Britannia 2021 power

[ptdunk]

So following on from Robert's modern made Britannia update,

http://www.airgunbbs.com/showthread....ia-2021-update

[IMG][/IMG]

Bear in mind most surviving Britannia's are doing around 7.5ftlbs. you could maybe get it higher with a stronger spring and fettling but it would probably be a bugger to cock and the trigger would get heavier as the spring got stronger, and adjusting the trigger lighter would just reduce sear engagement etc.

I have a used Britannia which I've given a bit of a tune, (new spring, sleeved transfer port, piston sleeve and fitted spring guide) which is doing 6.8 ftlbs but cocks and shoots beautifuly.

I'm not convinced by old or second hand reports of Britannia's doing '10-12ftlbs' and have yet to hear of these powers being measured on a modern chrono.


Before he got his chrono he adjusted the pressure in the gas ram to be as high as possible, whilst keeping it comfortable to cock it.
He assumed it would be quite powerful.
My Britannia has a stroke length of 75mm and a 28mm cylinder and a swept volume of 46181mm2,
His new one has a stroke length of 85mm, has a 27mm cylinder and a swept volume of 48667mm2 so very similar.

When he finally got his Chrono working it turned out it was doing 7.5ftlbs!
He did say it shot very smoothly with hardly any recoil at all.

I was surprised by this but also reassured that Fred Cox knew what he was doing. Maybe that is the maximum it will comfortably do, with the amount of leverage the barrel and cocking lever will give dictating how powerfull the spring or gas ram can be.

I think slow-runners fantastic Zealandia designs with the cylinder at the rear, but with a two stroke cocking system allowing for a stronger spring, is where the Britannia may have evolved to, if it hadn't gone down the swanny in 1908.

Needless to say I have passed on the Zealandia design ideas to Robert as requested. fingers crossed he gives it a go but that's probably asking a bit much!

interesting stuff, and many thanks to Robert for sharing.

Matt

[Shed tuner]

interesting... I feel it should be doing a bit more though...

What are the transfer port dimension please. Also the piston weight ? And what piston seal is he using ?

[Garvin]

Wouldn't you expect it to creep up to over 8 ft/lbs once run in, if it's doing 7.5 now?

[ptdunk]

interesting... I feel it should be doing a bit more though...

What are the transfer port dimension please. Also the piston weight ? And what piston seal is he using ?

Tranfer Port is tapered, about 5.5mm at piston side, 20mm long then tapers off to 3.8mm at the barrel.
He used this 3.8 centre drill which explains the shape. looks like it might be a lot more than 5.5mm at piston side judging by the tool.

[IMG][/IMG]


piston weight:120gms

Piston seal is Werkstof-S https://www.murtfeldt.com/products/p...-greennatural/

This a damaged one to give an idea of the shape.

[IMG][/IMG]

How much difference to power do you think the tranfer port can make?
Mine was 3.5 and taking it down to 3mm dropped the power a touch but sweetened the shot cycle a whole lot.

Cheers,
Matt

[Binners]

Great threads and amazed at the workmanship....

[Shed tuner]

I think the tapered port will be costing at least a foot pound, probably a good deal more.
If it's 20mm long, I'd be starting at 3mm parrallel and working up. Maybe he can just drill and tap, and have a threaded port insert ? Some medium loctite will seal the threads.

120G piston is also just too light for a 27mm bore - although it's probably fine if he wants to keep it at current power levels. With a more conventional port volume, that'd be bouncing pretty badly. I'd look to add 50g to the piston as a starting point. Then again, is the front of the ram in there ? In which case the weight of that (excluding the rod) needs to be added, so maybe it's fine.

Piston seal looks fine - that's unlikely to be the problem, unless the attachement mechanism has created a lot of lost volume in the centre.

So sort the port first, then test with the trio of (or two of the three) JSB pellets (7.3, 7.9, 8.4) and see if there is a pattern of energy/weight, then we can go from there.

[dvd]

I had one more than 35 years ago. I did not know then what I know now and wish I had kept it.
I do recall it being rather powerful (177) with a strong spring, it was not very shootable like that. I think it had two cocking levels and was ok to shoot with the short stroke mode. However it changed completely when the piston was pushed all the way back, power went up, trigger became much harder and the shots were vertically strung. This was probably due to my bad shooting technique since I did not allow for the stronger angular recoil relative to the barrel.
I used to like the tapered barrel, the flip down rear sight and the easily replaceable spring.

