Airsporter RB2 tune

[Oddbob08]

hmm.. that just sounds like it's the wrong piston - maybe worth sticking up a dimensioned sketch of yours ?

I'll just go get my measuring irons and box browny out and see what I can come up with.

I've just spoken to John Knibbs (not the man himself) and they weren't aware of any different pistons. Neither have I had time to see if the latching rod can be unscrewed to increase it's length, fnurr fnurr.

[severnsider]

Much talk of the above, but why do you consider the interior of the pellet skirt, the length of the transfer port, and the distance between the transfer port and the pellet to be "lost volume"? Surely it's simply "volume additional to that of the compression tube/cylinder" and as such is exactly the same air as that in the cylinder which is compressed and used to shift the projectile along the barrel?
Or is it the view that this comparatively small volume is somehow responsible for absorbing energy that would otherwise be transferred directly to the projectile once the piston starts to move? If so, I don't get that idea either, as it's still the same air which is in the cylinder and the volume of air in the cylinder still has to be compressed in any case to get things moving.
I can see an argument for wishing to reduce so-called "lost volume" if the piston stroke was so tiny and the barrel so long that the piston had reached the end of it's travel a LONG time before the projectile exited the barrel (e.g. the combined volume of the barrel and transfer port was greater than the swept volume of the cylinder) but otherwise I'm lost........
Am I missing the blindingly-obvious (quite possible, I know!) or is it more of a subjective issue?
Please educate me!

[evert]

The pressure that propels your pellet up the bore is highly dependant upon the ration between the swept volume and the volume that transfers this air to the pellets. Small changes can have large effects.

[severnsider]

The pressure that propels your pellet up the bore is highly dependant upon the ration between the swept volume and the volume that transfers this air to the pellets. Small changes can have large effects.

I shall have to take your word for it Sir! I had mistakenly assumed that the pressure in the compression tube = pressure in the transfer port = pressure in the barrel, but rather obviously (I suppose) the transfer port restricts the flow so that the compressed air is used to best advantage for the length of barrel, calibre etc. I'm still failing to grasp the significance of a long transfer port though. One might argue that a 600mm long barrel also represents a lot of lost volume compared to a 300mm barrel. I shall go away and do some reading on the subject!

[evert]

A very important factor in the behaviour of a piston rifle is the pressure buildup before the pellet releases and accelerates.
Transfer port volume, length and diameter affect both flow AND compression buildup..

[Oddbob08]

A thread I came across on the collectable thread suggests that BSA fitted grub screws to the tp of some rotary breeched rifles, mine doesn't appear to have one but they have been found on all three calibres used by BSA. Screwing this in or out affects the pressure in the tp and ultimately the power.

Going back to my problem, if I may (please say if I'm 'hijacking a thread'), I've been to the shed and measured my RB2 piston.

Front to rear of the piston itself is 134.9mm, the slot is 100.7mm long and ends 14.8mm before the tail-end of the piston. The rod extends 56.6mm past the end of the skirt and the bevelled cut (which latches with the sear) is 11.3mm from the tip. The piston diameter is 28.8mm at it's widest. I forgot to measure the cylinder of the gun, specifically how far past the cocking slot in the cylinder the cocking slot in the piston goes.

I can't tell if it has a steel top hat fitted or this is just where the cocking rod is secured.

[look no hands]

A thread I came across on the collectable thread suggests that BSA fitted grub screws to the tp of some rotary breeched rifles, mine doesn't appear to have one but they have been found on all three calibres used by BSA. Screwing this in or out affects the pressure in the tp and ultimately the power.

Going back to my problem, if I may (please say if I'm 'hijacking a thread'), I've been to the shed and measured my RB2 piston.

Front to rear of the piston itself is 134.9mm, the slot is 100.7mm long and ends 14.8mm before the tail-end of the piston. The rod extends 56.6mm past the end of the skirt and the bevelled cut (which latches with the sear) is 11.3mm from the tip. The piston diameter is 28.8mm at it's widest. I forgot to measure the cylinder of the gun, specifically how far past the cocking slot in the cylinder the cocking slot in the piston goes.

I can't tell if it has a steel top hat fitted or this is just where the cocking rod is secured.

