2400kt, Bottled, Regulated

[rsterne]

Sorry haven't run a Disco/22XX setup at those low pressures, with the exception of an <500 fps bottle gun running on about 500 psi with a 2240 spring with only 1/16" preload.... so I can't offer any advice on spring choice.... However, trying to maintain a given (relatively high) velocity (eg 900 fps) while lowering the setpoint pressure (eg from 1250 down to 1050) is a perfect recipe for wasting air from hammer bounce.... You are moving from the knee of the curve up onto the plateau, and that is exactly where hammer bounce occurs.... It is for this reason you often get more shots with a higher setpoint pressure on a regulated gun.... Read over my thread on tuning a Regulated PCP again....

Bob

[aPpYe]

So you are saying I am wasting air, even with bounce completely eliminated by the SSG?  I will admit to never actually testing shots per fill...   At 890fps with the gun as it is currently, it is definitely not on the plateau...   If anything I cannot quite reach a plateau, as 1 turn out is already about 870fps.  The idea was to stick with that velocity. 

[nervoustrigger]

Hmm, this is an interesting topic.  If appye's tune is now up on the plateau, it makes sense that air use and hammer bounce may be worse.  But I don't see the correlation with the higher ES.  Up on the plateau, having a bit more or less hammer strike will do little to the velocity, meaning that although it is wasteful of air, it will be more tolerant of little things like friction or sear drag that would tend to drive up the extreme spread.

I wonder what is causing it.

[rsterne]

If you are not on the plateau, you will not be waisting air.... What I meant was that when you reduced your setpoint from 1250 to 1050 (pre SSG), you may have moved up onto the plateau, the fact that you were experiencing hammer bounce would confirm that suspicion.... The plateau is were the dwell is so great that little if any velocity is being added to the pellet, and eventually it may be exiting the barrel while the valve is still open.... That could still happen with the SSG in place, what can't happen (assuming it's tuned correctly, with enough preload and a gap) is for the hammer to bounce.... However, with the right (wrong) combination of spring and preload you could still get enough dwell to be dumping air after Elvis has left the building.... When you lower the setpoint pressure, you lower the velocity where the plateau occurs.... Are you absolutely sure you are not on the plateau at your lower setpoint?.... Did you try various preloads before you installed the SSG to confirm that?.... What velocity is the new (lower) plateau at?....

Jason, I agree that the ES should not increase on the plateau, it should decrease.... Likely something else causing the increased ES.... drag somewhere....

Bob

[BigTinBoat]

I don't want to derail the SSG thread with questions about my specific setup, so I am posting my question here...  First, I was very successful in making and installing an SSG with my drill press, bench grinder, and some dremeling. I used my smartphone to make a super slow motion video (240 frames per second) of my gun shooting before installation, and it recorded an average of four 4 bounces per shot.  It recorded NO bounce at all with the SSG installed.  I am able to achieve the same 900fps with 10.34gr JSB's as before, and hammer bounce has been stopped cold.  Awesome!

How were you able to "film" this? Do you have the video posted somewhere we can see it?

[aPpYe]

I was thinking drag somewhere too ...  When I finally chose my spring/rod length combination for the SSG, I was using this really bunged up rod that I had marred up quite a bit... The gun sure seemed to be shooting with a tight ES though.  I did not really look too hard for it, maybe 5 shots, take it apart, try another spring, 5 shots ...  So the wide ES may or may not be due to my having worked on the gun.  Maybe even some metal shavings have made their way in between the tube and the hammer.  Anyway, I noticed the widened ES when I remade the SSG with a brand new, not bunged up section of rod.  It is nice and smooth...  I will pull it apart and make sure all is clean.  I may even take the time to polish up the rod and bolt with some fine sandpaper...  I am thinking drag might be with the tube and hammer though... I will find out tonight.

Part of what made me so excited about the SSG was it seemed it would give me the ability to go with an even lower set pressure, seeing as how hammer bounce was mitigated.

As far as how I filmed it, I used my phone.  This particular phone is the iphone6 that my work issued to me.  It has  "slo mo" function to the camera that allows me to film at 240 fps vs the standard 24.x fps of standard video.  The result is a video was initially filmed at 240 fps but plays back at 24fps... you get a video that is in super slow motion.  It is a really nice way to detect FOR SURE if you are getting hammer bounce.  Most newer smartphones (from maybe the last 2 years) have this feature...  I know because I have about a solid hour's worth of my daughter shaking her hair around in super slow motion!  If it is not built in, then look for "high speed camera" in your app/play stores...   It was nice, because it slowed down the entire shot (including bounce) to about a full second, allowing me to actually count the bounces.

