Testing new discovery

[KnifeMaker]

Kirby, the smaller spru plate hole is great suggestion and works. I make my own. Usually .124


would love a decent indoor range close by. I did go to one near Austin a few years ago in Flugerville. Poor air handeling. Got a lung infection from *(&^!


Still it is 75 miles from here. GRRRR!

[KnifeMaker]

This is the very first group I got testing the device. I said nothing about it being on there. Still Top Secret at the time.


This bullet shoots well, but not normally this well. LOL


Click on pic to bring it up larger. You can see two bullet that were pushed thru the barrel to check bore dimensions.


The bullet is a shortened 257420 from an Arsenal Mold @ 960 fps for 102.55 fpe.     


Mike

[Nvreloader]

Mike
Can you please check your OEM Archery damper, see if there is a small number stamped on the non bolt end.
I stripped the one of my QB, had another air leak again,
so I am getting spec's from both of them.

Here is what I have found so far, 2 different sizes, Long and Short,

Long
3/4" PVC pipe x 10-1/2" long, sealed PVC Caps on each end, one end has bolt for bow attachment
weighs in at 15.40 oz total, (#15 stamped on cap end),

Short
3/4" PVC pipe x 6 -3/4" long, sealed PVC caps on each end, one end has bolt for bow attachment
weighs in at 13.86 oz total, (#14 stamped on cap end),

Both are oil filled, loosened the nut end on the bolt and had oil seepage around the bolt threads.
There is movement inside, can hear and feel it, don't know what inside yet.....

The question I have is: Why the weight difference -vs- length etc?

Tia,
Don

[anti-squirrel]

I think the focus on weight and materials and mass is deviating.  The focal point is coming up with a device that dampens vibration.  Even lightweight dampeners will work as long as they are placed on the barrel to dampen vibration.

[grand-galop]

In my opinion, the weight of the device has not so much to do with dampening... The moving weight inside the device have important role if it matches closely to the weight of the projectile..

[rsterne]

My gut feel is that the more powerful the PCP, the heavier the mass of shot needed to dampen the vibration.... Since our velocities are typically within a narrow range, FPE varies with bullet weight.... There may be a linear relationship between the two, or it could be a square or cube root function, exponential, or something else.... It may well be that if the barrel is stiff enough, they are no advantage.... but on a 7/16" Crosman barrel 24" long a light one may be all you need to work great.... I built two dampers, one with twice the weight of shot.... but realistically they won't be properly tested on a wide variety of guns until I retire in 2021.... By that time, you will all know what works and what doesn't.... 

Bob

[KnifeMaker]

Don, there are no numbers on mine. Remember it was a prototype. Well before they went into production.


So I guess you could say this is my second time testing this device. LOL


Not sure if Mr. Bob is correct. My barrel is extremely stiff. Hanging the 14 oz device and a 18650 battery and Convoy C-8 light on it only dropped the poi by 1/1 mill. at 50 yards. Actually a little less than one tenth mil.


Most barrels would show a full mil. or more.


I would love to tension it. 


Mike

[rsterne]

Mike, by "stiff enough" I meant "crowbar stiff".... like a 1" or larger diameter bull barrel.... I have some barrels here that have 20mm OD CF sleeves on them, and while they are "stiff" I would still expect to see some effect with the damper.... What I was trying to convey is that the more limber the barrel, the more likely it is to have a larger effect.... JMO, having never used one yet....

Bob

[HunterWhite]

In electronics, a capacitor and or a coil and a resistor are used to dampen oscillations. The mechanical equivalent is a spring and a mass and something to tone it down a bit like a shock absorber ( or oil ).  I think that the formulas are the same, only the names have been changed.
First you need to know the frequency of the vibrations ( oscillations ), then the mass of the barrel and whatever you have attached to it. The springyness of the barrel. I should think that you could get an estimate of the harmonics much like a tuning fork. Strike the barrel with a hard object and record that with a microphone,  then display that on an oscilloscope.  That should give the fundamental frequency of the barrel.
The barrel is both a spring and a mass. The springyness may be subdued by tensioning the barrel, the mass can be adjusted with an external added weight.

