R7 FAC Power Curve

[Jinks]

Hi all, new to the forum and new to Rapids!

I recently purchased my first Rapid, a .22 Mk1 FAC after years of wanting to be a Rapid owner. I carried out a full seal exchange as soon as I got it as many of the o-rings were showing their age, and everything works as it should since doing this. I then got the chrono out and filled the bottle to 200bar to monitor the power curve and to see what the ideal fill pressure would be.
I fired 56 shots over the chrono, but of the whole string of shots, there was no noticeable sweet spot where the power curve flattened, the velocity just constantly dropped shot after shot from the first to the last.
First shot was 921fps/34ftlbs, last was 698fps/19.6ftlbs using 18.1gn barracudas.
I have tried adjusting the hammer spring tension to various settings, but still get no noticeable sweet spot.
Is this something a bit of tuning can sort out, or is this standard for FAC Rapids?
I was hoping to see at least a small sweet spot where the velocity would be pretty constant like on all my unregulated sub 12 rifles.
Thanks in advance,
Jinks

[Jinks]

Power curve graph for reference

[Ca_Varminter]

Sounds like a hammer/valve spring balance problem....
What power level are you wanting to achieve??
A Regulator my be the answer...
But All Road leads to KevG!!!!!

The Rapid Doctor should be able to supply or fix your FAC Rapid!!

Good Luck,
Stuart

[Squinty]

If not leaking through the barrel or external o-rings its what I call a dynamic leak, losing air pressure shot by shot.
Several reasons or combination of.
a) firstly hammer spring adjustment but you say it doesn't make any difference, if was ok before how much power was it making?
b) knackered valve, bent stem or  bad seating
c) worn bottle valve threads, the bottle tilts in the holder when you rest on it

edit... breaking the o-ring seal and leaking at the bottle holder
worse with a bottle mounted bipod and can allow the bottle to touch the barrel throwing out accuracy.

[Jinks]

Thanks for the replies,

Unfortunately i don't know what power it was making before I replaced seals as I didn't fire it due to leaks and condition of seals.
Im quite confident there are no leaks now, but as you state I may have a problem with something I missed (bent stem / valve seatng etc)
All shots were fired free hand over the chrono, so no resting on bottle.
Think I'm going to contact KevG to have a look at it to get the best out of it.

Thanks again,

Jinks

[KevG]

I would turn the power down a bit,you maybe thrashing it.
kevG

[Jinks]

Big thanks to KevG!
After a discussion on the phone, have now got it sorted.
As KevG suggested, the Rapid was being thrashed! It had a double hammer spring in it (apparently fitted as standard on some FAC models), and even when the adjustment screw was turned to minimum, it was still pushing 34ftlbs with the 18gn baracuda's on the first shot and rapidly dropping in power after that. I removed the inner spring and then machined a delrin spring guide to fit the outer spring only, leaving the end cap of the spring guide thicker to act as a spacer so that it could be machined down until the desired power was reached. After a few sessions over the chrono, and reducing the thickness of the spacer in between shot strings, I have now got it running sweet at 30ftlbs, with the first 40 shots being in single figure ES from a 200bar fill.
Again, big thanks to KevG for his time and advice, and didn't need to even send the Rapid to him!

Cheers,

Jinks

[P barrett]

Nice work mate hope you enjoy it now, ATB jazz 😁

[KevG]

if you have a lathe I would make a valve chamber extension,this can really help.on testing it compers to having an extra 3 inch to barrel.
kevG

[angrybear]

According to the owners leaflet Theoben recommend 220bar (3200psi) as FAC fill pressure, a bit more starting pressure should get you a few more shots.