Playing with the Extreme .308

[Jeremy1982]

I was thinking the same thing.I can get a good hundred yard shot here at the house but thats it, want to take it some where I can stretch it out.I have a nikon with a bdc on it..if I lower the mag to a 7 power the 3rd dot is 153 yards so I wanna see how it does.Even at 150 it will be more powerful then my 909 at point blank range.

[Fur-n-dirt]

Springs and more Springs!

Efunda.com has a cool spring rate calculator.  I was off by 10% based on a vendor specs.

By the way, the "light" spring that came with my extreme is calculated to be ~11.5 LB/in

[Fur-n-dirt]

Life is good.  ;-)

I'm already converging on the optimal pressure for the new spring selection.

I chose to start with a century spring #2894 which is ~7.7 lb/in.   By the way, the stock "light" spring that came with the gun is estimated to be ~11.7 lb/in based on an on-line calculator.

So, this spring is pretty light.

Shooting a 109 grain HP ; using a chrono at 10 feet away.

Below, is a plot of velocity vs. number of shots as a function of initial pressure.   ~3700 seems to be close to the bell curve.   Not bad since I did not want to fill this gun close to 4000 psi.   This pressure will allow me to shot a lot more, and  still pack a punch!

I believe the next step will be to shoot the gun at the range and check accuracy from 3650 to 3750 psi fills...

I'm up to ~.7 FPE/cuin.  much, much better.

[rsterne]

With your fills to 3750, your total ES is only about 24 fps or less.... I would suggest you try filling the gun to 4000 with that spring, you may be able to get 5 shots within a 4% ES.... Considering you are well over 1000 fps, that 0.7 FPE/CI isn't all that bad.... A heavier bullet to drop the velocity into the mid 900s will increase the efficiency even further.... I would look for something in the 140-150 gr. range....

Bob

[Fur-n-dirt]

With your fills to 3750, your total ES is only about 24 fps or less.... I would suggest you try filling the gun to 4000 with that spring, you may be able to get 5 shots within a 4% ES.... Considering you are well over 1000 fps, that 0.7 FPE/CI isn't all that bad.... A heavier bullet to drop the velocity into the mid 900s will increase the efficiency even further.... I would look for something in the 140-150 gr. range....

Bob

Thanks Bob...

You know what surprised me, the chrono numbers shooting in the morning around 65 f was about 50 to 70 fps slower than shooting in the afternoon at 85 F.. 

I have ideas, but I wish I knew the 1st order effect.

The numbers presented above were during the afternoon session.  Same bullets, same chrono, same everything..

[rsterne]

Sound like the lube on the hammer is thinning out at higher temps....

Bob

[Allen A]

Air Density

[rsterne]

How is the air density going to affect the velocity being produced by a shot of air that has already been compressed to 300 bar?....

Bob

[wwonka]

PV=nRT maybe? Keep everything constant but increase temperature, and the pressure rises enough to account for some change in energy. 60 fps seems a lot though, but it's part of it I'm guessing.

[rsterne]

Actually, the way the gun is tuned now, increasing the pressure will reduce the velocity.... Changing the temperature in a PCP doesn't affect the velocity in any significant way, but it will reduce the shot count.... The "T" in your formula is absolute temperature (Kelvin or Rankine) and a 60*F change in temperature will only change the pressure 10%....

Bob

[wwonka]

Changing the temperature in a PCP doesn't affect the velocity in any significant way, but it will reduce the shot count....

Correct about the absolute scale.

Interestingly, there have been some pretty thorough postings by people using guns much like this one (The Haley .257 and another Extreme .308 I believe) who have noticed speed changes with changing pressure. Unrepentantsinner wrote about this on another forum, and so did Cricket (both heavy-duty big-bore guys).


EDIT: here is the thread where somebody else shooting an Extreme .308 noticed velocity changes with temperature, and so did someone shooting a Haley .257
http://www.network54.com/Forum/79537/thread/1366590422/Some+observations+of+temperture+and+PCP%27s

[rsterne]

I would suggest the most likely cause is changing hammer velocity due to viscosity changes in the lube.... on hammer or spring.... or some secondary reason like sear friction....

Bob

[wwonka]

Just posting again in case this helps... here is the thread where two other different people: shooting an Extreme .308 and a Haley .257 noticed significant velocity changes with ambient temperature.

http://www.network54.com/Forum/79537/thread/1366590422/Some+observations+of+temperture+and+PCP%27s

[Fur-n-dirt]

I'm off to the range right now, but I'm going to dig into this further.  In the afternoon, the cast bullets were warm/slightly hot to the touch, in the morning, cold.  I was going to check for CTE differences.

Just a thought for the moment, maybe not even worth writing about..

The spread was 45 to 70 fps depending on pressure fill and variability.

[Fur-n-dirt]

Actually, the way the gun is tuned now, increasing the pressure will reduce the velocity.... Changing the temperature in a PCP doesn't affect the velocity in any significant way, but it will reduce the shot count.... The "T" in your formula is absolute temperature (Kelvin or Rankine) and a 60*F change in temperature will only change the pressure 10%....

Bob

I didn't fill the gun cold and shoot the gun hot. 

I filled the gun to the EXACT pressure when the gun was cold and hot.

[rsterne]

Even more reason for the velocity change to be due to hammer effects....

Bob

[wwonka]

Even more reason for the velocity change to be due to hammer effects....
Bob

I guess we'll find out sooner or later what the major contributing factors are. Not to hijack the thread though, it does catch my interest what you say about "hammer effects".

Question: what is a short list of things (polishing, lube, etc) that you would suggest specifically to be done to a PCP hammer (the common cylindrical type that slides through the breech pushed by the spring) - to improve performance?

I've avoided any lube on my hammer due to worries about o-ring contamination and/or potentially explosive mixtures - but is there any you would recommend? What about polishing? (I polished my hammer with fine-grit sandpaper, and didn't really notice much effect - so maybe I did it wrong).

[CarsonRatSniper]

I would recommend a dry graphite or moly coating on your hammer. Brownells sells a 2 part moly coating system that works great.

[Fur-n-dirt]

I'm off to the range right now, but I'm going to dig into this further.  In the afternoon, the cast bullets were warm/slightly hot to the touch, in the morning, cold.  I was going to check for CTE differences.

Just a thought for the moment, maybe not even worth writing about..

The spread was 45 to 70 fps depending on pressure fill and variability.

Well, I did a quick hand calc, not reasonable that it's a CTE effect.   Based on diameter,  I calculated a thermal growth of almost 2/10 of a mil for a 35 F delta for lead.

I was just curious if the bullet sealed better,  that the velocity could increase.  I don't think so anymore..

[wwonka]

I would recommend a dry graphite or moly coating on your hammer. Brownells sells a 2 part moly coating system that works great.

Thanks for the info by the way...