An o-ringed mystery

[Ribbonstone]

A bit of a mystery for you.

Have been playing with this 2300s HiPac pistol.


At one point I set it up like this:


Striker...spring...SHORT spring guide...o-ring...end cap.

Got this shot string:
8.4gr. Pellets
2150psi-1250psi

446
450
455
459
466
472
478
483
480
485
486
485
490
491
491
490
492
496
498
493
490
492
494
491
489
481
478
468
471
460
454
445

10fps = 498-488 = 13 shots
20fps = 498 -478 = 21 shots (lets call that 21 X 4.44foot pounds = 93.3 foot pounds)

Then I took the same parts ( a different/longer guide) and reversed them.


Jammed the o-ring into the striker...put in a longer spring guide head first...then spring...then end cap

Set up this way, the weight of the spring and of the guide are added to the striker weight (because they move together as a unit).
 
Had drilled the end cap to let the guide poke out just a bit to act as a cocking indicator.


Same pellets
Same pressure
same parts, just in a different order:

438
440
450
453
458
464
469
470
473
469
470
476
474
472
474
472
474
472
475
478
472
476
471
472
473
471
468
466
463
460
459
455
451
448
436

10 fps = 478 -468 = 21 shots
20fps = 478 – 458 = 27 shots (lets call that 27 X 4 foot pounds =  109 foot pounds)

Evidently it makes a difference.

So...WHY?

[Motorhead]

Hammer carrying the extra weight of guide pin has obviously slowed down the hammers acceleration so impact at valve stem is carrying less inertia yet the dwell has increased from the extra weight.

That what my mind comes up with

[Ribbonstone]

I'd have expected the extra weight and longer dwell to increase air use and decrease shot count...but the opposite happened.

Love it when results do not meet (or are reversed in this case) expectations; it makes me think.


Lets believe the numbers. Somehow, its getting more good shots at a slightly lower velocity...pretty good indications of a faster opening/closing valve.

Certainly there is more moving mass in the striker.  The o-ring inside the striker, spring, and guide move with the hammer, and add mass.
The pre-tension on the spring is the same in both cases.
Never had to take the gun apart...just the end cap...so didn't change the valve or transfer port.

Keep coming back to that o-ring inside the striker as doing something.  I THOUGHT it would act a tiny bit like a dead-blow hammer, no rebounding quite as fast until the guide compressed the o-ring.  Evidently it does NOT do that.

Perhaps it softens the blow somehow.

Do know this. Have two other Crosmans set up with the same moving spring guide/cocking indicator arrangement.  In the next week, will try an o-ring between the reversed guide and the inside of the striker and see what happens.  Maybe this gun is a "one off"...maybe the results will be the same.

[bstaley]

best guess: More mass = more momentum, less velocity(hammer). = longer dwell, but less lift (valve).

How that would translate to velocity and consistency would depend on a whole lot of other factors.

[Ribbonstone]

Was hoping it would be more predictable.  Not so much for the overall shot count, but for the count inside 10fps.  The "plateau" of 10fps shots surprised me.

Will play a bit more...certainly do not know what's going on, but I'll take advantage of it none the less.

Anyway, while not great, the overall efficency for both set ups isn't too different.

[rsterne]

Half the spring weight is moving mass in both cases.... You increased the hammer mass by the weight of the guide and O-ring in the second case.... That SHOULD have increased the airflow, upping the power, and reducing the shot count, as you mentioned.... Perhaps you have a bit of friction where the guide is in the hole in the end cap?.... My guess is that the O-ring is damping the energy transfer to the valve stem somehow....

Since you are using the same spring at the same preload, you have the same hammer ENERGY in both cases.... With the guide inside the hammer, the MOMENTUM is greater, however.... I would like to see the same experiment done without the O-ring on either end.... Make up for the difference in length on the head of the spring guide, perhaps?.... or don't bother and just try it without the O-ring....

Either way, there is something interesting going on that we need to understand....

Bob

[Ribbonstone]

No time today....but will get to it this week.  Think I'll just make a guide with a head that's fatter by the thickness of the o-ring to keep the pistol in the same velocity area.

Also plan on testing the o-ring another gun ( a .177 HiPac built on a 2260) that has the same type guide/cocking indicator.  Lot faster gun (12 foot pounds) an higher pressure.

[Ribbonstone]

Going to have to settle for  summation. Tried 7 different variations, TWICE each, for 14 shot counts @ 30 shots each string…be darn if I’m going to type  420 velocity readings.


Shot the following strings, twice for each:

No o-ring, guide to rear.
No o-ring, guide to front.

One o-ring, guide and o-ring to rear.
One o-ring, guide and o-ring to front.

Two o-rings, both 0-rings and guide to rear.
Two o-rings, one o-ring in front – one in back, guide to front.
Teo 0-rings, both o-rings and guide to front.


First off
 THERE IS NO DIFFERENECE in shot count for 20fps variation.
There is SO LITTLE difference in shot count at 10fps variation as to be meaningless.

THERE IS A DECIDED PREFERENCE in 5 feet per second shot count when the guide is head forward and the 0-rings in the striker head.


Summing that up…no difference in over all shot count, but the best 5 fps variation string is with the o-rings inside the striker with the guide reversed.

Be darn if I know why.


Numbers:
All using 8.4g JSBs

(yes the vel. Increased as I added o-rings.  The striker pre-tension increased, so you have to expect both a higher operating pressure and a bit higher energy.)

Guide to rear, no o-ring: 461fps
5 fps = 8shots
20fps = 25 shots

Guide to front, no o-ring: 463fps
5 fps = 7 shots
20fps = 25 shots


1 o-ring, 0-ring an guide to rear:468
5fps = 8shots
20fps = 24 shots

1 0-ring, guide and o-ring to the front: 468fps
5 fps = 10 shots
20fps = 25 shots


2 o-rings, both o-rings and guide to the rear: 478fps
5 fps = 8shots
20fps = 24 shots

2 o-rings, guide to front and one o-ring on each end of the guide: 471fps
5fps = 2 shots
20fps = 24 shots

2 0-rings, guide to front and both o-rings at the front: 478fps
5 fps = 11 shots
20fps = 25 shots

The WORST perfornace was with an o-ring on each side of the guide.  The  overall 20fps string was just as good, but shots were up and down so much that only two shots in a row fell within 5fps.

The BEST perfroance was with both o-rings jammed into the striker with the guide reversed (so it acts as a cocking indicator).  That allowed 11 shots inside of 5 fps, but the over all 20fps shot count was the same (as it was in all the trials).


SO.
WHATWEVER THE O-RINGS DO WHEN SANDWITCHED BETWEEN A REVERSED GUIDE AND THE STRIKER, IT SEEMS TO LEVEL THE SHOT COUNT AT THE PEAK. Get more very low variations shots, but no more shots over all.

Still do not know why.  To be honest, and rather tired of taking the arse end of the gun apart and swapping  things around.

But will test a second gun set up the same way,  That one manages 12 foot pounds and runs at a bit higher pressure, but it is the Same Crosman HiPac set up so the results would be more like Apples to bigger Apples than Apples to Oranges.

[Motorhead]

Rabid TINKERHOLIC Syndrome
It's sickness for which there's No cure ...

[Ribbonstone]

Agree. Am a tinkerholic.

Considering that there is no difference in 20fps variation shot strings, but there is a difference in 5 fps shot strings, am hoping this o-ring trick also works on the other guns. I may not get the +30% increase in 5fps shots with the bigger/higher volume guns....but then again, won't know until I try it.