Airsporter RB2 tune

[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

[maximus]

Sounds like a nice result. My two tap loading
Airsporter S's do well with FTT 5.53 for Accuracy so could be worth a go in your RB2. 👍

[Shed tuner]

Sounds like a nice result. My two tap loading
Airsporter S's do well with FTT 5.53 for Accuracy so could be worth a go in your RB2. ��

yes, I'm going to try them, and also some defiant vintage 5.6mm and the ubiquitous superdomes

[Hsing-ee]

The RB2 is the most disappointing rifle ever. I was hoping that it would be an equal to the 77 which the SuperStar nearly was but no way with the RB2, its a clunker.

I can't BELIEVE the transfer port is 3 cm long, isnt that the longest TP in the history of airguns? I mean apart from those trapdoor guns.

Is there any way to machine down the block so its not so Long?

I think this must the the problem with my home made sliding breech Original Diana 50, the TP is too long... as well as there being no seal on the breech and a wierd 45 degree chamfer on the breech which must be alot of dead space where its not needed. Standard cocking effort but only 2.5 fpe!!!!!

So sad the RB2, an expensive rifle of its time, is only capable of being a short range plinker.

[maximus]

But the superstar and the rb2 have similar powerplants so how can one be a contender and t'other be a clunkers??

[Hsing-ee]

But the superstar and the rb2 have similar powerplants so how can one be a contender and t'other be a clunkers??

Something to do with barrel resonance and the trigger? Big Seth had one back in the day and did everything he could to make the RB2 shoot straight and it ended up in the bin I think.

My BSA Firebird PCP with the same breech type arrangement had an accuracy problem, and after a phone call to John Bowkett (gosh I actually spoke to him, and got free useful advice!) I reseated the barrel and it shot very accurately.

Is it a matter of seating the barrel on a 'Sporter?

The BSA Stutzen RB2 is a truly beautiful rifle but horrible to shoot and inaccurate. My old 1973 BSA Meteor can outshoot it.

[Shed tuner]

The BSA Stutzen RB2 is a truly beautiful rifle but horrible to shoot and inaccurate. My old 1973 BSA Meteor can outshoot it.

but does respond to a fairly serious tune, as above . i think it should be accurate too, when it all settles and i test fully.

not much you can do about the TP length, it's intrinsic to the design. you could maybe save 5mm but no more, but you cant really machine back the existing design. i'm tempted to make a sliding breach version from a scrapper action i have someday, just because. and stick a rekord trigger in it.

[Geezer]

but does respond to a fairly serious tune, as above . i think it should be accurate too, when it all settles and i test fully.

not much you can do about the TP length, it's intrinsic to the design. you could maybe save 5mm but no more, but you cant really machine back the existing design. i'm tempted to make a sliding breach version from a scrapper action i have someday, just because. and stick a rekord trigger in it.

So you want to make a crap Pro-Sport?

[WILBA]

So you want to make a crap Pro-Sport?

Pro sports are finiky also. I hated mine as it was hold sensitive as hell and the underlever hurt my hands if i had a good go down the range. Id rather a tx anyday. The prosport inderlever is the most uncomfortable i have used.

[look no hands]

Thank you Jon for putting this info up and for having a go at tuning it, it's nice to read something different for a change and something that is close to my heart, like I said on the other thread, I can't believe how different the RB2 shoots to the Superstar and considering the piston has a centre rod like the TX/97 so in theory should have some sort of shooting characteristics like them, it certainly would be nice to see someone build a superdoopersporter with sliding breech/rotating piston, especially if it's in a beautiful rifle like the stutzen.

Pete

[Shed tuner]

So you want to make a crap Pro-Sport?

it's better, as the lever can be returned before loading.. so much more comfy to use. Shot cycle and trigger are not quite there though

[TonyL]

As always, very concise and informative, Jon.

[TonyL]

But the superstar and the rb2 have similar powerplants so how can one be a contender and t'other be a clunkers??

Hi Maximus, although I have had a few shots with Superstars (and very nice they were, too), I've never stripped one. But have worked on the Mercury and Supersport.

I'm guessing that the Superstar internals are more closely related to the Supersport. These, seemingly, can be made to shoot nicer, especially the break barrel with the shorter TP. Triggers more fiddly to deal with, though.

[look no hands]

Hi Maximus, although I have had a few shots with Superstars (and very nice they were, too), I've never stripped one. But have worked on the Mercury and Supersport.

I'm guessing that the Superstar internals are more closely related to the Supersport. These, seemingly, can be made to shoot nicer, especially the break barrel with the shorter TP. Triggers more fiddly to deal with, though.

Yes Tone the Supersport is identical inside to the Superstar, it's just the cocking linkage is all different.

Pete

[maximus]

Probably the main difference between the RB2 and Superstar is the central latch rod in the RB2 piston making it heavier. The RB2 stutzen I had seemed accurate enough too and I had owned a superstar prior to that.