Britannia 2021 power

[ptdunk]

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

How do you mean? It’d be great if he didn’t have to go too far and redrill.

Thanks,
Matt

[Shed tuner]

How do you mean? It’d be great if he didn’t have to go too far and redrill.

Thanks,
Matt

he needs to stop drilling now
my gut tell me that any more than 3.2 probably won;t be optimal when the correct piston weight is used.

Simply put, if (all other things being equal, and they are not, but with the JSB domes they are about as close as you can get), if the rifle generates more power with lighter pellets, it's means the piston is bouncing too much. Lighter pellets will move faster and reduce the back pressure (this is why the all things being equal caveat is so important - differnt skirt thicknesses, pellet start pressure etc all influence it), reducing the energy lost through piston bounce.

So if you piston is bouncing too much you can
i) up the preload. Will also give more power - but in this case due to cocking effort, he doesn't want to - that's fine.
2) increase the piston weight. Will also give more power.
3) open the port.. unlikely to give any more than a very small increase in power once you get much above 3mm unless the bounce is horrendous, but it may well reduce the felt recoil.
4) increase spring preload (swap for a longer softer spring, so cocking effort is unchanged, but spring force on the piston when uncocked is higher, reducing bounce). Tricky with a ram, which is why I suggested trying a spring

clearly the reverse of all the above applies if more energy is realised with the heavier pellet.


However... oppening the port is the last thing I'd do when I see the piston bounce, because the smaller ports help cushion the final (not the first) forward stroke of the piston, making the gun softer to shoot. So I'd have left it at 3.0mm, done the pellet test, added piston weight, etc. 3.1 or 3.2 may be optimal, but 0.1mm is quite significant on a long TP.

[ptdunk]

he needs to stop drilling now
my gut tell me that any more than 3.2 probably won;t be optimal when the correct piston weight is used.

Simply put, if (all other things being equal, and they are not, but with the JSB domes they are about as close as you can get), if the rifle generates more power with lighter pellets, it's means the piston is bouncing too much. Lighter pellets will move faster and reduce the back pressure (this is why the all things being equal caveat is so important - differnt skirt thicknesses, pellet start pressure etc all influence it), reducing the energy lost through piston bounce.

So if you piston is bouncing too much you can
i) up the preload. Will also give more power - but in this case due to cocking effort, he doesn't want to - that's fine.
2) increase the piston weight. Will also give more power.
3) open the port.. unlikely to give any more than a very small increase in power once you get much above 3mm unless the bounce is horrendous, but it may well reduce the felt recoil.
4) increase spring preload (swap for a longer softer spring, so cocking effort is unchanged, but spring force on the piston when uncocked is higher, reducing bounce). Tricky with a ram, which is why I suggested trying a spring

clearly the reverse of all the above applies if more energy is realised with the heavier pellet.


However... oppening the port is the last thing I'd do when I see the piston bounce, because the smaller ports help cushion the final (not the first) forward stroke of the piston, making the gun softer to shoot. So I'd have left it at 3.0mm, done the pellet test, added piston weight, etc. 3.1 or 3.2 may be optimal, but 0.1mm is quite significant on a long TP.

Thanks so much for this Jon, that makes sense and I’ve learned something very useful.
I’ve passed you advice on.

It’s a shame he opened it to 3.3mm, I did ask him to hold fire yesterday but he’s keen and didn’t realise the significance fully.

He did test a 10gr pellet which put out more power than the super dome but I told him as it is a different shape it’s not an ideal comparison and to wait for the JSB’s.
With them he can check for ‘piston bounce’ ie. lighter pellets giving higher power than heavy ones and take it from there.

Nice one,
Matt

[Shed tuner]

FYI, superdomes are quite hard, and quite large, so he probably get an extra 1/2 FP just by using something softer (like the JSBs), unless he has a relatively large bored barrel.

[45flint]

The quest for power? Seemed perfect at 7.5? Insanely cool gun!

[ptdunk]

The quest for power? Seemed perfect at 7.5? Insanely cool gun!

I know, but it’s a rare opportunity to fine tune a classic system so we might as well.

[45flint]

I know, but it’s a rare opportunity to fine tune a classic system so we might as well.

You may find that it was fine tuned 100 years ago?

[Jerry Cornelius]

You may find that it was fine tuned 100 years ago?

That would be a very interesting result in itself.

[45flint]

You may find that it was fine tuned 100 years ago?

Just Chronyed my new to me Haenel 3 - 7.6 FPE. So sweet to shoot and Laser. I think my Diane 58 was 7.5 as well when you think it should be 12!

[slow_runner]

You may find that it was fine tuned 100 years ago?

Very probably. Medicine practice and other areas of expertise were also at their peak 100 years ago?

[45flint]

That would be a very interesting result in itself.

Very true.

[ptdunk]

FYI, superdomes are quite hard, and quite large, so he probably get an extra 1/2 FP just by using something softer (like the JSBs), unless he has a relatively large bored barrel.

I’ve got all 3 JSB’s so might see how my own project Britannia is doing.

So is the aim to have it setup so that all 3 weights of pellet generate a similar Ftlbs output? (Different velocities obviously) and then choose based on shot cycle/accuracy etc?

Cheers,
Matt

[Shed tuner]

I’ve got all 3 JSB’s so might see how my own project Britannia is doing.

So is the aim to have it setup so that all 3 weights of pellet generate a similar Ftlbs output? (Different velocities obviously) and then choose based on shot cycle/accuracy etc?

Cheers,
Matt

Yup.
trouble is with the old guns, and two of my britanias fall into this categyory, they seem to have somewhat larger size bores, so it kinda messes with the results a bit. I'd test with the 3 jsbs, plus those superdomes, as a loose bore fit blows any results out the water.

One of my britanias does love JSB though.

[ptdunk]

So just as an update, 7.5ftlbs to 9.5 ftlbs.

The 2021 Britannia started with a power of around 7.5 ftlbs with superdomes, but with quite a large transfer port.
Cocking effort was as much as the owner was comfortable with.

Robert reduced the transfer port to 3.3mm x 25mm and tested with softer JSB's in 10gr, 8.44 and 7.87.
The power actually dropped a bit, but the lighter pellets were producing noticeably more power than the Heavies, apparently indicating a heavier piston would help.

So Robert added 40g to the piston, bringing it from 158g, up to 198g.
This had the effect of bringing the power for all 3 weights of pellet up to 9.5 ftlbs with a nice shooting cycle and good consistancy.

He's very happy and is going to leave it there and move on to finishing the gun nicely.

Armed with this knowledge I am in the process of trying something similar with my project Britannia but using a steel top hat/half spring guide to add weight without further compressing the spring. (Robert used a gas ram so could add weight and keep the preload as he originally set it.)

If it works I'll do a separate post for a possible non-permanent Britannia tune.

Huge thanks to Shedtuner who advised and guided the process, I've learned some very valuable stuff that I hope to put to good use.

I'm very happy we've been able to help this guy out.

Cheers,
Matt

[Lakey]

Very interesting thread this.

Loads of information to work through. Plenty of possible applications when it comes to maximizing the output of oldies.

Lakey