Spring Gun tuning - Top Hat advice

[evert]

That extra material makes it possible to machine it to spec.
You might need to do some fine adjustments on the lathe to eliminate taper when you machine it to size.

[Mr.Fixit-Norm]

That extra material makes it possible to machine it to spec.
You might need to do some fine adjustments on the lathe to eliminate taper when you machine it to size.

Yes you’re right, and it looks like I owe HPC a mental apology, much further down the listing past all the other adverts is a bit about the tolerances and 12mm is +0.2-0.7mm. Looks like I’ve some machining to do

Norm

[evert]

Looks like I’ve some machining to do

Thats the fun part

[Mr.Fixit-Norm]

Thats the fun part

Haha , yes, but only when you get it right

[Mr.Fixit-Norm]

I managed to spend some time in the workshop this afternoon and successfully machined the 12.2mm delrin rod down to 11.95mm, this gave an approximate 0.05mm gap between that and the spring. It wasn’t that easy to do, even using a live tail stock the rod had a tendency to spring away from the cutter when I was machining the middle part. ( I’m guessing that’s what evert was referring to ? )

I overcame this by taking very fine cuts, ending up with a rod that was a nice smooth fit in the spring. I then machined the old guide off the plug and while it was still centred in the lathe, drilled a 10mm hole right through.

I then machined 15mm of the end of the Rod down to 10.1mm which allowed for a tight fit in the plug ( careful application of a mallet )

I trimmed the guide down to 115mm which I guessed to be the maximum I could get away with but that was a bit too close for comfort when cocked so I took another 5mm off.

I found a nice new shiny 2mm thick plated washer which was a perfect fit in the end of the piston, not to tight so I couldn’t get it out but tight enough so it won’t move sideways , with the top hat thickness this gives 5mm of pre-load.

Once assembled with a decent amount of moly grease on the spring, guide and top hat, and back in the stock I found it was marginally harder to cock but not enough to be an issue. When I fired it there didn’t seem to be any twang whatsoever and despite the heavy trigger it seemed smoother and with virtually no recoil !
Not sure if the lack of recoil is a real effect or due to my rose tinted safety specs - lol

I didn’t have a chance to chrono it but I’ll try that tomorrow. I don’t know if what I’ve done will affect the power, I’m not worried if it doesn’t as the exercise was to quieten and smooth it, but it’ll be interesting to see what the result is.

Unfortunately I didn’t take any pics of the spring guide but it looks just like the original one but 110mm long rather than 49mm

Norm

[Mr.Fixit-Norm]

Put a few shots over the chrono today and surprisingly the power is unchanged at an average of 8.7ftlbs

The fps variation is about the same as well, but it’s definitely lost the twang and there is also less recoil, so all is good

I’m currently putting JSB Exact pellets through it as apart from a few old Bisley Magnums, these are the only 0.22 pellets I have. The thing I’ve noticed is the heads on the JSB‘s are a bit tight to get into the breach. They’re 5.52’s and I’m wondering if 5.51’s or 5.50’s would be better, as one of the JSBs I put over the chrono went into the breach much easier and that one pellet gave 9.3ftlbs compared to most of the others which gave 8.5-8.8.

I know it’s not about power but accuracy and they are quite good but I’m concerned that if the heads get marked when they go into the breach, it might affect accuracy.

Any thoughts appreciated.

Norm

[TonyL]

Sounding good and suitably de-twanged, Norm.

If you can, get hold of some RWS Hobby and JSB RS. These tend to give higher outputs in lower tuned springers. If the power isn't uncomfortably close to the limit (doubting it will be), it sounds like you'll have plenty of scope to raise power by increasing the preload, if required. Maybe a 5mm increase to start with and then going in 2mm increments?