Forty years ago I would have reveled at the chance to decipher this conumdrum, but the hamster in my noggin is completely unwilling to comply anymore.

[K.O.]

I think it will be  set by the Modulus of elasticity/bore diameter-O.D./length/fpe interdependence's...

plus a choke might throw in a fractal generation type element...

[HunterWhite]

Kirby, good point about the choke,
maybe that's part of the mystery behind choked barrels being unkind to bullets.

Several have suggested that the effect is more pronounced with more power, I agree that more mass accelerating will increase the amplitude of the oscillation.

I know that in a powder burner you can see the seer release, then the firing pin strike the primer, then the primer charge ignition followed by the powder ignition,  then the projectile starts to move, on some guns it may actually stop once or twice.  The vibrations from all of these events will traverse the length of the barrel 5 to 8 times before the projectile leaves the barrel. Airguns are much slower and the barrels are much less rigid.

Brass is a lot less springy than steel,  perhaps that is why the old brass barrels were accurate.

I wonder if anyone ever tried sleeving a steel barrel with a brass or copper tube

[rsterne]

While I think there may be a relationship between barrel stiffness/length/mass and bullet velocity/mass.... I wouldn't even begin to try and calculate it without a decade more knowledge than I have at the moment.... Having said that, I have spent the last couple of days coming up with a working design (actually two) for a coaxial damper that will slide over the muzzle and attach to the barrel with 1/2"-20NF threads at the muzzle.... Here is the smaller one, designed to work with 1/2" to 16mm OD barrels.... Click to enlarge....



The outer tube is 1.25" OD x 0.035" wall aluminum tubing.... The "spool" is turned from a piece of 1.25" OD aluminum rod, the length to be determined by the volume required for the lead shot.... It is first drilled to 29/64" and then drilled out with a 41/64" drill, leaving 1/2" of length to be tapped 1/2"-20NF.... The OD of the spool is turned to be a slide fit inside the tubing, and then the center portion is turned down to 3/4" diameter, leaving an annular space of 0.215" for the lead shot.... The O-ring grooves on the OD are turned to 1.00" diam. x 0.125" wide to fit #120 O-rings.... A mandrel with 1/2"-20NF threads on the end could be used to hold it for turning the OD.... There is a single screw in the side to prevent the tubing from sliding once installed....

There is an internal O-ring groove at the end opposite the threads.... This groove is 0.740" diameter x 0.150" wide.... The size O-ring you select would depend on the OD of your barrel.... For a 1/2" barrel you would use a #206, for a 5/8" or 16mm barrel a #016, and for a 14mm barrels a #113.... You might need a Metric O-ring for other sizes, just select one that gives you a snug fit.... The purpose of this O-ring is just to keep the damper centered on the barrel at the back end, and supply a bit more support.... The mounting is provided by screwing it onto the barrel threads up against the shoulder.... like a "backwards" LDC.... If you wish, you could drill the ID to just clear your barrel, and cut the internal O-ring groove to whatever diameter works for you.... this was a "universal" design....

The internal length would be selected based on the weight of lead shot you wanted.... The volume is 0.65 CI per inch of length.... Using #7.5 or #8 shot (which weighs 4 oz. per CI), and filling it about 3/4 full of shot, would require 1/2" of internal length for each ounce of shot, and LOA of the damper would be 3/4" longer than the internal length.... If you wanted 4 oz. of shot, the damper would be 2" long inside, and 2.75" LOA.... I would use a small funnel to fill it about 1/3 full of oil and then add the shot, while holding it threaded end down, with about a 1/4" gap between the back of the tube and the front of the rear flange.... Shake it to get the air out, and continue adding oil.... If you want it full of oil, you could slide the tube back leaving about a 1/16" gap (small enough to contain the shot) and submerge it in the oil.... When full to the desired level with oil, slide the tube back, install the single screw in the side, and clean it off and you're done.... Install the internal O-ring in the groove, slide it over the barrel, and screw it on until it tightens against the shoulder on the barrel....

