Velocity Loss ?

[Yogi]

SOP: check both the breech seal and piston seal.  Possibly a nick in the seals?
Otherwise send them to MotorHead to figure out!

-Y

[Mark 611]

When I apply tar on a spring its usually in the center 5 to 6 coils where the spring is at its weak point or unsupported state, after the piston and guide have separated, this is usually where vibration or buzz will occur, properly fitted guides usually do not require heavy amounts of tar or other lubes of choice,  any other lubing on the spring beyond the center should be a film state of lube, its just there to help with friction from metal to metal contact, if to much lube is used on ur spring u risk the lube to migrate to places u don't want it to go! to much lube on ur spring in certain states of air temperature can cause the velocity to slow down, usually colder weather, if lube has gotten past ur piston seal in hotter temps u can usually smell something or have detonation or dieseling, applying lubes properly is trial and error, each gun is also different, but once u have done it enough u know how and where the proper lubes need to be applied

[Airnut]

I used the tar that came with the JM kit.  Is that a no-no or did I overuse it?

Opinions will very on what the best lube is but I don’t think that is the issue. My thinking is that did you chrony the HW30S-1 to soon after you tuned it and maybe had to much lube on the seal causing a little dieseling on the first test giving you a higher velocity reading? I myself will not chrony a newly tuned gun until I have at least 50-100 shots thru it with no signs of dieseling. This way I can get an accurate base line.

[Mark 611]

agreed, this why I test fire them for function and accuracy then tear them down to clean them out so I can properly fix them!!!!

[Seanh]

Do the guns normally stay inside the house or at school? Had they had a chance to alcomate to the outside temperature? I've got one rifle that will give me lower velocity as soon as I take it outside ... outside temperatures about 90, inside temperature 68 to 72

[Mark 611]

Actually 90deg temps should help with higher velocities in a spring gun! but I agree with acclimation before shooting!

[nced]

Actually 90deg temps should help with higher velocities in a spring gun! but I agree with acclimation before shooting!

That would seem to be the case.
In the past I had issues with "temp related poi shifting", At a spring field target match in Virginia both my brother and I had a 1" poi shift at only 20 yards with our .177 R9s after zeroing during snow flurries and the temp rising to upper 50s by match end. We were was using traditional molly paste and tar for lubing so I started messing with oring sealed piston caps at that time to minimize the amount of lube and "rubber" sliding against the receiver. I thought that perhaps the lube viscosity and the "rubber" durometer was changing. The change to an oring seal and minimal lubes definitely gave improved poi stability so I've been using oring sealed piston caps ever since.

After moving to NC about 12 years ago I "discovered" non-dieseling Dupont Krytox "space station lube" and have been using the stuff ever since. After the move from West Virginia to North Carolina I tested "temperature related velocity changes" years ago (during a NC winter month) with oring piston seal plus Krytox lube. The velocity was tested  at mid 20s and mid 70s and the velocity for the "mid 20s" was 10ish fps HIGHER than "mid 70s". My only unscientific reasoning was that cold air is denser than warm air so there was "heavier air" being compressed at the "mid 20s". Anywhoo.....10ish fps variation is insignificant for my shooting (I don't shoot my springers at mid 20s  )  so there have been no other low/high temp velocity tests of oring seals and Krytox lubes since.

[Oldgringo]

I found my notes from 9/4/16 and I did not transpose numbers.  Those original readings were with the JM kit AND seal.  So I reckon I'll take her down and see whats to see.  I wonder if the R7 and HW30 use the same piston seal, anybody know?  I have the OEM springs for both 30S' and the R7 and they are three different lengths by just a little.

[Mark 611]

Yes Greg their the same piston and breech seal! I have been using Air seals Australia piston seals in my HW's and really like them!

[Oldgringo]

I've taken her down and cleaned up her innards.  There's nothing wrong with the mainspring or either seal that I could see and/or feel.  That may have been the first one I DIY tuned.  It was gummy with JM's heavy tar but - it's not anymore.

Now I have another problem.  The little bitty screw that holds in her innards broke during reassembly..  It's part no. 541 on PA's schematic.  I wonder where one might find a replacement?

[Oldgringo]

I've taken her down and cleaned up her innards.  There's nothing wrong with the mainspring or either seal that I could see and/or feel.  That may have been the first one I DIY tuned.  It was gummy with JM's heavy tar but - it's not anymore.

Now I have another problem.  The little bitty screw that holds in her innards broke during reassembly..  It's part no. 541 on PA's schematic.  I wonder where one might find a replacement?

