HW97K Piston Weight Reduction

[Sqwirl57]

So I'm mostly thinking out loud here but figured I'd write it down and see what the experts think.

Today I re-tuned (snipped 2 coils, nothing crazy) my 97K from 12fpe, down to 10.5fpe with 8.44 JSB Exacts. It shot well which is what I was hoping for but the gun still has that 97K snap to it. For fun, I tried the JSB 10.3gr pellets. Well wouldn't you know it they grouped almost identically and possibly a bit better over a 20 shot group. My big takeaway from that was that the gun felt less snappy and the reticle stayed on target MUCH better through the shot cycle.

This got me thinking that maybe my piston may be a bit heavier than nessasary for those 8.44gr pellets. Would tossing the piston in the lathe and turning away some of the piston fat be beneficial for me. Obviously I don't want to go too light but I can't imagine losing 10٪ or so would do anything other than make the gun shoot nicer.

Any thoughts or concerns?

 

[Bear-of-Grayling]

It seems to me if the piston was lightened the spring would just push it faster. Probably a wash.

 

[Sqwirl57]

It seems to me if the piston was lightened the spring would just push it faster. Probably a wash.

Yes but I feel like that pocket of air under pressure between the piston and pellet would slow it down as well. Less weight, less momentum. Too light (from what I understand at least) and the piston will actually bounce back more off of that built up pressure until the pellet starts moving. Although I may be misunderstanding the "piston bounce" concept.

I'm sure there is a lot of back and fourth between playing with the piston and tweaking the spring though.

 

[Bear-of-Grayling]

I think Weihrauch has the internal design and balance correct. The move to heavier pellets is the key. This is why after 30 years of being a die hard 177 fan and always going to heavier and heavier pellets, I moved into 22 caliber and never looked back.

 

[DIYEngineer235]

Yes but I feel like that pocket of air under pressure between the piston and pellet would slow it down as well. Less weight, less momentum. Too light (from what I understand at least) and the piston will actually bounce back more off of that built up pressure until the pellet starts moving. Although I may be misunderstanding the "piston bounce" concept.

I'm sure there is a lot of back and fourth between playing with the piston and tweaking the spring though.

I've actually been working on a problem similar to this. The problem with what you're thinking is that the "cushion" won't develope as you think. Assuming the hole to the barrel is .177" diameter, that will release all pressure before a cushion can form. The resistance of moving the pellot down the barrel is just too low. I'm still working on the math, but if the opening were sealed, max pressure with a 30lb/inch spring would be reached at around 5 inches. The pellet would start moving at 2-3 inches of plunger travel. The pellet is already out of the barrel by the time the plunger reaches half its travel distance, so at that point, there is no air restriction to form an air cushion.

Now, reducing poston weight will reduce recoil, but also loose some pressure. Removing that mass reduces the kenetic energy of the plunger moving forward. However, it would move faster too, so it would be something of a wash with energy transfer efficiency. The best way to remove mass with minimal impact to the strength of the plunger is to just drill holes in it.

 

[Sqwirl57]

I think Weihrauch has the internal design and balance correct. The move to heavier pellets is the key. This is why after 30 years of being a die hard 177 fan and always going to heavier and heavier pellets, I moved into 22 caliber and never looked back.

Yeah I hear you there. I love my .22 97K for general plinking/hunting purposes. Its been nothing but a great gun for me.

My .177 on the other hand I'm looking to make more of a project out of. Weihrauch definitely gives the gun a good balance out of the factory but I think that is so that it will work well over a wide range of performance. It does well at 10fpe and it does well at 17fpe. There has to be a way to tip the scales so that it does amazing at 10.5fpe and less so at 17fpe though, right? This is my FT gun so higher power is useless to me. My TX is currently outshooting it and I'd like to amend that.

The BEST way that I can think of (because people have already done it) is to lose air volume. Smaller piston, shorter stroke, yada yada. Unless I get real creative with the lathe or drop $300+ on a lost volume kit, I'm sort of stuck with what I'm trying to do.

 

[Sqwirl57]

I've actually been working on a problem similar to this. The problem with what you're thinking is that the "cushion" won't develope as you think. Assuming the hole to the barrel is .177" diameter, that will release all pressure before a cushion can form. The resistance of moving the pellot down the barrel is just too low. I'm still working on the math, but if the opening were sealed, max pressure with a 30lb/inch spring would be reached at around 5 inches. The pellet would start moving at 2-3 inches of plunger travel. The pellet is already out of the barrel by the time the plunger reaches half its travel distance, so at that point, there is no air restriction to form an air cushion.

