TX200 piston seal lost volume

[BTDT]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

[cooper_dan]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

I'll be no rush to get away but it's too late, I've already taken the plunge and made a modification.

The best I can describe it is that before, I could shoot the thing all day no problem. After changing the breech seal, the shot cycle felt more 'sudden' through my leading hand. And where my face touches the cheek piece my teeth felt like they were rattling and skin was tingling. It was actually fairly uncomfortable, hence making this change so quickly.

[cooper_dan]

Actually Jim, the modification is reversible. So maybe we could measure it post modification, then strip it down and undo the modification to measure what I thought was 'slammy'. Could be an interesting exercise and I won't be in any rush

[Hawkeye20]

good afternoon cooperdan, if i was you i would put it back to factory as it seems all your "mod" has done is make the gun more bouncy

[cooper_dan]

good afternoon cooperdan, if i was you i would put it back to factory as it seems all your "mod" has done is make the gun more bouncy

Its hard to go back to standard, and I guess I didn't properly explain my reasoning. There are 2 main points:

1- Why did I change the breech seal?
My favourite pellets in this gun are by a clear margin, die 54 JSB exact. I'm talking 12mm groups (10 shots) at 45 yards. They are basically magic in this barrel. However die 54 have a slightly longer skirt than other dies, so they ever so slightly stick out of the barrel (just a fraction)
With the standard o-ring seals, they compress when firing and allow the compression tube face to touch the face of the barrel. Unfortunately this means that fraction of pellet sticking out gets clipped off and leaves lead swarfe behind. This then gets behind the seals/round the comp tube etc and is just a mess.
The one piece Weihrauch seal I've put in there is harder than the o-rings, so the compression tube is always held slightly away from the barrel. I've put about 200 shots through with this new seal and not had any clipping or lead swarfe. So one problem solved.

2 - Why mess with the static compression ratio
Unfortunately the fix above came with the downside of a harsher shot cycle, which to me feels 'slammy' (although I could be dead wrong about this, I just know there was a definite change and I don't like it). I think because I no longer have the squishy o-rings in there to soak up some of the perceived force.
My TX has been short stroked to 80mm. So with the 25mm diameter piston and new breech seal, should be almost identical to an old Weihrauch HW77 or HW97. However its a lot harsher (side by side comparison).
Looking through the rifles, the clear difference to me is that the Weihrauchs have a much shorter and smaller diameter transfer port, and a flat faced piston seal.
Looking at the static compression ratios (simplified calculations), the Weihrauch is approaching 1000:1. My TX200 (including the lost volume in the seal is nearer 200:1. By filling in the seal volume, it rises to 400:1, therefore should get me a bit closer to the HW77/97 numbers.

Basically I'm trying to make small adjustments so that I can keep using these pellets.

[Rickenbacker]

I have no idea what's going on with the different approaches to making the breech seal up (although I've always thought that two o-rings was an unusual idea/solution - therefore KT must have had a good reason for it).

But just a thought regarding the die 54 pellet skirts sitting proud of the barrel - I take it you can't 'thumb' them all the way in - so have you tried a pellet seating tool?

[nick-nic]

Would a very thin shim between the standard o rings help Dan.

[cooper_dan]

I have no idea what's going on with the different approaches to making the breech seal up (although I've always thought that two o-rings was an unusual idea/solution - therefore KT must have had a good reason for it).

But just a thought regarding the die 54 pellet skirts sitting proud of the barrel - I take it you can't 'thumb' them all the way in - so have you tried a pellet seating tool?

I did try a pellet seating tool but it didnt have any kind of depth stop. I ended up pushing half the pellets quite far into the bore. One that limited how far in the pellet could go is worth considering though...

[TonyL]

Its hard to go back to standard, and I guess I didn't properly explain my reasoning. There are 2 main points:

1- Why did I change the breech seal?
My favourite pellets in this gun are by a clear margin, die 54 JSB exact. I'm talking 12mm groups (10 shots) at 45 yards. They are basically magic in this barrel. However die 54 have a slightly longer skirt than other dies, so they ever so slightly stick out of the barrel (just a fraction)
With the standard o-ring seals, they compress when firing and allow the compression tube face to touch the face of the barrel. Unfortunately this means that fraction of pellet sticking out gets clipped off and leaves lead swarfe behind. This then gets behind the seals/round the comp tube etc and is just a mess.
The one piece Weihrauch seal I've put in there is harder than the o-rings, so the compression tube is always held slightly away from the barrel. I've put about 200 shots through with this new seal and not had any clipping or lead swarfe. So one problem solved.

2 - Why mess with the static compression ratio
Unfortunately the fix above came with the downside of a harsher shot cycle, which to me feels 'slammy' (although I could be dead wrong about this, I just know there was a definite change and I don't like it). I think because I no longer have the squishy o-rings in there to soak up some of the perceived force.
My TX has been short stroked to 80mm. So with the 25mm diameter piston and new breech seal, should be almost identical to an old Weihrauch HW77 or HW97. However its a lot harsher (side by side comparison).
Looking through the rifles, the clear difference to me is that the Weihrauchs have a much shorter and smaller diameter transfer port, and a flat faced piston seal.
Looking at the static compression ratios (simplified calculations), the Weihrauch is approaching 1000:1. My TX200 (including the lost volume in the seal is nearer 200:1. By filling in the seal volume, it rises to 400:1, therefore should get me a bit closer to the HW77/97 numbers.

Basically I'm trying to make small adjustments so that I can keep using these pellets.

I know you know what you're doing, Dan. And that you can certainly shoot and you know what you want from the gun. But I think I'd be tempted to increase the stroke slightly, if do-able. Maybe 5mm. And let this slightly larger volume work slightly less hard than the reduced volume. There'd be hardly any difference in piston time. If by later fitting the parachute seal the efficiency was increased, you could then go even lighter on the spring?

[cooper_dan]

I know you know what you're doing, Dan. And that you can certainly shoot and you know what you want from the gun. But I think I'd be tempted to increase the stroke slightly, if do-able. Maybe 5mm. And let this slightly larger volume work slightly less hard than the reduced volume. There'd be hardly any difference in piston time. If by later fitting the parachute seal the efficiency was increased, you could then go even lighter on the spring?

You read my mind. I've got a 4mm lower stroke piston sat in front of me waiting to go The only thing stopping me is that the length of stroke does have a slight impact on my aim points, even if the muzzle energy is the same. I won't pretend to understand why it happens, but it does.

If tomorrow's test doesn't work, this piston will be going into the gun until the new seal arrives.
The shoot tomorrow doesn't matter too much, as you can drop a couple of bad scores over the course of the series. But Sunday there are some major bragging rights up for grabs so I won't take any risks for that one.

[TonyL]

If you're in no hurry to get away after this Sunday's springerfest, Dan, I could record the shot cycle and see what it is that you're feeling as 'slammy'.

As usual from The Gentleman of Springer Deity, what a lovely, generous offer.