Smaller longer ports for softer shot feel ?

[TopDog]

David, very very interesting....

I was having a conversation with BlackBeard about the graphs / models, the basis of which was "what should be we looking to achieve" ?

e.g. minimal velocity of the gun at the time of the pellet exit, or minimal displacement or minimal acceleration etc.
I think you have a system now where we can measure guns, and potentially use this to identify the specific characteristics of the ones we subjectively "like".

I'm also amazed how many little bounces there after before the gun comes to rest - cleanly in the first the piston is still moving, but not sure of the others are just the spring moving around.

Hi John

Well. I got sucked into what was going on inside and devised a system to measure piston displacement directly during the firing cycle. This was mainly to try to tie in some aspects of the acceleration traces with piston "crash" or whatever terms one likes to use for the piston hitting something solid (or very nearly solid like a thin layer of VHP air) to try to find the optimum pre-load force. Still am not sure what I mean by optimum since we need the action to be fairly still when the pellet exits for minimum hold sensitivity and larger pre-loads are better for this, but the very high pre-load forces do tend to exacerbate the crash effects thus disturbing the "feel-good" element! This is an ongoing investigation (along with all the others!). Anyway, some configurations clearing show that the piston "crashes" a number of times but correlation of the number of times with the number of acceleration ripples (which itself varies a lot, some of my rifles are extremely tame!) leaves a bit to be desired, scientifically speaking! I have procured a higher resolution data capture device (12 bits rather than 8) to improve the chances of resolving the crash/no-crash situation!

I have done this on two models so far, a HW80 and a HW95 and it involves milling a slot in the cylinder and attaching a contact carrier to the piston skirt which taps off a voltage from a wire parallel to the piston axis and is very repeatable. Hopefully will do a 77 soon too. All gets a bit expensive in ruined hardware eventually! Still, it's cheaper than golf and far better than bowling and watching day-time TV so not complaining!

It is all very time-consuming too but at least provides the quantitative data on which to base logical steps in modifications! So it's make a mod, test and evaluate, take the next step in mods, test and evaluate etc with a clear record all the way!

As a parting shot on this post, the velocity and displacement traces are derived from the acceleration trace and a lot of detail information is lost on the way so my main aim is to try to relate what's on the acceleration trace with what one observes in the real world! I can use wider bandwidth accelerometers to show the higher frequency vibrations too but they tend to mask the shock accelerations and usually end up showing the same velocity and displacement traces with some small amplitude vibration ripples on them so only use these generally to identify vibrations, whether in the action or the sensor system!

Progress is always slow since I have to balance my skills and the capabilities of my Mini-Lathe with my requirements so often sub the difficult stuff which puts a time delay in the loop!

I will also try to tie together a lot of the findings of all our fellow investigators since there are some gems in there! I will post other bits of data from my hundreds of tests to illustrate specific points as we go along!

atvb all
David

[BTDT]

I have experimented with 8.4mm and 12.4mm (both 2.9mm diameter) transfer ports, each with four levels of preload. The piston stayed in the cylinder throughout; only the ports and preload were changed. The rifle was my 1985 HW77 in .177".

From piston release to pellet exit, the only significant difference between the two port lengths was in the recoil displacement (hence the piston stroke), which was around 1.8% greater with the longer port. Post pellet exit, the longer port gave in the region of 1.4% less surge displacement (hence piston bounce), and the piston landed around a millisecond sooner.

With the exception of the lowest preload (23.93 lb/f), the shorter transfer port gave the harsher piston landing. The low preload recoil velocities were virtually carbon copies, apart from a slight deviation immediately following piston landing.

One interesting result of this test was that the post piston landing spring activity was much subdued with the longer port; it would almost certainly feel the more refined. The muzzle energy penalty of the longer port was minimal; hardly worth bothering about.

[slugger]

i bleddy knew it ... a bit of "lost volume" isnt the end of the world in terms of shot feel versus drop in velocity

[TonyL]

I have experimented with 8.4mm and 12.4mm (both 2.9mm diameter) transfer ports, each with four levels of preload. The piston stayed in the cylinder throughout; only the ports and preload were changed. The rifle was my 1985 HW77 in .177".

From piston release to pellet exit, the only significant difference between the two port lengths was in the recoil displacement (hence the piston stroke), which was around 1.8% greater with the longer port. Post pellet exit, the longer port gave in the region of 1.4% less surge displacement (hence piston bounce), and the piston landed around a millisecond sooner.

With the exception of the lowest preload (23.93 lb/f), the shorter transfer port gave the harsher piston landing. The low preload recoil velocities were virtually carbon copies, apart from a slight deviation immediately following piston landing.

One interesting result of this test was that the post piston landing spring activity was much subdued with the longer port; it would almost certainly feel the more refined. The muzzle energy penalty of the longer port was minimal; hardly worth bothering about.

Absolutely brilliant, Jim.

I realise that these findings relate to just one example of one gun, but does seem to answer a lot of questions. I'd been wondering for some time if there may be advantages in any way with longer ports, mainly regarding to firing cycle perceived feel, despite the lower efficiency that may be prevalent.

I sometimes feel that people may sometimes look at how efficient a 25mm 77 is and feel that they have to copy some of its design, the short port being one of them. But it's the overall design, the complete package, that make these so good and not JUST the short port.

Maybe time to cease chasing the "short port holy grail" and not modify? Although further experimentation is always a good thing......just think if the designers and manufacturers had thought that the pinnacle of spring sporting air rifle design had been reached in the very early 80s, before the 77 came!