Now, reducing poston weight will reduce recoil, but also loose some pressure. Removing that mass reduces the kenetic energy of the plunger moving forward. However, it would move faster too, so it would be something of a wash with energy transfer efficiency. The best way to remove mass with minimal impact to the strength of the plunger is to just drill holes in it.

An engineer! Perfect haha. I'm a mechanic by trade so my general system is logical thought followed by guess and check when doing things like this this.

Ok so I don't know the math behind it all but I do have some numbers for you if that will help.

The transfer port is definitely under 0.177" and if what I have found is true, it is actually closer to 0.114". Comparing that hole to a pellet, I believe this to be accurate. The compression tube itself is a total of 6" long but the actual stroke (I believe) is only 85mm or 3.35" long.

 

[Bear-of-Grayling]

Clearly the shorter stroke is a solution to make the firing behavior in a rifle this heavy almost recoilless. Which appears an objective for you. With that optimal accuracy should be achieved. Instead of skeletonizing the piston why not lengthen the trigger engaging bar on the piston to shorten the stroke.

 

[Sqwirl57]

Clearly the shorter stroke is a solution to make the firing behavior in a rifle this heavy almost recoilless. Which appears an objective for you. With that optimal accuracy should be achieved. Instead of skeletonizing the piston why not lengthen the trigger engaging bar on the piston to shorten the stroke.

That is exactly what I did to my TX actually and it works like a charm. I just had to compensate with an additional 2 coils to get the power back up some. Forr whatever reason, longer rods are readily available for the TX but I can't seem to locate one for the 97. I assume it is because the TX has an unnecessarily long stroke whereas the 97 does not.

 

[EMrider]

I’ve always felt that JSB 10.3s had a better shot cycle than 8.4s at any power level. Those are my primary pellet in any springer that shoots above 10fpe.
R

 

[Sqwirl57]

I’ve always felt that JSB 10.3s had a better shot cycle than 8.4s at any power level. Those are my primary pellet in any springer that shoots above 10fpe.
R

I may go that route but this is a gun I use for FT so I am pretty limited for power. At 70fps slower, even with an ever so slight edge for BC, we're talking an additional inch of drop at 50yds. At 55yds, its nearly 1.5" more. That may not seem like a big deal, but when I'm guesstimating my range on a target that is 45mm in diameter that definitely makes things harder.

The gun already shoots the 8.44s very well. Im mostly just pushing my luck trying to get every drop of precision I can get out of it.

 

[Bear-of-Grayling]

Question? Has anyone ever seen or know of a stock with the addition of harmonic dampeners? What a difference they made to the Archery world.

 

[Bear-of-Grayling]

I wonder how a MDT aluminum stock loaded with harmonic dampeners would change the springer world. The receiver could be fastened much more securely to the stock. I have been enjoying shooting 22 long rifle out to 300 yards lately. My Savage B22 Precision has the MDT stock and suppressor. What a difference from a wood stock model. Hitting 3” discs at 300 yards. Take this stock and with the addition of harmonic dampeners like Mathews uses in their bows.....

 

[Bear-of-Grayling]

It was previously mentioned that springers will eventually make their comeback. If so, it will take a breakthrough in technology. Maybe a springer with a Aluminum Chassis and a Harmonic Dampening system. The receiver screws can be tightened to 60 in lbs. I think I will build one. (Borrowed pictures)

 

[thumper]

Alright so first off, lets talk about WHY it felt better with the 10gr pellets. What changed was the bounce point, and the duration of it. Too short, and it gets slammy feeling and too long and it get's bouncy and hold sensitive. Just right, and its smooth and quick like a "snick".

How do the 8.4gr fit in the bore compared to the 10gr? Are they less snug? Fit and sealing make a big difference at these power levels, sometimes more than pellet weight alone. I shoot the hard and tight fitting boxed Crosman Premier Lites. They are lighter weight, but offer good resistance and efficiency. If I pop in a less snug JSB, I can feel the difference (in a bad way) even though an Express is 7.9gr like the CPL 7.9gr.

Lightening the piston only helps when the cycle is slammy to begin with, or it's just too heavy to begin with. Which IMO the 77/97 is not. Too light and it will bounce and you'll be stuck adding weight back or drilling the transfer port to compensate.

