Hw99S - Loss Of Power.

[Cedric 132]

After cleaning and polishing the internals of my new HW99S .177 (to try and get some of the 'twang' out of it) I had it chrono'd. 720fps, 8.4 gr pellets = 9 1/2flbs.

 

Added a tophat (approx 1mm) plus a piston sleeve - result - 702fps but very consistant, no more than a 2lb variation. I tried to add a 3mm spacer washer but couldn't get the gun to cock so took it out.

 

It's a bit mystifying. Could the sleeve (plastic and not tight) cause the spring to slow down to that extent?

 

Do spacing washers increase power by adding to the compresion a fraction or could they stop the spring extending to it's full length and using all it's potential power?

 

Is it unusual for a HW99S to be so low powered out of the box?

 

I can't really send it back to Wierhauch now as I've been farting about inside it!

[PJWmids 35]

when i put to many power washers in mine it wouldn't cock. had to reduce them. mine is .22 and after the rifle bedded back in it stayed more consistant. cant comment on the power output of the .177.

[paulk73 151]

if its new it will take time to run in and get leaded. its the lead from the pellets that lubricate the barrel, so it takes a while for the barrel to get a good coating. just be patient. put loads of pellets through it mate.

[oxo 499]

Take the bits out you have added and chrono the standard gun, check the breech seal is not nicked too.

[Elmerfudd 5]

In .177 you ideally want it at around 10.5fpe. I you send me your address I'll send you some power bands to put on the rear spring guide. You'll also find that there's not much room for the tophat with the standard spring guide, reducing it to 70mm gives you more scope in the piston.

[Cedric 132]

Elmer - what do the power bands do and where do they fit? I did check that the spring guide had plenty of room before it got near the top hat. I'm going to take the stock off and mount it securely so I can cock it and try and see spring compression etc.

 

I've ordered a Titan spring in case I don't have any joy fixing this one. As I've fitted a piston sleeve to try and reduce the 'twang' I need a top hat or at least a thin washer to stop the end of the spring chewing up the tabs that hold the sleeve in place.

 

I'm not too bothered about having it in the high 11's as long as it shoots sweet.

[Elmerfudd 5]

They fit over the spring guide at the rear and add preload. You can work out how much you can add by counting the coils on your spring. A Titan spring will increase the power, but may be a bit harsh for such a light gun. A tophat with slip washer is a good idea too as that will make it more efficient.

[Cedric 132]

I made a top hat (about 1mm protuding above the spring) and tried adding a slip washer (approx. 3mm). The rifle would not cock at all. It came to a dead stop and wouldn't lock. I tried to look and see if the spring was fully compresed and it didn't appear to be, so I took out the slip washer and then it cocked but only just, needed an extra tug at the end of the cocking stroke. I thought of shortening the spring a little but, surely, that would defeat the power objective.

[Elmerfudd 5]

That sounds more like the spring guide and top hat meeting than spring binding. The guide is 80mm iirc with only 100mm available in the piston. The piston weight is a fair chunk of what's left.

 

I reduce the spring guide length to 70mm, remove the weight and use a 25mm tophat. Although you lose mass in the piston, the increased acceleration makes up for quite a chunk of that. Added preload takes it further and you end up with around 10.5 with less felt recoil.

[Cedric 132]

I've had it to bits again, a closer inspection showed the top hat and spring guide were pretty close so I reduced the guide by about 10mm. Seeing that I now had plenty of clearance I put the 3mm spacer washer back in, re assembled - it wouldn't cock again !! I sat and stared at it for a while, working out all the dimensions in my head - then a bliding light !!!. You have to cock the trigger to get it back in and I had forgotten to unset it, a click of the trigger and the rifle cocked fine ! What a numpty !

 

I haven't chrono'd it yet, I'll see what it's doing in the morning.

 

I didn't know there was a piston weight. Is it loose up inside the piston?

[SimonC_Here 36]

it's held in place by an o ring up in side.