Cocking effort

[HectorMedina]

Hector,

Thats good intel, thanks.

Would it be possible to give us some/any details on how you go about "tuning for optimal stability" ? 

Thanks again.

Tuning for Optimal stability implies shooting a lot with the gun and defining what is the best pellet for the rifle and at which velocities. It also starts from the premise that you are capable of making your gun shoot to any MV with a standard deviation of 4 fps or less in a 20 shot string.

In some rifles you will find that the pellet that shoots excellently at 800 fps will not shoot as well at 900 fps or, viceversa.
So, all tuning should start with a good knowledge of the specific gun and its likes/dislikes.

Then, from all the possible MV's you need to establish at which MV the pellets are most stable, that means that they will "go to sleep" while rotating as exactly as possible on its axis without any extraneous motions (Nutation, Precession nor Yaw). You can do this in two ways:
1.- Shoot at long range (55 yards) at rice paper squares (rice paper is the paper that is used to roll cigarrettes), analyze the tears in the paper and see which tears are more evident than others, note the MV at which those pellets were shot (usually it takes at least 5 shots at each MV)
2.- Get a laser flashlight and locate it near the muzzle pointing at the target. Shoot and observe the pellet's path (not as difficult as it sounds with a good quality scope focused to the mid-point of the trajectory). Choose the MV that yields the smallest corkscrews. If you find an MV that shows ABSOLUTELY no corkscrew at all, congrats! you have a fine barrel in your hand! It is not as common as you would think.

You could also get a high speed camera, but that would be more expensive than the laser flashlight.

Now set the gun to that MV and tune the harmonics out of the muzzle jump using whatever harmonics tuner you have.

Once you have done that, you have tuned for "Optimal Stability".

Hope this explains the process in general terms.




HM

[HectorMedina]

Hector,

Very interesting comments. I am not familiar with the notion of "short stroking" or "skinny pistons". Is there a thread or article somewhere that describes how it is done? How does a skinny piston get involved in short stroking? Skinny and short don't seem to be directly related terms. I would seriously like to understand more about these spring pistons. If I can use a D48 platform to achieve the level of cocking effort I want and also provide an accuracy potential that I like that would be great.

Short stroking and sleeving (skinny pistons) are mutually exclusive. You either use one or the other.

Short stroking is when a piston's stem is made longer than normal so that the stroke of the gun will be shorter than normal.
Some people like to use the OEM pistons and just alter the stem length, I prefer to make my pistons from scratch:



From left to right: Diana 48-54 T-01 trigger Full Power (FP) and Sub-12 ft-lbs (WFTF), Diana 48-54 T-05 Trigger FP and WFTF, Diana 48-56 T-06 Trigger FP and WFTF, Diana 430 FP

Short stroking has a number of advantages:
Quicker shot cycle
Less stress in the spring
Reduced cocking force (what started this thread)
Less wear on the compression chamber

It DOES need a re-inforced steel breach seal because the compression cycle is so fast that the temps reached at the seal can easily "flame cut" the seal.

On the other end of the spectrum, there is the skinny, skirtless piston:



That relies on a reduced diameter piston (22 mm's in this photo) at full stroke to reach the needed swept volume. A smaller piston is also a lighter piston, so lighter springs can be used to achieve the same power.
Usually this also needs altering the transfer port in the compression cylinder.

BOTH methods work well when all the factors are properly balanced. IMHO, the disadvantage of the skinny piston method is the need to restrict the use of the rifle to a relatively narrow set of conditions, of which atmospheric pressure is the main restriction, derived from altitude changes.

Because of this most of my target gun developments have folllowed the short stroking methodology.

Shooting ½"groups at 30 yards from the sitting position is quite possible with all my WFTF FT guns, and they require only 22# of cocking force. Groups at 55 yards usually hover around the 1" and sometimes, when I read the wind well, 3/4".

Hope this clarifies the ideas.

Keep well and shoot straight!






HM

[Harpoon1]

Hector,

Thats good intel, thanks.

Would it be possible to give us some/any details on how you go about "tuning for optimal stability" ? 

Thanks again.

Tuning for Optimal stability implies shooting a lot with the gun and defining what is the best pellet for the rifle and at which velocities. It also starts from the premise that you are capable of making your gun shoot to any MV with a standard deviation of 4 fps or less in a 20 shot string.

In some rifles you will find that the pellet that shoots excellently at 800 fps will not shoot as well at 900 fps or, viceversa.
So, all tuning should start with a good knowledge of the specific gun and its likes/dislikes.

Then, from all the possible MV's you need to establish at which MV the pellets are most stable, that means that they will "go to sleep" while rotating as exactly as possible on its axis without any extraneous motions (Nutation, Precession nor Yaw). You can do this in two ways:
1.- Shoot at long range (55 yards) at rice paper squares (rice paper is the paper that is used to roll cigarrettes), analyze the tears in the paper and see which tears are more evident than others, note the MV at which those pellets were shot (usually it takes at least 5 shots at each MV)
2.- Get a laser flashlight and locate it near the muzzle pointing at the target. Shoot and observe the pellet's path (not as difficult as it sounds with a good quality scope focused to the mid-point of the trajectory). Choose the MV that yields the smallest corkscrews. If you find an MV that shows ABSOLUTELY no corkscrew at all, congrats! you have a fine barrel in your hand! It is not as common as you would think.

You could also get a high speed camera, but that would be more expensive than the laser flashlight.

Now set the gun to that MV and tune the harmonics out of the muzzle jump using whatever harmonics tuner you have.

Once you have done that, you have tuned for "Optimal Stability".

Hope this explains the process in general terms.




HM

Hector

With the right lighting, I've noticed the occasional corkscrewing. It's been so random, I always wrote it off as just an irregular pellet. I also like the "reading the paper tears" idea. I used that often years ago when "tunning" a particular bow and arrow combination. Never thought about using it for checking pellet stability
Good stuff!

Thanks for the thoughtful, in depth reply. It was very helpful.

[dwalk]

my 52 and 54 are not as difficult to cock as is my AA ProSport underlever...