Fair warning, that piston shell is harder than woodpecker lips. And keep in mind that the diameter of the piston shell is what keeps the piston straight in the tube, so just reducing it's diameter wouldn't be a good idea. The best bang for the buck if you want to lighten it, is have a slot cut out lengthwise. or two. But IMO it's unnecessary.

The older guns had 25mm pistons. These were inherently slightly lighter, and more efficient due to the smaller diameter taking less spring to push. They can shoot a little sweeter because of this.

My advice, is find the pellet your barrel favors and that you can keep available. And then tune your gun specifically to it and you'll end up as good as you'll get.

Don't be afraid to drill the transfer port if you feel like you have bounce, as the 97 already has a port on the smaller side at 3mm. I run mine at 3.2-3.5mm and have less swept volume with the 25mm piston. My old Venom tuned has a port dang near 4mm! But it also has a 10mm longer stroke, but with a 25mm piston.......i'm going down the rabbit hole here......

 

[Teber66]

I wonder how a MDT aluminum stock loaded with harmonic dampeners would change the springer world. The receiver could be fastened much more securely to the stock. I have been enjoying shooting 22 long rifle out to 300 yards lately. My Savage B22 Precision has the MDT stock and suppressor. What a difference from a wood stock model. Hitting 3” discs at 300 yards. Take this stock and with the addition of harmonic dampeners like Mathews uses in their bows.....

If you want to see a stock that will blow your mind as far as dampening recoil( centerfire) you need to try a Foundation stock if you ever get the chance. They are 100% micarta. Not sure how one built for an airgun would feel but I bet it would dampen it quite a bit.

 

[MBG]

HW97 tune - What would you do? | Piston & Spring

I have recently started shooting a really very nice HW97 that I got from a fellow SWEFTA member, so far I have netted two 3rd places and a win in the SWEFTA spring gun league and now winter is here it is time for a spruce up. Piston is 26mm and most of the internals look standard, it is / was...

shooting-the-breeze.com

 

[Sqwirl57]

Alright so first off, lets talk about WHY it felt better with the 10gr pellets. What changed was the bounce point, and the duration of it. Too short, and it gets slammy feeling and too long and it get's bouncy and hold sensitive. Just right, and its smooth and quick like a "snick".

How do the 8.4gr fit in the bore compared to the 10gr? Are they less snug? Fit and sealing make a big difference at these power levels, sometimes more than pellet weight alone. I shoot the hard and tight fitting boxed Crosman Premier Lites. They are lighter weight, but offer good resistance and efficiency. If I pop in a less snug JSB, I can feel the difference (in a bad way) even though an Express is 7.9gr like the CPL 7.9gr.

Lightening the piston only helps when the cycle is slammy to begin with, or it's just too heavy to begin with. Which IMO the 77/97 is not. Too light and it will bounce and you'll be stuck adding weight back or drilling the transfer port to compensate.

Fair warning, that piston shell is harder than woodpecker lips. And keep in mind that the diameter of the piston shell is what keeps the piston straight in the tube, so just reducing it's diameter wouldn't be a good idea. The best bang for the buck if you want to lighten it, is have a slot cut out lengthwise. or two. But IMO it's unnecessary.

The older guns had 25mm pistons. These were inherently slightly lighter, and more efficient due to the smaller diameter taking less spring to push. They can shoot a little sweeter because of this.

My advice, is find the pellet your barrel favors and that you can keep available. And then tune your gun specifically to it and you'll end up as good as you'll get.

Don't be afraid to drill the transfer port if you feel like you have bounce, as the 97 already has a port on the smaller side at 3mm. I run mine at 3.2-3.5mm and have less swept volume with the 25mm piston. My old Venom tuned has a port dang near 4mm! But it also has a 10mm longer stroke, but with a 25mm piston.......i'm going down the rabbit hole here......

Thanks Thumper, all good info as always!

The main reason my thinking jumped straight to piston reductions was because the pellets feel absolutely identical when getting loaded in. Being that they are both JSBs with identical head sizes and (I'm assuming) very similar, if not the same, composition then a slight reduction in piston weight may may the gun feel a but better with the 8.44s

I have also seen others recommend playing with the TP a hair so I will definitely keep that in mind as I move forward. Thanks again!

 

[thumper]

You could try a different spring setup as well. If you are running a lightish wire spring with a lot of preload now, swap to a heavier wire with little preload.

Or go the other way, depending on where you are now.

That can change the feel a good bit in itself, and is the least invasive thing to change.

Endless tinkering with these things as you know. Keeps it interesting.