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Messages - rsterne

Pages: [1] 2 3 ... 81
1
Airgun Ammo / Re: .308 cal 106 gr. Airgun Bullet at NOE
« on: November 19, 2017, 09:12:17 PM »
Spitzers are not necessarily much better for BC in the Subsonic range.... The point is very important when Supersonic where there is a shock wave surrounding the nose.... and you want the shape of the nose to be inside that shock wave.... Subsonic things work a lot differently....

Bob

2
Ask Bob / Re: Tuning a Regulated PCP
« on: November 19, 2017, 04:35:06 PM »
As you lower the pressure the plateau velocity will lower as well.... Only if you are below the knee of the curve will the velocity increase with reduced pressure.... If that happens then you can assume you are somewhere on the downslope.... ie more hammer strike would increase velocity....

Bob

3
Airgun Classifieds / Wanted QB78 Tube and Front Cap - Got It !
« on: November 19, 2017, 11:42:25 AM »
For an experimental project, I need a QB78 tube and the front cap that threads over it and retains the CO2 plug.... If you have one you wish to donate, please PM me....

Looks like I have one coming.... Thanks for looking....


Bob

4
Ask Bob / Re: Tuning a Regulated PCP
« on: November 19, 2017, 10:26:35 AM »
Without the ability to try different hammer spring preload settings, it is pretty tough to know where you are on the power curve.... If you are up on the plateau, then reducing preload will decrease air use without a loss in velocity.... If you are down on the downslope, then any reduction in preload will lose velocity....

Bob

5
Airgun Ammo / .225 cal 35 gr. RF & HP at NOE
« on: November 19, 2017, 08:54:52 AM »
New Group Buy at NOE for a 35 gr. GC bullet in .225 cal, and also a smooth sided 37 gr. version.... both with a Lyman HP pin available in one cavity.... http://noebulletmolds.com/smf/index.php/topic,2298.0.html

Bob


6
Airgun Ammo / Re: .357 cal 136 gr. BBT at NOE
« on: November 17, 2017, 11:15:46 AM »
Just a reminder that this is at last call.... http://noebulletmolds.com/smf/index.php/topic,1629.0.html

Bob

7
Airgun Ammo / Re: .308 cal 106 gr. Airgun Bullet at NOE
« on: November 14, 2017, 07:03:05 PM »
According to the Kolbe Twist Calculator, anything 26" or faster should work fine....

Bob

8
Airgun Ammo / Re: .308 cal 106 gr. Airgun Bullet at NOE
« on: November 14, 2017, 03:58:04 PM »
I got my mould today, and it looks beautiful....



It came with three sets of pins, for the HP (above) a flat nose, and a cup point (shallow cone HP).... When I get a chance to cast some (next month?) I will do a report on the diameter and weights as cast....

Bob

9
Airgun Ammo / Need BBT HP Moulds?
« on: November 13, 2017, 08:32:34 AM »
The NOE Store has sold out of the HP versions of the .22 and .25 cal Bob's Boattail Moulds with a Lyman HP pin in one cavity.... If you are interested in seeing an inventory run, please post in this thread....  http://noebulletmolds.com/smf/index.php/topic,2316.0.html

Please state the size(s) that you are interested in....

Bob

10
OFF TOPIC / We Will Remember Them
« on: November 11, 2017, 08:09:30 AM »
High Flight

John Gillespie Magee, Jr

Oh! I have slipped the surly bonds of earth,
 And danced the skies on laughter-silvered wings;
 Sunward I've climbed, and joined the tumbling mirth
 Of sun-split clouds, --and done a hundred things
 You have not dreamed of --Wheeled and soared and swung
 High in the sunlit silence. Hov'ring there
 I've chased the shouting wind along, and flung
 My eager craft through footless halls of air...
 Up, up the long, delirious, burning blue
 I've topped the wind-swept heights with easy grace
 Where never lark or even eagle flew --
And, while with silent lifting mind I've trod
 The high untrespassed sanctity of space,
 Put out my hand, and touched the face of God.

In memory of F.L. John Sterne, D.F.C.... and all those who paid the ultimate sacrifice.... We Will Remember Them !!!

Bob

11
Airgun Ammo / Re: Review of NEO molds
« on: November 10, 2017, 08:31:42 AM »
I have used the .22 cal 30 gr. BBTs on Marmots and Ground Squirrels, and the HP versions are devastating, with accuracy comparable to a good pellet.... I have also used the .25 cal 50 gr. BBTs with similar results....

Bob

12
Airgun Ammo / Re: Published BCs of the various pellets
« on: November 10, 2017, 08:19:53 AM »
Unless the Standard is given that the BC is calculated with, such as G1 or GA, the number is questionable.... When the BC is calculated, it SHOULD be corrected to standard atmospheric conditions, but I'm betting most don't bother.... Notice that anyone can submit data to that list, there is a submission form at the bottom....

