About what psi is behind the pellet of a break barrel when it starts to move?

[Earl]

I know that depends on the gun.
Is it more than 2000 or 3000 psi?
Does the psi start out high and get less as the pellet travels down the barrel?

[Motorhead]

If I may ....
What you describe is correct, pellet sits like a cork in a bottle until there is enough pressure to break it loose collapsing the skirt and sending it down the barrel.
* The fit / snugness / lead hardness ALL have a bearing on when this release happens.
 Pressure within a spring piston air rifle IIRC read years ago generally in under 1500 psi.
Because the firing cycle / compression producing time is SO SHORT controlling the "When" is a science within itself and for those seeking the most consistency will get pellets that first are sized correctly for the barrel and will in theory release somewhat equally and in the end ... accurate too.

[rsterne]

From what Cardew said, the pressure in a springer peaks at about 1500 psi and the temperature at about 800 *F....

Bob

[Earl]

Thank you.
I wondered about the pressure.
I had not considered the temperature of 800 degrees.
What is the melting point of the lead alloy pellets?

[Insanity]

Approximately 621°f but it happens so fast that there is not enough heat soak time to melt the lead.

[Hoosier Daddy]

What Scott (Motorhead) said makes sense.. take a .22 for example.
 If the pellet head size is 5.50 it would take less PSI than a 5.53... larger size = more friction.
But the bore of the barrel is a factor. a big bore with a small pellet is detrimental to accuracy, and a large pellet in a small bore induces drag and effects speed.
I have several tins of H&N FTT pellets of the same weight with different size that are each favored by different guns.

[Mole2017]

It would take some doing to compare these with a springer, but here are two pellets I shot out of my BSA R10 (a PCP) with the Huma regulator set "high" (150 bar) and "low" (I'm a little foggy here. It was the lowest setting--70 or 90 bar?).

Both of these are JSB 8.44 grain domes. These were recovered from squirrel bodies before and after some regulator adjustments. You might think the subtle extra flare of the skirt of the pellet on the left is from impact. However, notice the length of the rifling marks on the skirt--it came out of the gun that way. So, much higher pressure than 150 bar (~2200 PSI) stands a chance of really "blowing" some skirts.

[rgb1]

 Earl,
 Gererald Cardew covered the topic in his first book, "The Airgun from Trigger to Muzzle".
 It was published in 1976 and can now be downloaded as a .pdf  (9.4 MB)  here.....


  http://www.mediafire.com/file/5mcemcyutrm/The_Air_Gun_From_Trigger_To_Muzzle_s.pdf/file


 His explanation starts on page 43, hope this helps.




                                                                                     Ron

[Tom Tucker]

I would think that it varies from gun to gun.  I think higher-powered guns reach much higher pressure than Cardew's test gun (which produced only 5 FPE from a swept volume of 32.2 CC).  In "Internal Ballisitics of Air Guns" by Domingo Tavella, a Beeman RS2 was tested.  He writes "The highest chamber pressure and temperature are 29,248 kPa [4299 PSI] and 1452 K, respectively, while the highest values for the breech are 27,298 kPa [4013 PSI] and 1,506 K." (p. 28)

https://www.researchgate.net/publication/274638905

[Earl]

WOW!
Good information!
I will need some time to read more of the books but I get the general idea.
How many degrees F is 1506 K?
Thanks

[Insanity]

2251f

[rsterne]

I have read that article previously, and AFAIK, those were calculated numbers, and were never actually measured.... I suppose it is possible for there to be transient moments where they could be true, but confirming that would to me seem impossible.... The very act of measuring them would alter them, I would think....

Bob

[Motorhead]

Approximately 621°f but it happens so fast that there is not enough heat soak time to melt the lead.

 But it DOES melt the lead chaff / dust created and why some spring piston guns LEAD FOUL the first few inches of there barrels.