The lump at the bottom of the piston is where the rod is attached to the piston, I had to build up the space around that lump so that the delrin top hat could sit on a flat surface, regarding the adjustable transfer port, I was once told by Tony Wall at SFS that he used to back the screw out so the TP was fully open and then tune the rifle from there, apparently you can then us less spring.

Pete

[Shed tuner]

regarding the adjustable transfer port, I was once told by Tony Wall at SFS that he used to back the screw out so the TP was fully open and then tune the rifle from there, apparently you can then us less spring.

Pete

I'd agree with that, but you still have excess lost volume, hence the need to sleave for greater efficiency

But high lost volume and a grub screw choked flow is a really bad idea, so backing out the screw and reducing spring helps a bit...

[gary_c_ham]

what oversize O'rings did you use on the rotating breach?

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

[Shed tuner]

what oversize O'rings did you use on the rotating breach?

sorry I couldn't tell you the spec, I just went through my fairly large box of o-rings

( i use the same tecnique when selecting springs - folks on here have given up being annoyed by my useless inormation on spring specs "from the box" too)

[MartynB]

Could you convert one to use a 77 like sliding cyclinder ?

[Shed tuner]

Could you convert one to use a 77 like sliding cyclinder ?

That is my plan sometime... see post #7 onwards ( I know it's a long time ago !)

[MartynB]

That is my plan sometime... see post #7 onwards ( I know it's a long time ago !)

Be interested to see how you get on

[Phil Russell]

Your piston issue is interesting. If the rod will not latch on the trigger sear then either something is stopping the piston going back far enough or the rod is too short, or both. I have reread the topic and you said you started with a bag of bits. Do you know for certain that the piston is for this rifle? I assume not.
So ... can the piston latch with just the piston and the trigger block being used? Or have I missed your comment on this? Clearly if the rod will latch then there is enough length available on the rod beyond the end of the piston. So the fault llies in something preventing the piston getting to this point. If the rod is too short at this point then either there is something on the trigger block impeding piston travel or for some reason the rod is too short. If the latter, can you rule out the possibility that the piston is the wrong one for the RB2?
If I remember correctly, Airsporter end blocks come apart. Has it been apart and assembled 'out of sync' with the screw threads? Has someone messed with it? Is it the correct end block or could it be from another model? I am not sure from Chambers pictures if the trigger block parts are common to all models. Maybe your piston is from another model?
Does anyone have a working RB2 and could measure the piston and let you know. Might be worth it to see if someone can provide a definitive, working spec for the piston.
Cheers, Phil

[Oddbob08]

Your piston issue is interesting. If the rod will not latch on the trigger sear then either something is stopping the piston going back far enough or the rod is too short, or both. I have reread the topic and you said you started with a bag of bits. Do you know for certain that the piston is for this rifle? I assume not.
So ... can the piston latch with just the piston and the trigger block being used? Or have I missed your comment on this? Clearly if the rod will latch then there is enough length available on the rod beyond the end of the piston. So the fault llies in something preventing the piston getting to this point. If the rod is too short at this point then either there is something on the trigger block impeding piston travel or for some reason the rod is too short. If the latter, can you rule out the possibility that the piston is the wrong one for the RB2?
If I remember correctly, Airsporter end blocks come apart. Has it been apart and assembled 'out of sync' with the screw threads? Has someone messed with it? Is it the correct end block or could it be from another model? I am not sure from Chambers pictures if the trigger block parts are common to all models. Maybe your piston is from another model?
Does anyone have a working RB2 and could measure the piston and let you know. Might be worth it to see if someone can provide a definitive, working spec for the piston.
Cheers, Phil

It's doing my head in lol...... If I take the end block I can push the piston in, it will latch. The RB2 doesn't have the threaded section, it is secured by two small allen bolts. I can assemble it without a spring, but the piston washer is not in contact with the breech, and the piston will not draw back far enough to latch. I don't think I've mentioned before, but if I remove the cocking slide and link, I can push the piston back far enough to latch. Stick the spring back in and the piston is too far forward to get the pin in for the cocking link. As there are no new pistons available, I can't just splash out on a new one to try. I am of the opinion that that this is not the right piston.