[ptdunk]

I think the tapered port will be costing at least a foot pound, probably a good deal more.
If it's 20mm long, I'd be starting at 3mm parrallel and working up. Maybe he can just drill and tap, and have a threaded port insert ? Some medium loctite will seal the threads.

120G piston is also just too light for a 27mm bore - although it's probably fine if he wants to keep it at current power levels. With a more conventional port volume, that'd be bouncing pretty badly. I'd look to add 50g to the piston as a starting point. Then again, is the front of the ram in there ? In which case the weight of that (excluding the rod) needs to be added, so maybe it's fine.

Piston seal looks fine - that's unlikely to be the problem, unless the attachement mechanism has created a lot of lost volume in the centre.

So sort the port first, then test with the trio of (or two of the three) JSB pellets (7.3, 7.9, 8.4) and see if there is a pattern of energy/weight, then we can go from there.

Briliant, thanks Jon.

[ptdunk]

However it changed completely when the piston was pushed all the way back, power went up, trigger became much harder and the shots were vertically strung. This was probably due to my bad shooting technique since I did not allow for the stronger angular recoil relative to the barrel.

I didn't know you had one David, it's funny what you said about the vertical stringing at high power.....

I did some slo-mo footage that I didn't include in my Britannia Video. From the side it showed that at full power the recoil was enough to dislodge the thumb catch back for a fraction of a second, which allows the barrel to drop. It happens so fast you can't see it normally. on my nice Britannia it does it so consistently that it doesn't matter, it's still ridiculously accurate.

Here you can see it go back on the frame the trigger is pulled back.

[IMG][/IMG]

On my scruffy one I'm tuning up I put a stronger spring behind the thumb catch for a firmer closure, but I bet it does it as well.
Robert said he noticed when he made the gas ram stronger the barrel was dropping on each shot and the poi was lower.

I wonder if a bit of air escapes? I'll hold down the thumb catch and see if it has higher power...

Cheers,
Matt

[ptdunk]

I'd be starting at 3mm parrallel and working up.

Well he doesn't waste time.
he driled it out and put in a 3mm insert so it's 3mm all the way through. Power dropped a bit, tomorrow he's going to open it up 0.1mm at a time,heading towards 3.5mm.

Does he need to think about piston wieght at the same time or just keep opening it up till the power rises?

I suppose in an ideal world you’d just keep opening it up 0.1mm at a time until the power peaked then started dropping. Then remove the insert and re-drill it to the optimum size.

Cheers,
Matt

[slow_runner]

I didn't know you had one David, it's funny what you said about the vertical stringing at high power.....

I did some slo-mo footage that I didn't include in my Britannia Video. From the side it showed that at full power the recoil was enough to dislodge the thumb catch back for a fraction of a second, which allows the barrel to drop. It happens so fast you can't see it normally. on my nice Britannia it does it so consistently that it doesn't matter, it's still ridiculously accurate.

Here you can see it go back on the frame the trigger is pulled back.

On my scruffy one I'm tuning up I put a stronger spring behind the thumb catch for a firmer closure, but I bet it does it as well.
Robert said he noticed when he made the gas ram stronger the barrel was dropping on each shot and the poi was lower.
I wonder if a bit of air escapes? I'll hold down the thumb catch and see if it has higher power...Cheers, Matt

Which is where the Jongman Zealandia excels. The breech barrel lock up is consistent, constant and never budges. Lever that barrel with an extension and you could bent it and still not move or unlock that latch.
The beauty of the over centre adjustable link. Dont you think so?
A break barrel air rifle that does NOT droop and has the constant accuracy of a fixed barrel.
Pull the trigger before latching? The barrel is independent of cocking, ergo no barrel slam.
The stock or grip may but hardly likely as the cocking is one movement and there is no requirement or tendency to finger the trigger . Finger the trigger? An interesting term

[Shed tuner]

Well he doesn't waste time.
he driled it out and put in a 3mm insert so it's 3mm all the way through. Power dropped a bit, tomorrow he's going to open it up 0.1mm at a time,heading towards 3.5mm.

Does he need to think about piston wieght at the same time or just keep opening it up till the power rises?

I suppose in an ideal world you’d just keep opening it up 0.1mm at a time until the power peaked then started dropping. Then remove the insert and re-drill it to the optimum size.

