Spring Gun tuning - Top Hat advice

[Mr.Fixit-Norm]

Hi Folks,
I’ve been tweaking and sorting an old BAM DB4 over the last few weeks ( some may have seen my crazy snakeskin sprayed stock )
I got a chance to try it at a longer range than in the garden last week, and it’s reasonably accurate at 25yrds. ( Open sights ) However, although I originally stripped the spring, piston and compression chamber, cleaned and greased it all up, it still ‘twangs’ quite a bit

It’s running at an average of 8.7ftlbs with an average spread of 10fps, which I’m happy with ( it’s not a hunting gun ). I was reading a bit about top hats, spring guides and sleeves and having a lathe and some suitable Delrin rod I was wondering about
making a top hat for it and whether it would help with the twang ?
I haven’t done any measurements yet but i seem to remember when i took it apart there wasn’t much room between the spring and the piston so I’m not sure if there’d be room for a liner.

The questions I have are, if I do make a top hat, how thick does the end have to be, I assume were it goes into the spring it has to be a reasonably tight fit so the spring rotates on the delrin, also if it’s a reasonable thin end do I have to remove anything off the spring or live with a bit more pre-load? There isn’t a large amount of preload anyway.

i know it’s only a cheap chinese job but from what I’ve read on here and the UKCAF forum they can be turned into a reasonable plinker so any advice would be very gratefully received.

Norm

[Neil54]

A spring guide snug in the spring reduces most of the twang.
The top hat helps with the twist of the spring when compressed and uncompressed.
about 3mm proud of the spring would be usual for a top hat. Doesn't have to be that long about 12- 15 mm overall.
Make sure that spring will still compress without the top hat touching the guide. This is normally done by assembling the rifle with no spring, cocking the gun and making sure the sear engages correctly.

[Mr.Fixit-Norm]

A spring guide snug in the spring reduces most of the twang.
The top hat helps with the twist of the spring when compressed and uncompressed.
about 3mm proud of the spring would be usual for a top hat. Doesn't have to be that long about 12- 15 mm overall.
Make sure that spring will still compress without the top hat touching the guide. This is normally done by assembling the rifle with no spring, cocking the gun and making sure the sear engages correctly.

Thanks for the info Neil, that’ll certainly be useful , sounds like i won’t have to take anything off the spring which will be handy !

[TonyL]

As Neil says, Norm.

Your post didn't make it clear, but if you haven't home made a rear guide already (and if the standard guide is a sloppy fit) do make a rear guide. Also, try and make the rear guide as long as you can (ie so that the guide and top hat don't touch nod interfere with each other) to give the unsupported section of the spring more, er, support.

[Mr.Fixit-Norm]

As Neil says, Norm.

Your post didn't make it clear, but if you haven't home made a rear guide already (and if the standard guide is a sloppy fit) do make a rear guide. Also, try mad make the rear guide as long as you can (ie so that the guide and top hat don't touch nod interfere with each other) to give the unsupported section of the spring more, er, support.

Hi Tony,

Blimey, I thought I stayed up late !

The rear metal block that holds the spring in does have a short ( 1”ish ) guide to centre the spring but it’s nowhere near long enough, so yes I’ll try and extend that somehow. To be honest initially I just wanted to get it cleaned and lubed so that i could put it together and check out the accuracy, unfortunately lock down put a bit of a damper on checking it at anything other than 15yrds in the garden.
It was enough to convince me that it wasn’t too bad and I then concentrated on the stock.

Now i’ve had a chance to try it out at a slightly longer range I wanted to try and ‘refine’ it a bit, if that’s possible.

I’ll get started on it next week as the missus has Sunday and Mondays off so that’s us time rather than me time

Thanks for the advice,

norm

[TonyL]

Hi Norm, having a longer rear guide that fits the spring properly is the main part of the equation to reduce the twang. Be interesting to know how that existing "guide" is fixed in. Don't know if you can post pictures? Hopefully you'll be able to remove and substitute it for a custom, properly fitting guide. Or maybe use the existing "stump" as a base for the new guide?

Hope this helps and it'll be nice to hear how it turns out.