Wonder of all beholders, our local Tru Value had the screw!  The gun is back together all degreased and clean.  I reckon the spring is just tired, she's still shooting 7.33 gr. AA Field Diabolos at 600 fps.  So, I'm not gonna' fret about it no more.

[Mark 611]

I would say ull probably find 1 at some hard ware store or a place that sell fasteners, I would use a grub screw with a allen head 

[Oldgringo]

I tried a couple of shots yesterday and she's still at ~600 fps with the 7.33 gr. AA.  If I pull the JM spring and see what she does with an OEM spring and the velocity comes back up, that confirms that the JM spring is tired.  I reckon I can then take the top hat and guide from the JM spring and insert them into the OEM spring, can't I? 

OTOH, the rifle has peep sights and I don't shoot her any further than 30 yards, just don't worry about it.

[Mark 611]

well u know u can probably space up the JM spring with some Delrin washers and get more power out on it! just my thinking!

[nced]

I tried a couple of shots yesterday and she's still at ~600 fps with the 7.33 gr. AA.  If I pull the JM spring and see what she does with an OEM spring and the velocity comes back up, that confirms that the JM spring is tired.  I reckon I can then take the top hat and guide from the JM spring and insert them into the OEM spring, can't I? 

OTOH, the rifle has peep sights and I don't shoot her any further than 30 yards, just don't worry about it.

"JM spring is tired"
I guess it's possible (especially if "heavies are shot")  but my experience over the years is that the JM springs I've used hold their temper till the spring breaks after 10s of thousands of shots. Decades ago I bought a Maccari "soft spring kit" for my Beeman R10 after the factory spring broke and kept track of the shot count and velocity. The R10 started with a 7.9 grain pellets 910fps mv. After a couple years and 20,000 shots, determined by counting empty Beeman Trophy tins, the pellets were still clocking 905fps (more variation can come with atmospheric changes). Because of that spring I've been only using Maccari springs for my kits except for the two Vortex springs I tried after moving to North Carolina which sagged with less than a case of CPLs. LOL....the early Vortek PG2 kit (didn't have closed spring ends) lost 80fps mv shooting less than a case of CPLs (roughly 4500 shots). A few years later I bought a bare Vortex spring for my R9 and it also sagged after 2500 shots losing 30fps in less than two months.

As was mentioned by Mark, perhaps there is enough "free space" to add a couple washers to increase the spring preload and restore some of the velocity.

[Oldgringo]

Thanks, Mark and NCED.  Washers are certainly a thought.  The plot thickens, what is the real cause of velocity loss and does it matter?  This is a nice little open sighted air rifle that is used for plinking in our 30-yard deep backyard....and taking an occasional Brown-Headed Cowbird off of the bird feeder.

[BackStop]

Thanks, Mark and NCED.  Washers are certainly a thought.  The plot thickens, what is the real cause of velocity loss and does it matter?  This is a nice little open sighted air rifle that is used for plinking in our 30-yard deep backyard....and taking an occasional Brown-Headed Cowbird off of the bird feeder.

I think I agree with you.   As long as the velocity doesn't keep dropping off and it is doing the job you need it to do... why worry or fuss with it?   

[nced]

Thanks, Mark and NCED.  Washers are certainly a thought.  The plot thickens, what is the real cause of velocity loss and does it matter?  This is a nice little open sighted air rifle that is used for plinking in our 30-yard deep backyard....and taking an occasional Brown-Headed Cowbird off of the bird feeder.

Hummm.......I also take out cow birds on my bird feeder!

Velocity degradation can come from multiple sources such as a sagged/broken spring, thickened lubes (especially with low powered springers), bad piston seals so on and so forth. IMHO, only a teardown and inspection can determine the cause.

Does the velocity reduction matter you ask. Only if it negatively affects your shooting. LOL, when I received my .177 HW95 I deliberately detuned the gun straight from the box to reduce the CPL velocity from 880fps to 850ishfps. This velocity reduction makes the HW95 easier to control, yet out to 50 yards the trajectory remains almost the same.   

[mikeyb]

Did all get new springs and seals just before your previous test? If yes and this new test is many months later after the new springs have taken a preload "set", isn't this a case of "spring scragging"?

https://www.pyramydair.com/blog/2006/04/spring-talk-some-facts-about-mainsprings/

[Oldgringo]

Did all get new springs and seals just before your previous test? If yes and this new test is many months later after the new springs have taken a preload "set", isn't this a case of "spring scragging"?

https://www.pyramydair.com/blog/2006/04/spring-talk-some-facts-about-mainsprings/

I dunno'?  When I took her down this second time, there seemed to be plenty of spring pre-load as I slowly backed out the spring compressor.