FYI

[Tom Tucker]

I believe you're right, Bob.  It would be very difficult to measure the instantaneous pressure.  Maybe NASA could do it 

I don't think they are very far off though.  I've always been a pneumatic and CO2 guy, but I've been thinking about getting into springers.  It's amazing how efficient they are.  I believe that they are the only type of air gun that takes advantage of adiabatic compression in a meaningful way - by operating not only on the amount of air to be compressed and the compression ratio, but also on the rate of compression and the resultant rise in temperature.

I'll share some of my back-of-the-napkin type of calculations.  I've been thinking about these 2 pistols - 1 SSP, 1 Spring.

HW40/Beeman P17 with full compression (25mm x 0.8mm disc added to face of piston)

6.7" .177 cal barrel
45cc swept volume
~0.83cc compressed valve volume
797 PSI calculated
Tested energy: 3.27 FPE with 7.33 grain pellet

HW45
6.7" .177 cal barrel
21.8cc swept volume
unknown compressed volume (which will vary as the piston lands and then rebounds)
unknown peak pressure
Typical energy: 5.0-5.2 FPE with 7-8 grain pellet

In order to make 50% more power with half of the air, the peak pressure in the HW45 must be very high indeed.  If both guns were pneumatic, I would say (and I could be wrong) that the HW45 would need 9X as much pressure as the HW40 in order to accomplish such a feat.  Since the HW45 is a springer, it may need even more peak compression when piston bounce and thermal losses are brought into the calculation.

Then there is the diesel effect to consider.  I believe Cardew in his second book was able to determine that the healthy HW35 (freshly rebuilt and settled in) he tested, released an air mass from the muzzle that was 12% greater than the mass of the air originally in the cylinder, presumably because of spent gases from combustion of lubricant oils.  So perhaps, we should add 12% to the swept volume when playing with these numbers...

I love learning about these guns. 

[rsterne]

No question, Springers are much more efficient than any of the Pneumatics (CO2, pumpers, PCPs)…. as a result of adiabatic compression.... All the others have adiabatic expansion (and the accompanying cooling) to some extent during the shot cycle....

Bob

[nced]

No question, Springers are much more efficient than any of the Pneumatics (CO2, pumpers, PCPs)…. as a result of adiabatic compression.... All the others have adiabatic expansion (and the accompanying cooling) to some extent during the shot cycle....

Bob

Yup.....springers are "HEATERS" and PCPs are "COOLERS". 

[rsterne]

Comparing the FPE output to the volume of air compressed may not be a particularly "fair" way to do the math.... In a PCP, the compression takes place slowly, and the air then cools to ambient temperature, with the power source (compressor or pump) external to the airgun's "power delivery system" (valve and barrel)…. The energy comes directly from the expansion of the compressed air itself, which cools again during the shot.... In a Springer, the energy comes from releasing a compressed spring, where the energy stored in the spring (steel or nitrogen) is released suddenly, which causes the heating by compressing the air charge.... They are in reality, two completely different processes....

Bob

[robertr]

 Nevermind.

[Madd Hatter]

Earl,
 Gererald Cardew covered the topic in his first book, "The Airgun from Trigger to Muzzle".
 It was published in 1976 and can now be downloaded as a .pdf  (9.4 MB)  here.....


  http://www.mediafire.com/file/5mcemcyutrm/The_Air_Gun_From_Trigger_To_Muzzle_s.pdf/file


 His explanation starts on page 43, hope this helps.




                                                                                     Ron

I remember reading that book many decades ago. May even have it with some of the books I'm storing out in the shed. Need to go out there and check thru the boxes.

[grand-galop]

If the springer are more EFFICIENT, why the companies are not developing different geometry or ways to canceling the movement of the shot cycle??

Daystate is supposed to have a 2 opposed piston springer later this year...

I would love a springer shooting as smooth as a PCP with the same power levels..

As a PREPPER GUN, it would sell like CUPCAKES...