I have a similar design using 1.375" OD tubing for 17mm-20mm barrels.... I can post that here if you wish.... Obviously these designs are not intended to be used with a shroud.... They could, however, be reversed, and threaded on to project ahead of the muzzle like an LDC.... Used that way, it could be used to tension the barrel against a shroud, with or without Belleville washers.... It could also be used to just locate the front of a shroud by machining a shoulder on it to fit inside the shroud ID....

Bob

[Hobbyman2007]

Even though it’s a little more complicated to make , why not add a 1/2”x20 ( or 1/20x28 )male threaded joint or use a bushing . This would allow you to screw on your favorite attachment after the device .

[Nvreloader]

Bob

Thank you for that design, It looks sharp etc,

Question for you?
Would it not be better to have a design that is made to slide along the length of the bbl,
to find the node points, for best accuracy?

If it works out that the end of the bbl is the place, you are Fat, Dumb and Happy, as they say,
but, what if the sweet spot is a number of inch's along the bbl etc.

I am basically referring to the limber Gauntlet/Crosman wet noodle type of bbl's etc.

That is what I am testing right now, started at the breach and working towards the muzzle,
in 1" steps, with the 7.46 oz Mercury damper I am using now.

I just have to figure out what amount of weight is required for the best results etc.
I can adapt this Mercury damper, so I can change weights, safely, for the next test/step.

Tia,
Don

[rsterne]

Denis, if you made the front, threaded end twice as thick (1-1.25") and then used a 1-1.5" long threaded bushing, with the front half of the thread to match your LDC, that would work.... Screw on the damper, and then screw the bushing into the (longer) threaded hole, and butt it up against the muzzle.... then screw on your LDC.... I can't see how it could work with the damper ahead of the muzzle....

Don, I think the idea of the damper is to have the weight as far forward as possible, in order to maximize its effect for the minimum weight.... I think the concept is quite different than a solid weight being used to move the position of the node.... The idea, as I understand it, is to "capture" and deaden the vibration by spreading it out over time while minimizing its amplitude.... If you put it at a node, would it in theory not vibrate at all?.... I could be wrong, of course....

If you want to slide it along the barrel, just drill it straight through at the barrel clearance diameter, and use setscrews both ends, with no internal O-ring.... The back end would have to be increase to 1/2" thick instead of 1/4", to allow room for the setscrew behind the outer O-ring.... That modification could also be used anyways if you didn't want the internal O-ring.... This design would not work behind the muzzle on a gun where the air reservoir is full length and close to the barrel.... but perfect for a bottle gun....

Bob

[KnifeMaker]

Mike, by "stiff enough" I meant "crowbar stiff".... like a 1" or larger diameter bull barrel.... I have some barrels here that have 20mm OD CF sleeves on them, and while they are "stiff" I would still expect to see some effect with the damper.... What I was trying to convey is that the more limber the barrel, the more likely it is to have a larger effect.... JMO, having never used one yet....

Bob

Mr. Bob, I agree completely. I wish I could go higher on the dia. However, the LDC I have here from Neill attaches to the shroud. No room in it for even one more tube of cf. GRRRR!!!


Mike

[rsterne]

Stop the larger CF tube just shy of the back of the LDC.... As long as it goes into the receiver it will still do great !!!

Bob

[rsterne]

Here is the larger version, for 17-20mm barrels.... Click to enlarge....



Same procedure applies, except this one is 0.70 CI per Inch of internal length.... so you will only need about 0.47" inside length per ounce, for 3/4 full of lead.... At 1/2" inside per ounce, it will be about 70% full of shot.... The ID is drilled to 51/64", and the three O-ring sizes for different barrel ODs are #018, #115, or #208, fitted into an internal groove that is 0.890" diameter.... I plan to make one like this to fit my CF sleeved 20mm OD barrels, to try instead of the adjustable brass tuning weight....



That tuner weighs 7 oz. complete with the steel mount, which tightens in place on the 1/2"-20 NF thread on the muzzle....

Bob

[KnifeMaker]

Very nice workmanship Mr Bob!

[Hobbyman2007]

Bob,
If you look at reply 138 , I show pics of the damper I made . It’s pretty much identical to what you have drawn . It screws on to the barrel forward of the barrel and at this point I can tell you that I won’t shoot any of the rifles it fits on without it .