As I mentioned in (might have been the Mainspring Data Thread, might have been one leading UP to that thread), if all other factors are correctly in place, I think the design of the spring, understanding its power-releasing and surge-combatting characteristics, is the key to further improvement. Many of us lay shooters really don't know, scientifically, enough about this most vital and fundamental component. If only we had a greater cross-section of quality springs, in different configurations, to play with.

[Black Beard]

David

Nice work.

The 80 looks like the computer model I've got but the Longbow doesn't- has it got a silencer on?

The longbow also looks like it has a bit of piston crash as the negative recoil at the end of the first stroke has a spike instead of a smooth dip.

[Shed tuner]

Great info Jim...

Slugger, the prob on a TX is the port is a rather large 3.8mm, not 2.9mm as in Jim's tests, and that amount of volume at 10mm long does make a difference...

[slugger]

thats true....but from my own messing about with various seals it soon became apparant that a bit of lost volume isnt always a bad thing, its vertainly not the be-all and end-all
what i did also find with the flatter faced seals was the consistency was very 'delicate' in terms of temp changes/bit more lube versus bit less lube .. in short ive been drinking single malt and cant remember what else i was going to say hic.......sorry

[HW777]

I have experimented with 8.4mm and 12.4mm (both 2.9mm diameter) transfer ports, each with four levels of preload. The piston stayed in the cylinder throughout; only the ports and preload were changed. The rifle was my 1985 HW77 in .177".

From piston release to pellet exit, the only significant difference between the two port lengths was in the recoil displacement (hence the piston stroke), which was around 1.8% greater with the longer port. Post pellet exit, the longer port gave in the region of 1.4% less surge displacement (hence piston bounce), and the piston landed around a millisecond sooner.

With the exception of the lowest preload (23.93 lb/f), the shorter transfer port gave the harsher piston landing. The low preload recoil velocities were virtually carbon copies, apart from a slight deviation immediately following piston landing.

One interesting result of this test was that the post piston landing spring activity was much subdued with the longer port; it would almost certainly feel the more refined. The muzzle energy penalty of the longer port was minimal; hardly worth bothering about.

Which leads me to ask you Jim,

What is your favourate rifle shot cycle wise ?

[BTDT]

i bleddy knew it ... a bit of "lost volume" isnt the end of the world in terms of shot feel versus drop in velocity

As TonyL rightly points out, this is just one test with one rifle and one pellet, and the result may differ with other rifles, wider transfer ports, or even different pellets. There's a lot more testing to be done before conclusions can be drawn, not least, whether the two port lengths have an effect on accuracy.

Which leads me to ask you Jim,

What is your favourate rifle shot cycle wise ?

When the bulk of my shooting was offhand, I very much favoured a quick action, though these days I shoot from rested positions, and prefer slower actions. Most of my springers are set up to have pretty much the same shot cycle, and I honestly can't single out any as being my favourite on the basis of the shot cycle.

[Black Beard]

Had a play with the computer....

The longer transfer port definately stretched out the time of the cushion of air at the end of the second forward stroke. A smaller diameter transfer port made the pressure higher overall but short transfer ports made it shorter and more intense. I did 1 to 20mm transfer ports on a model of Jon's TX with a slightly longer stroke.

I would have thought a sall transfer port would come out similar to a longer one but it didn't. Don't ask me why.

[Shed tuner]

Had a play with the computer....

The longer transfer port definately stretched out the time of the cushion of air at the end of the second forward stroke. A smaller diameter transfer port made the pressure higher overall but short transfer ports made it shorter and more intense. I did 1 to 20mm transfer ports on a model of Jon's TX with a slightly longer stroke.

I would have thought a sall transfer port would come out similar to a longer one but it didn't. Don't ask me why.

Very interesting again, thanks Andy.

Looks like I wasn't just dreaming this up then

[Shed tuner]

PS Andy, can the effect be replicated by adding lost volume (i.e. to the seal).

Asking as I was just playing with a very high lost volume seal (vac25), and one interesting effect was that I found I could see my pellets in flight really easily, at much shorter ranges than I would normally - sub 30 yards. Thinking it's that second fwd stroke air cushion at play....

ATB - JB

[BTDT]

PS Andy, can the effect be replicated by adding lost volume (i.e. to the seal).

Asking as I was just playing with a very high lost volume seal (vac25), and one interesting effect was that I found I could see my pellets in flight really easily, at much shorter ranges than I would normally - sub 30 yards. Thinking it's that second fwd stroke air cushion at play....

ATB - JB

The obvious solution, Jon, would be for me to get hold of a vac25 seal, and measure the shot cycle.

What rifle was the seal in?

[Shed tuner]

The obvious solution, Jon, would be for me to get hold of a vac25 seal, and measure the shot cycle.

What rifle was the seal in?

I was it in a TX, obviously Although that particular seal has now been machined down to get rid of the lost volume as I needed it for my sleaved down pro Elite, which already has a long port and is somewhat soft shooting.


When I order my vac26 and I'll get a 25 too

[Shed tuner]

Hmm, been thinking about the high lost volume vac25 seal making it easier to see pellets.

I can only think it's because the large protruding lip limits the fwd stroke - and also gives it a slightly softer landing (rather than hitting the almost solid flat face or a regular seal). I guess this is why an o-ring head is even "harsher" than a regular seal - the carrier (normally metal) hits. (All of this is talking about the third phase of piston motion, the second fwd stroke).

I've got all my guns in different but good states of tune now, so I'm going to do some hold sensitivity tests when I get chance to see what effect the different tunes have.