The Standards used to calculate the BC with, using the Cd which is the important thing, doesn't accurately reflect pellet drag in the Transonic range (MACH 0.8-1.2).... This leads to differing BC values depending on the velocity.... The GA model is an improvement over the G1 model for pellets, but until we get a better standard the test velocity should be specified as well.... You can bet that some of the incorrect data in that chart is because widely differing methods, or velocities, were used to do the calculations.... For example, the .22 cal Exact Heavy is shown with a lower BC than the Exact, and anybody who has tested those two pellet, on the same day under the same conditions, can tell you that is a crock....

Bob

13
Airgun Ammo / .308 cal 106 gr. Airgun Bullet at NOE
« on: November 06, 2017, 06:28:22 PM »
Brand new design I didn't even know about, now in the store....

http://noebulletmolds.com/NV/index.php?cPath=30_515

RG2 and RG4 moulds available, they come with flat nose (106 gr.), cup point, and hollow point pins (100 gr.)....

Bob

14
Ask Bob / Re: How Barrel Harmonics Can Affect POI
« on: November 06, 2017, 04:39:04 PM »
Wish I had the time.... but I barely have enough time to do the projects I want.... and I don't have enough time to shoot in the summer because of running the Motel.... Thanks for the kind words....

Bob

15
Ask Bob / How Barrel Harmonics Can Affect POI
« on: November 06, 2017, 03:30:41 PM »
There have been some recent discussions about the use of Harmonic Barrel Tuners to dial in a PCP for a given pellet and velocity.... Here are some generic charts showing what happens when the pellet arrives at the muzzle of a barrel that is vibrating in the vertical plane.... The first chart shows an example of the angle the muzzle is pointing, relative to the boreline at rest when the trigger is pulled.... I drew the chart so that a pellet travelling at 900 fps would arrive at the muzzle at the point where the barrel is at its highest.... with that being a bit over 2 MOA (it might be more or less, depending on barrel stiffness)....



You would think that would be the worst possible case, because the barrel is the furthest from its position at rest.... but in fact it is pausing at that moment, because is it reversing direction.... In the second chart (a detailed view of the last peak in Chart 1), each dot represents the time the pellet arrives at the muzzle, with a dot every 10 fps from 800 to 1000 fps.... 900 fps is at the peak, because I chose the vibration frequency to make it so.... You will note that there is hardly any difference in the angle the pellet leaves the barrel for quite a large ES either side of 900 fps.... That intuitively might tell you that is the optimum velocity for a barrel vibrating at this frequency.... However, that is incorrect also....



The next chart shows the POI of the pellet (I chose a BC of 0.036 to get these numbers at the various velocities).... If the gun is sighted in at 100 yards when the velocity is 900 fps, then at 800 fps it hits about 6.5" low, and at 1000 fps about 4.2" high.... At 800 fps, a 20 fps ES changes the POI about 1.5".... At 900 fps, it makes about a 1.0" difference.... and at 1000 fps, that 2% ES changes the POI only about 0.7"....



When you combine the flatter trajectory of faster shots with the angle of the muzzle at the instant the pellet leaves the barrel.... you get an interesting result, as shown below (gun again sighted in at 900 fps at 100 yds.).... The slower pellets (eg. @ 800 fps), arriving after the peak of the barrel vibration, shoot lower on the target.... but the faster (1000 fps) pellets, arriving when the barrel is still travelling upwards, ALSO shoot lower on the target.... This increases the vertical dispersion for pellets travelling slower, and decreases it for pellets travelling faster.... Now, for that same 20 fps ES, pellets at 800 fps have vertical stringing of 2.1" (instead of 1.5").... those at 900 fps are still about 1.0".... but pellets at 1000 fps, with that same 2% ES, only have a vertical stringing of less than 0.3" (instead of 0.7")....



It is important that you do not interpret this to mean that faster is always better.... The concept I want you to understand is that by using a movable weight (or changing a weight) attached to the muzzle, you can change the frequency at which the barrel vibrates.... If you want to shoot at 900 fps, then you want the barrel to be on the upswing, but nearing the top of it's movement, when the slowest pellet arrives at the muzzle.... This adjustment is arrived at by trial and error, but can reduce the vertical dispersion of your groups once you find it....

Think about it this way.... The slowest pellet in your shot string arrives at the muzzle when the barrel is nearing the highest point on the swing, but still on the way up.... It is launched on a higher angle, but going slower, drops more.... Conversely, the fastest pellet arrives earlier in the vibration cycle, when the barrel isn't pointing quite as high, but going faster, drops less.... The combination of these factors can reduce the vertical stringing of the shots when they impact a target downrange.... I hope this explanation helps you understand the merits of tuning the harmonics of your barrel for the velocity you are using....

Bob

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