Cheers,
Matt

yes, he defo needs to think about the piston weight. with a 3mm port and the very light piston, it'll bounce horribly. this is why it's insightful to test with some light and some medium weight pellets - and JSB are idea, as the pellets are as similar as can be, with only the weight changing.

I'd not change anything until he can do the variable pellet weight test. my guess is around 3,2mm port and +50g of piston weight. he may also be able to up the ram pressure a bit.

what I'd expect him to find (as the power has dropped a bit) is that it favours the lighter pellets. so before touching the port, I'd add some piston weight. this should give a quite significant power increase. then retest. recoil will go up a bit. he should then be able to add some ram pressure, without making it harsh, as the smaller port will cushion the secord forward piston stroke, and the extra piston weight will have reduced the distance by which it's bounced back too.

It'll take a few iterations, but not many...

ATB - JB

PS He may not want to, but I'd also try it with a spring.. they do behave quite different, and it could well be easier to cock swith a spring for a given power output.

[dvd]

Hi Matt,
I never looked into all the possibilities why it was stringing at high power, at the time I put it down to the extremely amplified dynamics of a strong spring pushing a piston angularly to the barrel, causing it to flip downwards. If it was consistent at anything it was always shooting lower than the point of aim on high power, lock up was not an issue
Come to think of it, there are more angular arrangements that seem to work fine such as the Walther lp53, but the weaker spring still allows for decent grouping with those dynamics at that power.
Perhaps there is a limit to how strong a spring can be before recoil that is not in line with the barrel starts to take its toll on point of impact. This is wher the fine tuning with ports and weights comes in to dampen recoil to more acceptable levels.
It was interesting to learn that you saw the latch moved rearwards, perhaps indicative of the substantial forces at play when the barrel and cylinder are not in line. It could be that the mating surfaces of the latch and the barrel are worn down to the point that there is only the latch spring pressure holding it over the barrel without actually forcing the barrel down.
I do recall that mine did not go all the way forward and over the breech hump when closed and lock up was very solid.
A stronger latch spring might overcome the inertia of the latch, but lockup needs the latch to actually force the barrel down.
A wedge detent with a lever to collapse the spring, a la Diana 65, would be foolproof.

[ptdunk]

yes, he defo needs to think about the piston weight. with a 3mm port and the very light piston, it'll bounce horribly. this is why it's insightful to test with some light and some medium weight pellets - and JSB are idea, as the pellets are as similar as can be, with only the weight changing.

I'd not change anything until he can do the variable pellet weight test. my guess is around 3,2mm port and +50g of piston weight. he may also be able to up the ram pressure a bit.

what I'd expect him to find (as the power has dropped a bit) is that it favours the lighter pellets. so before touching the port, I'd add some piston weight. this should give a quite significant power increase. then retest. recoil will go up a bit. he should then be able to add some ram pressure, without making it harsh, as the smaller port will cushion the secord forward piston stroke, and the extra piston weight will have reduced the distance by which it's bounced back too.

It'll take a few iterations, but not many...

ATB - JB

PS He may not want to, but I'd also try it with a spring.. they do behave quite different, and it could well be easier to cock swith a spring for a given power output.

Ok he's on it.
Started opening the transfer port and power went up, 8.2 ftlbs at 3.2mm, 8.4ftlbs at 3.3mm, but less consistent but at this stage he's only using superstars.
He's paused now to order the jsb rs, express and exact in 4.52.

Turns out with the top hat on the end of the gas ram strut the piston is about 218g which sounds heavy enough.

any more power in the gas strut and its difficult to cock, I doubt he'll want to put a spring in but I'll mention it.

So when do you know when to stop opening the transfer port? Surely you have to go too far to know when velocity has peaked? Then put in a fresh plug and 're-drill it?

I'm quite excited that the power is going up.
Poor bloke's in for a frustrating wait for those pellets

Thanks,
Matt

[Shed tuner]

I'd defo not touch the port until you do the pellet test.

218g piston weight is a little on the light side for a 27mm cylinder. I find a 25mm TX is optimal at around 200g, but that's with more like 38cc of air, so less likely to bounce... but it's in the right ballpark for sure. It may be it's just 20g or so short of optimal.

ref the TP size, yes, you can go too far and then revert.. but the pellet test will tell you in advance