Originally Posted by
Shed tuner
A couple of threads have touched on this, and as l've just finished mine, I figured I'd share.
First, it's worth assesing what is wrong with the rifle, and what you hope to acheive, so you can guage success.
accuracy - unusually, this was a relatively low prority for this tune, as this is a looker / plinker
shot cycle - eash - it was nasty. Lots of recoil and noise. Not quite as bad as a stirling HR81, but headed that way. Highest priority to improve.
cocking effort / power - would much rather sacrifice power and have a low effort / calmer cycle - I don't hunt, and am not likely to use it past 20 or so yards
first things first after stripping that stupid maxgrip rail off...
The piston seal was in decent nick, and an OK fit, but on the loose side. Clearly not literally loose, but probably 1lb of force to move it. Experience tells me this is quite power efficient, but doesn't help in acheiving a soft cycle. Now you see why I laid out the parameters above ? So swapped the seal for a slightly tight (but not excessively) Maccarri seal
The TP is long, around 30mm, and 3.1mm. There is also a restrictor, but that will only reduce flow, not lost volume. Also, the system has some lost volume behind the pellet too. I surmised that reducing it as much as possible, whilst still maintaing flow, was required. Once again, experience comes in.. went with 2.7mm (and not just because I found some 3mm OD, 2.7mm ID brass antenna tubing in my spares bin, nope, it was all based on experience). Fitted the brass tube, loctited, machined / tidied up the id, job done.
Whilst I had the RB2 breech out, i replaced the o-rings with some slightly larger. Makes it a little stiff to rotate, but not overly.
Next there's the piston - it's a big lump. Too big. So I cut a couple of slots in it, opposite one another, at 90 degress to the cocking slot. I forgot to note the weight before and after, but I shaved over 50g. I was worried that this might be a bit much, as the 29mm bore needs mass to stop bounce, and with my port restrictions it might cost me too much power. OTOH, it would be easy to add a steel TH to get 25g back if needed.
The spring.. I found a softish spring that was a nice tight fit on the guide. remember power wasn't a factor. I had to add a beer can sleeve inside the piston, as it was a fair bit smaller. I made up a small delrin TH too. Greased up, and assembled.
After settling down, and adding in a couple of slip washers, I ended up with the following.
12 gn hobbies - 10 FP
13.43 gn FAPs - 10.1 FP
14.5 gn LRGs - 9.3 FP
16 gn JSBs - 9.8 FP
Cocking effort I describe as medium - more than it should be for this power output, but not excessive.
Far more importantly however, all the noise was gone - no twang, no crashing sounds, and the recoil was noticably lower, quicker, and "calmer".
I think a 2.8mm port would be spot on, but this is close enough. Just scoped it up this morning, and accuracy is looking fine with Hobbies - well under 1/2" at 15 yards, and it's still settling in. Need to test some other pellets.
Hope this is of use / interest...
Cheers - JB
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