...Aaaand I just figured out how I can share it.  I published a couple on youtube.  It is really cool, because the youtube player allows you to slow down the resulting video by half again and still hear the audio.  Here is a link to the PRE-SSG video:



... And here is a link to a video I made at night with the ssg installed...



I think I could have made it without so much background noise if I didn't do it from my enclosed deck.  I had a wall to my left and a ceiling above me.  It can probably be made to sound cleaner if I had done it out in the open...

[BigTinBoat]

So you "audio" recorded it. I thought you could see the hammer bouncing. While maybe that is "hammer bounce", I don't think it is hitting and opening that valve 5 times.

[aPpYe]

Well, I could try filming the cocking pin thingie as well, but it sure sounds like it was opening the valve several times... I could hear the "farty" sound every time the gun would fire.  What else would it be?

Also, here is another hammer bounce that was recorded at full power.  The daytime shot from the previous post was at a much lower velocity and with a lighter pellet.  I don't remember what exactly, likely a 8.44gr jsb exact fired at ~800fps, versus the video below is a 10.34 fired at ~900fps...

[BigTinBoat]

  What else would it be?

Not sure, but wondering where you are holding the mic? and wondering why I don't hear any report?

[aPpYe]

Hmm.. I will post a normal speed version of the file as well, so you can hear the difference.  It IS the report that you are hearing, just slowed down to where you can hear the individual bounces.  I was not holding any microphone, just filming the shot with my phone.  Maybe the phone was a foot or two above the gun when it fired.  Tomorrow is my day off, so I will film some daytime shots, hopefully complete with a visual of the actual hammer bounce if I can get the phone to pick it up...

[BigTinBoat]

Maybe it is "bouncing" 4 times.


Doesn't look like it is opening other then on the first one here.

[rsterne]

I see at least a second opening of the valve.... small but definite (maybe 0.010-0.015").... possibly a third, or it could just be the seat material decompressing....

Bob

[WECSOG]

Looks that way to me, too.

[aPpYe]

Well, I have been sick for a while, so gun tuning keeps getting put off.  I also have not had the chance to film the hammer bounce... Shivering in my bed seemed like a better idea at the time... 

I am not so sure I am on board with the idea that it was not releasing air on each one of those pulses.  The gun for sure had that farty sound when it was fired.  Each pulse nearly as loud as the previous, and while this gun isn't really all that loud, the sound of air releasing from the barrel is considerably louder than just the hammer bouncing on the valve pin when the gun is dry fired with an empty tube.  That is the level of sound I would be hearing if the pulse did not open the valve, right?  For example, in this video, I hear FOUR separate releases of air, and a fifth pulse that might just be the hammer landing on the pin and not opening the valve...

Anyway, I put set pressure back to 1200psi (well, I put the shim back in anyway, I did not actually re-test pressure with that particular shim combo, but it is what I had before...) and also put the lighter valve spring in from the original c02 valve...  No difference in spread. 

It occurs to me that I did one other thing a couple of weeks ago that might be the culprit...  I did the barrel work that Jason (NervousTrigger) recommended to me, and perhaps I went a little too crazy with the lead/bore transition smoothing part...  I figured I would save some time by using a drill motor to run the sandpaper in the leade by wrapping some sandpaper around a q-tip and spinning it in there.  I might have gone too far into the bore itself and widened it somewhat.  If this is the case, it could be the cause of my wide spreads, right?

Anyway, I took the shim out again.  I still have the lighter (and slightly longer) valve spring in there... As I understand things, this might cause an undesired longer dwell, wasting some air, right?  Obviously I am still learning...


....Aaaand I am finding that my benjamin pump is now not able to put more than about 1700psi into the gun.  The pump definitely does not feel right...  I am likely going to be tearing the pump down and looking for broken parts or something... I am not sure what would cause it to "cap out" like that though, shouldn't it just leak or something?  Looks like I am gonna be learning about pump teardowns and whatnot in the near future.  I bought the pump brand new just two months ago and it has been in the house the whole time.  I have done nothing to it other than put a crappy harbor freight desiccant filter on it... I definitely need to buy some bulk beads and run the air through a soda bottle or something.

EDIT:  Another thing to report is that I can hear some air escaping on each pump, even at less than 1000 psi.  The more pressure I get into the gun, the more I hear the little squeak I am used to hearing at the very bottom of the stroke, where the air actually gets into the gun being replaced by the telltale whoosh of escaping air.  I also noticed I am getting a diminished return for every stroke, where I only gained about 100psi with 25 strokes, where that would normally take about 10 strokes or so...  Please tell me I can do this without specialty tools...

