Pellet Drop

**StellaArtois Sr** · 30-10-2018, 04:01 PM

My HW98, 5 shots spot on then suddenly the next 5 drop about 4 mildots, then they come back up again. I change the breach seal still does the same.

I thought it's probably the PTFE sleeve that I put in it, maybe its catching). I strip the rifle down only to find I hadn't put one in it. I look at the piston seal to seal its got a slightly rough patch on the edge but I have used worse. I check that the TbT guides are not over tight, they are fine. I re grease the rifle put it back together (thinking I should have changed the piston seal).
At the range the first 7 shots spot on, Yeeeeeah I thought then they dropped about 4 mildots again.
It goes through the chrony 11.75 ish for the next few shots then drops to 11.3 oh dear me I think to my self. I have changed the scope just to confirm it's not that. Have tried different pellets and it's just the same and cleaned the barrel. I tried tightening the axis bolt and now it's up and down like a whores drawers. I am perplexed I have a couple of things to try.
My thinking is now that leaves 2 things, the piston seal or the spring.
You may ask, did it do this before I put the TbT kit in, I can't be sure as I had only used it at 20 yards heard it boing and changed it quickly.
Over to you Guys for advice please, thanks in advance, Ade

**TonyL** · 30-10-2018, 06:24 PM

The variation between 11.75 and 11.3 ft.lbs will make very little difference to point of impact, Ade.

Maybe try skimming a tiny amount of depth from the new breech seal and ensure the detent spring is good and the detent itself lubed to ensure consistent lock-up?

**RobF** · 30-10-2018, 08:00 PM

How fast are you shooting it?

**tinbum** · 30-10-2018, 08:10 PM

A teeny, tiny smidgette of a decent lube around the rear edge of the piston seal will fix that.

**Shed tuner** · 30-10-2018, 08:27 PM

whilst that velocity difference in itself would no way cause that large a POI shift, it may be that they are both symptomatic of something that could, e.g. a change in firing cycle, timing, amount of combustion, sealing, etc.

Iffy lockup is definitely an option, e.g. too thick a breach seal - Sunbeam posted a thread not long ago where he has this issue too - a new breach seal cured his.

**T 20** · 30-10-2018, 08:28 PM

Read of this problem before --- it turned out to be the barrel detent Spring.

**tinbum** · 30-10-2018, 08:35 PM

See, if it had been a Diana a poor quality detent or that daft angled breech would have been my first thought, but HW's are usually very good. I guess if it's the 98 the extra weight could have an effect on it's consistency of lockup?

**T 20** · 30-10-2018, 09:02 PM

Originally Posted by T 20

Read of this problem before --- it turned out to be the barrel detent Spring.

Originally Posted by tinbum

I guess if it's the 98 the extra weight could have an effect on it's consistency of lockup?

The full story I was told was of a HW98 that was sold on here for £100 and had been professionally tuned but had the same problem described by the OP --- hence the low price.

Part of the professional tune involved chopping the barrel latch Spring down to make breaking the barrel easier.

It turned out that the cut down Spring was allowing the barrel to drop so lowering the point of impact, the weight of the 98 barrel didn't help --- a new latch Spring cured the problem.

I was told this story by the member who bought the gun for £100 and cured the problem --- he actually offered the gun back to the vendor for £100 plus the price of the latch Spring, but the vendor told him to keep it.

**tinbum** · 30-10-2018, 09:07 PM

Originally Posted by T 20

The full story I was told was of a HW98 that was sold on here for £100 and had been professionally tuned but had the same problem described by the OP --- hence the low price.

Part of the professional tune involved chopping the barrel latch Spring down to make breaking the barrel easier.

It turned out that the cut down Spring was allowing the barrel to drop so lowering the point of impact, the weight of the 98 barrel didn't help --- a new latch Spring cured the problem.

I was told this story by the member who bought the gun for £100 and cured the problem --- he actually offered the gun back to the vendor for £100 plus the price of the latch Spring, but the vendor told him to keep it.

That'll be a stage 9 tune then....

**Shed tuner** · 31-10-2018, 03:04 AM

Originally Posted by tinbum

That'll be a stage 9 tune then....

sounds like a pretty stupid idea to fix a problem I never heard of anyone having !

**StellaArtois Sr** · 31-10-2018, 01:35 PM

Originally Posted by tinbum

A teeny, tiny smidgette of a decent lube around the rear edge of the piston seal will fix that.

Hi Nick, are we talking of Bumslide or silicone grease. I have both so will try that first.

**StellaArtois Sr** · 31-10-2018, 01:40 PM

I have tried 2 different breach seals to no avail. I did check the detent spring and it moved freely enough, wouldn't know if it was over tight to be honest.
Going to try Nicks fix of a bit of lube on the piston seal, so will double check the detent again once it's in bits again.
It's a new rifle so it's giving me the hump I can't take it back under warrenty as I fitted a TbT kit to it straight away.

Many thanks for the ideas Lads it's much appreciated and I will update you soon.

**TonyL** · 31-10-2018, 01:52 PM

Originally Posted by RobF

How fast are you shooting it?

Nice thinking. Piston seal expansion after a few shots to warm up robbing power.

Stock screws all nicely snuggled in?

Also (and I've yet to work on a newer one with the later style front stock screw bracket).......is that secure?

Piston seal lube......moly, not the silicone grease.

**Phil Russell** · 31-10-2018, 02:00 PM

My first thought was '4 mildots is a lot' and so I ruled out straightforward power issues as it seemed to be something to do with barrel alignment with the cylinder. This could be the breach seal, but you say you have changed it. So my bet is, as mentioned above, the barrel lock up. Even though the spring looks Ok it might be weak and allowing some barrel movement on firing at times. I would change the spring, making sure all is squeaky clean with the lock up area and nothing is creating any drag on the plunger or breach face anywhere. I have at times added a small shim to a plunger to good effect in aiding lock up. Could the plunger be worn such that it can rotate a few degrees at times? Could the spring be catching on the plunger? Could the plunger be catching in its recess at times.
Cheers, Phil

**Muskett** · 31-10-2018, 02:17 PM

While going through all the tick boxes to find the fault or faults, don't forget test the scope and mounts. Maybe just try with another scope.

Does it go back on target after a rest, or after a while?
I'd run 50 pellets through it before testing so at least its shot in after open heart tinkering.

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