Adiabetic cycle ??

[grand-galop]

  Question for experimented guys in the subject or minded people..  It has been discuss before and i have download  the link for the calculation, but, if the stroke and spring being the same, is there more power to    SKEEZE BY GOING BIGGER in the bore??
Could it be more power aviable and  could
it controlling  the foward SLAM being more
accurate??  I have search for a 50mm bore break barrel  and have not found any...

[subscriber]

Alain,

Enlarging the bore while keeping the stroke and spring the same might sound like it may be more efficient, but the opposite is true:  A larger piston area will reduce piston slam very dramatically, because the piston's tendency to bounce will be exaggerated.  That bounce represents energy not imparted to the pellet.  This is due to, whatever pressure behind the pellet before that accelerates out of the way, acting on a larger piston area.  At that pressure, the larger bore would generate a proportionally higher force, decelerating the piston earlier in its stroke.

You could compensate for this increased piston bounce by making the piston proportionally heavier (and it happens for free to some degree with a larger piston).  The snag with that is that for the original spring, such a heavy piston makes the shot cycle slow.  It also makes the rifle more bouncy (harsh) because the piston has a higher percentage mass compared to the rifle.

A heavier piston would accelerate slower with the original spring, so the pressure rise may be slower than ideal for best performance. 

If your airgun has problems with the piston slamming the front wall of the cylinder, try shooting heavier pellets.  If that makes no difference, check the seal for wear or damage that cause to excessive blow-by.

If you looked at successful springers on the market, their stroke is typically between 3 and 4 times the piston diameter.  Most nearer 4X.   There is a reason for that.  It produces airguns that work well and are comfortable to shoot.

To make sense, a 50 mm piston would require a much heftier spring, with stroke length and heavy compression cylinder to match.  It would be a beast of an airgun, that would require a windlass to cock...

[grand-galop]

I got the hatsan 125 from 2008 when it was the same as the Walter Talon and i have to shoot JSB's  monster 13.43g to manage the slamming.. The SCARCITY of these pellets and the scope problems is what make me thinking about finding a solution.. i have a crosman phantom that would be a great candidate to experiment with.. I was thinking about cutting the end whwre the transfer port begin and machining a 50mm pipe over it and welding it to that piece.  Just to test the reasonning and ftheory..

[rsterne]

There is a relatively narrow range of bore to stroke ratios that is used in springers (3:1 to 4:1 for most)…. In addition, there is a definite relationship between the swept volume and the FPE (about 3 cc per FPE is typical).... There is also a relatively narrow range of spring force compared to piston area.... These are there because they work.... Building outside the "norm" is unlikely to produce an improvement, or the companies would have already done that.... JMO....

Bob

[grand-galop]

There is a relatively narrow range of bore to stroke ratios that is used in springers (3:1 to 4:1 for most)…. In addition, there is a definite relationship between the swept volume and the FPE (about 3 cc per FPE is typical)....

Bob

I take that in consideration before posting..
The 4.1 ratio is exactly what i was considering with  the phantom based platform...
 I know the cycle is very short and giving the best parameters to function the way i would like it, is not an easy ansewr.. I thaught getting bigger volume would TAMED the slamming and improuve the power and accuracy..

[Doug Wall]

Since the bore area of a 50mm cylinder is 4 times the area of a 25mm cylinder, with the same spring and stroke, the final pressure would only be about 1/4. Dropping the pressure by 75% doesn't do any good things for velocity or power

[subscriber]

Alain,

Your experience with heavy pellets is very instructive:

One thing you can do relatively easily, without destroying your airgun, is to drill out the transfer port and replace it with a threaded insert having a smaller ID.  You can then make a few TP inserts of different sizes to see which works best with available pellets.

As a short term experiment, you may be able to glue in a short length of thin walled brass tubing without permanently altering the rest of the hardware.  If you use a thin wicking type adhesive such as one of the Locktite products, you could remove the insert by direct heating to "melt" the adhesive at some future date.  Just don't let the "straw" protrude into the compression chamber...  Also, don't let the adhesive run in, or it may act as an "unlubricant", freezing the piston.

The TP probably has a diameter of 3 to 4 mm now.  The powerplant is too much for .177 (yours?), and may  use the same TP for .177 as it does for .22 and .25.

Reducing the TP ID to just under 3 mm would probably tame the piston slamming.  As the Hatsan 125 has a gas piston, you can't lop a few coils of the spring...

If you are concerned about a glued in TP blowing out, consider this:
Making the TP insert from brass will enable it to stand the heat generated when the air is compressed.
If it is blown out, it will pass down the barrel, or run into the back of the barrel at such low velocity that it won't cause any damage.  The net surface area for compressed air to act on when considering a 4 mm OD and a 2.8 mm ID is so small, that the actual force applied to it is very little.  Something easily carried by a glued joint 10 mm long.  Now, the TP may be twice that long, if it travels from the middle of the piston to the center of the barrel breech.

Brass tube like this:  https://www.mcmaster.com/8859k21 or this https://www.mcmaster.com/8859k19. 

Tubing more suitable could be found, if you could determine your TP ID.  This can be done by using the shank of a series of (clean) twist drills as go/no go gauge pins.  Find one that will go in, and one that won't.  Measure the undamaged shanks of those drills to verify their diameters.   If you don't have a good range of drill bit, order gauge pins form amazon, or McMaster.  They are cheap in the 2 to 5 mm range.

Now, the glue bond could not stand steady heat at over 400 degrees C. However, despite the air temperature possibly reaching closer to 1000 degrees C, the duration is so short that not much heat can be transferred to the TP.  It would be nice if it lasts 10,000 pellets, however, if it lasts only 100, and solves you problem while doing it, that would be useful information.  Then, you can either get a metal to metal adhesive that can stand higher temperatures, or machine your TP for a threaded insert...

[grand-galop]

Since the bore area of a 50mm cylinder is 4 times the area of a 25mm cylinder, with the same spring and stroke, the final pressure would only be about 1/4. Dropping the pressure by 75% doesn't do any good things for velocity or power

You are right...   
I dont know why i have 42mm in mind for the crosman..

Thanks for the right info...

[grand-galop]

Alain,

Your experience with heavy pellets is very instructive:


The TP probably has a diameter of 3 to 4 mm now.  The powerplant is too much for .177 (yours?), and may  use the same TP for .177 as it does for .22 and .25.

Reducing the TP ID to just under 3 mm would probably tame the piston slamming.  As the Hatsan 125 has a gas piston, you can't lop a few coils of the spring...

If you are concerned about a glued in TP blowing out, consider this:
Making the TP insert from brass will enable it to stand the heat generated when the air is compressed.
If it is blown out, it will pass down the barrel, or run into the back of the barrel at such low velocity that it won't cause any damage.  The net surface area for compressed air to act on when considering a 4 mm OD and a 2.8 mm ID is so small, that the actual force applied to it is very little.  Something easily carried by a glued joint 10 mm long.  Now, the TP may be twice that long, if it travels from the middle of the piston to the center of the barrel breech.

Brass tube like this:  https://www.mcmaster.com/8859k21 or this

I think i will go that route..
Tapping for a sleeve and reducing the TP might be a good alternative..

I can manage the piston slamming and getting the pressure higher behind the pellet..

Sound the best solution

Thanks for the advice..