Power problem Brocock Grand prix

[r4robert]

I bought a new .177 Super 6 Brocock Grand Prix pistol last month and have put around 250 pellets through it to date.

Today I decided to check the power with my chrono and was staggered to get a reading of around 1.8 to 1.9 Ft Lbs using Hobby pellets (7.0 grains). FPS was around 340-350.

I had filled the pistol to 190 BAR and everything is as it was when it left the shop.

Stranger still is that when I put some JSB Exact Heavys (10.4 grains) in it (heavy pellet for a pistol I know) the Ft Lbs on the chrono jumped to around 2.9-3.0!

I immediately suspected the chrono as having a fault, but when I shot Hobbys again, I got 1.9 and 340 FPS - just like the first time. I went to some Daystates at 7.9 grains and got 2.2 Ft Lbs.

It seems that the heavier the pellet the higher the power.

I realise that the Brocock is not regulated, but 1.8 to 3.0 is quite a 'swing'?

(I have since chrono'd my EV2 rifle and got 11.2 Ft Lbs, so the chrono seems to be working fine.)


SO, what's wrong with my Brocock? and why do I get such a big jump in power with the heavy pellets?

I appreciate that the 6Lb limit has to cater for any weight of pellet, but even with Exact Heavys 3.0 is a bit low?

Thanks
Robert

[r4robert]

And before you ask, I am re-calibrating the chrono for the weight of the different pellets!

Cheers

Robert

[Gerard]

I'm no physicist, and only very recently got into tuning a PCP since I bought an Atomic pistol, but it sounds to me like it makes sense. With the lighter pellets, especially if they have skirts on the weaker side or if the diameters are 4.49 or so instead of 4.52mm, or both, the initial blast of air is going to get them out of the barrel more easily than with the heavier pellets or pellets with larger or thicker skirts, or both. The longer the heavier pellet is in the barrel the more of the air flow (which is largely being wasted by the lighter pellets since they're gone before much of it can get out of the muzzle) it can take advantage of to get up to speed. Does this make sense? I've read a few posts where experts refer to .22" as being much more efficient in an unregulated PCP than a .177" pellet. And in looking at the literature with Brococks this seems to bear out; more shots per fill with the .22" than for a .177" in an otherwise identical pistol. At first it made no sense, but then it started to sink in. They have to actually waste more air with the lighter .177" pellets to get them up to spec speeds with a heavy initial blast than they do with the heavier .22" pellets, which hang around somewhat longer in the barrel to make better use of less air.

I'd recommend shooting heavy pellets from that pistol. If you were able to convert it to use 300bar then my suggestion would rather be to use light pellets and tune the hammer pin bolt as short as possible while still functioning to trip the valve, such as to get a minimal release of very high pressure air. That way your first 80 to 100bar of pressure drop would show good velocities with light pellets and you could just top up the pressure every couple of dozen shots. But they say not to fill beyond 200bar, so you're stuck with the heavy pellet solution, the heavier the better most likely. Of course I'm perfectly ready to have my reasoning shot down by experts!

Oh, and for context my Atomic is a .22", and I found that with the factory-set hammer bolt projection from the hammer face my velocities were starting near 500fps and dropping steadily until around shot 30 I was getting 350fps, using 14gr Hobby or Meisterkugeln. By turning out the hammer bolt until it had only 7.2mm projecting from the face (stock was around 12mm I think) I was able to achieve a rather long plateau with the velocity only starting to drop around shot 20. And velocity even up to 30 shots is reasonable, even though the pressure is down to less than 40bar remaining - checked when I start pumping up again and the valve squeaks when it starts accepting new air and I take a look at the gauge on my Gehmann FX pump. So I'd recommend keeping the hammer as short as possible while still firing the valve. Preloading the spring turned out to be counter-productive, making for more erratic velocities and a shorter overall cycle.

[oldboyshooter]

hello, i would suggest contacting brocock and asking what power your pistol should be? and if it is not doing just that as it is new to return to the shop you purchased from and let them sort it out with the company or get a replacement.

[gingernut]

I had this with my .22 Grand Prix.
Considerably more power with heavier pellets.
I think I settled on AA field in 5.51 in the end.

[manxteddy]

The booklet that came with my new GP .177 Brocock states ftb at 5.6.
I would contact by phone Brocock.
Graham