barrel port.... My testing is limited to 20 ft. indoors until the snow clears in a couple of months, so I tested for shift in the POI at various velocities.... I did the tests with the 7/16" OD Crosman barrel free floated, and again with it in tension, with the shroud free floated, and then with the shroud supported about 60% of the length out from the breech by a barrel band with an O-ring.... Group were 5 shots, and sizes were overall tighter with the barrel in tension, and slighty tighter again when supported by the O-ring barrel band, but to be truthful they are all just small ragged holes, so much further distances are required to evaluate the group sizes....

However, it was amazing to watch the group wander horizontally and vertically across the paper as I changed the velocity.... With the Exacts, the group moved 0.26" to the left from 638 fps to 871 fps, and then 0.11" back to the right at 1048 fps.... Once tensioned, the total movement left to right shrank to just 0.04".... The Heavies had 0.18" of horizontal movement with velocity, but it also was cut to just 0.04" when tensioned.... The vertical movements of the groups was about half the horizontal movement with both pellets, but it was reduced to about a third of the movement when tensioned, less than 0.04" with both pellets.... Here are the results....

Image

The vertical axis is the distance the group moved from it's average position in inches, and the horizontal axis is the velocity.... The upper graph is the vertical movement of the group, and the lower graph is the horizontal movement.... The solid lines are with the barrel tensioned, and the dashed lines are without the shroud, just the 7/16" barrel floated on its own.... The red lines are the Exacts, and the blue lines are the Heavies.... Overall, the barrel has only about one quarter of the deflection as velocity changes once tensioned, so just on that basis the experiment is a screaming success as long as the groups were not worse, and that is certainly the case, they are definitely tighter as well.... There is no doubt in my mind that tensioning the barrel in this manner drastically tames the whippy Crosman barrel....

I did some testing with the Heavies at 980 fps, with various tensions on the Bellevilles, both with and without the shroud support by the O-ring barrel band, and there is significant side to side, and some vertical, movement of the groups as I change the tension.... and as the group size changes as well, but not by a lot.... It appears at it's worst to be as good as before, and at best basically a tiny 1/4" hole.... Both with and without the O-ring 1 turn of tension on the Bellevilles (about 300 lbs.) seemed to be a sweet spot, which is why I used it for the POI testing....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Tue Jan 27, 2015 10:58 pm 
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That looks very promising indeed!


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PostPosted: Thu Jan 29, 2015 7:14 am 
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rsterne wrote:
The best method I have seen so far for stiffening thin barrels is carbon fiber tubes glued on.... but to get a 1/2" barrel up to 3/4" takes two layers and they aren't cheap (but they are STIFF)....
Bob


What if the first layer were to be an aluminum tube epoxied to the barrel with the carbon fiber tube being the final layer. An alternative to the tube could be a fibreglass wrap.
Delrin sleeve??

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PostPosted: Thu Jan 29, 2015 9:08 pm 
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When gluing on a carbon sleeve, you want something as stable as possible, delrin, or spacers, would allow the barrel and outer tube to move relative to each other.... I suppose you could use aluminum, but aluminum and carbon in contact can set up a dandy battery if they touch and get wet.... I'd sooner stay with steel or carbon, and if you are trying to save weight, then stay with the carbon....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Feb 06, 2015 12:54 pm 
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Location: England.
Bob have you some sort of high speed camera to record those results?

If so, at the point where its zero shift at muzzle does barrel flex towards the breach banana shaped. Sorry just noticed you've sleeved the barrel cant check.
Another way might be if you could rotate barrel say 90 degrees and retest. I think its flexing in the weak area along the barrel by tensioning. So if nip up nut its bending the barrel whichever way it wants to go?


Can you do two tests, this time leave power set at one level don't touch.
Just adjust the nut in 5 degree increments (less better) and record what happens.
Same again no sleeve and disc springs just adjust the nut in same way. Know you will be surprised!


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PostPosted: Fri Feb 06, 2015 9:58 pm 
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No high speed camera....

Barrel has a port in the bottom, not able to rotate it....

Meaningful tests with slight adjustments in tension and nut position will have to wait until the snow is gone and I can shoot outdoors....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Feb 06, 2015 11:06 pm 
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Cool plan Bob. Be nice to see the accuracy of the bbl tension at say 20 yrds, :?: seems a good starting point for a test. Whats your thoughts. We got time :D there still snow everywhere... :lol: :lol:

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PostPosted: Sat Feb 07, 2015 12:34 am 
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Three sets of tests, using 18.1 gr. JSB Heavies, which I know shoot well in this barrel in the mid 900s....

1. Set the velocity to the mid 900s, vary the barrel tension 1/12th turn at a time (30*) and record group size....
2. Using the best tension setting from above, try different velocities of 50 and 100 fps faster and slower and record group size....
3. Using the best velocity from above, repeat test one and see if the optimum tension changes....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Feb 08, 2015 5:12 pm 
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Location: England.
Bob you will need at least 1/24th of a rev increments even better 1/48th due to the TPI of barrel thread.
Need to fire off 5 shot groups at each position at range. I would suggest 50 yards.

If rotate nut 1/36th a rev, next one 1/24th you may have missed the sweet area and the 4 distinct patterns that will develop.
Once found that fine tune for best group which on your pitch thread your looking at sub 1 degree movement.
Personally I cant see how you can accurately rotate each position repetitively, these things your looking at moving the nut back and forth assuming it weighs 12 to 16gr max 0.7mm to see 4 shot patterns. So 0.7mm divide by 4 = 0.175mm, fine tune 0.01mm!


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