[aPpYe]

Okay, so I replaced a couple of orings (one extruded into a valve at the top, and the "bumper stop" was torn to bits) and the pump is in working order...  I kinda wish I had researched pumps a bit more originally.   Anyway, I think I will eventually be replacing all the orings with better ones if such a thing exists.  Anyone have any thoughts on material?  Rather than getting a kit, i was thinking I would buy them individually in say, ten packs.  All the data on oring materials kinda boggles my mind, and I am torn between the ones that have high heat resistance and those with high abrasion resistance since we are dealing with moving parts.  I would like to go with 90 durometer where it is good to do so to prolong life as much as possible... Of course, does 90 durometer really mean "durability" or just "hardness?"

Anyway, I think I know what may be the cause of the high ES...       When I was initially messing with the first iteration of the SSG, I likely had the end that pushes the hammer milled down off center a bit...  Or something.   Anyway,  there are now twin gouges at the top of the tube where the hammer hits the valve pin.  The gouges match the front part of the hammer where it has that bit removed that sort of looks like a bite has been taken out of it.  The piece' that is machined away that makes it not completely round?  Well, the corners of this bite are what have scarred the inside of the tube. 

So the gun is in pieces on my table again.   I dont know how I missed this before, but it makes more sense because I am 90 percent sure I still had nice tight spreads a good week after doing the barrel work that I was blaming this on a few days ago.  Pellets still generally feel tight as I push the bolt (with extended probe) closed too, so I am thinking the barrel is fine.

Anyway, I would like to save the tube if i can.  Initially I was thinking of running a hone, but that would introduce its own friction as the tube would no longer be "slick" like it is now...  So likely not a good idea, right?  It is too far to get in there with sandpaper or my Dremel, though I haven't actually tried that...  I was also thinking of cutting slots out that match the gouges, basically removing the section where the gouges are, so the hammer doesn't rub there.  I have filed down those same sharp edges of the hammer as well.  How far down can I remove those edges?  Maybe I could cut some slots in the hammer to avoid the gouges?

Also, since the gun is apart again, and the lighter valve spring didn't seem to change behaviour in any way that I could see, i should  put the heavier one back in, potentially reducing dwell time and saving air, correct?

I am grateful for any input you have...

[rsterne]

Valve spring and hammer spring are for the most part interchangeable.... less or one works the same as more of the other.... Having said that, the worst hammer bounce I ever got was on a gun where I tried a light valve spring.... In theory, on an unregulated PCP, a heavy valve spring may extend the shot string to a lower pressure....

Bob

[aPpYe]

Okay, so I might want to revisit some of my lighter hammer springs then...  The gun is not at all difficult to cock, but if it can be made easier...  Why not?   Good to know!  So my theory about extra dwell time wasting air doesn't really hold water?

Any opinions on my gouges?

[rsterne]

Well, extra dwell time does waste air.... but balancing the hammer spring to your lighter valve spring should fix that....

You shouldn't have to worry about gouges, only bumps.... If you fixed the corners of the hammer, and the cause of the problem, you should be good to go....

Bob

[nervoustrigger]

Appye, regarding your O-ring questions, really the only one in a typical high pressure pump that is worth splurging on is the one on the inner piston.  With the Benjamin pump in particular, this O-ring tends to fail more readily due to extrusion than anything else.  That is assuming you pump reasonably...do the 'ol jackhammer and heat may kill it.  Case in point, a 90 durometer polyurethane has worked well for me despite the fact it has the lowest temperature rating of any common O-ring material.  It would seem polyurethane's superior tear resistance wins out.  It's great in the area of abrasion as well.  Again, this is specifically about the Benji pump.   In fact, my particular pump ate an exotic Aflas O-ring in short order as well as a Viton.

Meanwhile everywhere else on the pump has been trouble free with common Buna-N, even on the outer tube which had been neglected by a prior owner and was pitted.  I smoothed it as best I could and it has held up fine.

But to address your other question about durometer, yes in general a higher durometer will wear longer in a sliding application.  It's also less apt to extrude and tear if the gap is large-ish.  Just be on the lookout for O-rings that need to slide.  I was unaware and installed a 90 durometer -210 on the Benji pump and it would not work right because it clung too tight and would not slide in the wide groove at the top of the 2nd tube.

[aPpYe]

Jason, thanks for the info, just what I was looking for!  The small inner piston ring was fine on my pump.  I put everything back together with that one and all was well.  The one that DID fail on me was between the second and third cylinder.  This guy here:



It is interesting that the 90duro polyurethane is holding out better.  To me this indicates that failures are not due to heat, but loose tolerances and extrusion of soft rings.