Author Topic: If You Give a Mouse a Cookie...  (Read 549 times)

mcoulter

  • Jr. Member
  • **
  • Posts: 92
If You Give a Mouse a Cookie...
« on: December 10, 2018, 06:20:04 AM »
…he will ask for a glass of milk... 

Besides being a great book for kids, it's really how I have felt since being fortunate enough to win a PCP here.  The gun I won was a Fortitude, but having it be delayed and delayed during the spring I was given the option of choosing another similarly priced rifle.  I ended up choosing the Hatsan Flash.  I received the Flash back in early July and shot it all summer.  While being far from a perfect rifle, I had a TON of fun with it over the summer.  So much so that I didn't shoot my "better" rifle, the FX Royale 400. 

I eventually decided to sell the Royale and use the proceeds to fund the purchase of a compressor and mid-level gun.  It didn't take long to find a very nice used shoebox f10.   Then had I had pretty much decided to get some variation of the Hatsan AT44 as my replacement rifle.  However, during the pre-black Friday sales I found a great deal on the Kral Bigmax from PA.  Well I went for it  knowing very well that there is very little information online about this specific model.  The Kral information that IS being shared is generally AFTER people's rifles have been worked over by Troy Hammer  :-) 

I have had the Bigmax out shooting a couple of weekends now and have just begun to crack it open to see how it's put together.  My short-term goal is to reduce the velocity a bit. 

This is how it was shooting out of the box:

https://photos.app.goo.gl/zghkE2SMGxLay5T2A



Given how it was assembled, I could only reduce the hammer spring tension by 1/2 turn.  That dropped the velocity perhaps 10 fps.   I'd like to eventually get it down to about 870 fps or so.  Currently, the gun sounds like it's wasting a lot of air.  Access to the HS requires degassing.   Given this loss of air with each tweak, I'm hoping to get some advice on options to lower the velocity.  (The gun does have the power adjuster, but in my stubbornness I'd like to start with adjusting the valve behavior for better air usage.)

Here's what it looks like:




Degassed and gauge removed:



Remove the rear air coupling (yes, it came buggered up like this...)






Rear bottle block is just held on with a couple of allens:




Back of bottle block:



Back of receiver - hammer spring screw is flush and can't be backed out any more:




Set screw hold that hold the hammer spring adjuster:


Hammer spring details (19 coils?):





















Depth of spring hole in hammer:



Length of spring and adjuster:




Amount of preload on the spring:




I started looking up springs online last night and quickly became frustrated finding that all the sellers who had good variety only sold their springs in quantities of 10 and/or had pretty high minimum orders.

My thoughts are to:

1) Find few similar OEM replacement springs that I can cut to different lengths

2) Find shorter & stiffer springs to play with that option. 

Yup, 1 & 2 are kind of the same thing I suppose so any advice from the pros here would be most welcome! 

:-)
« Last Edit: December 10, 2018, 09:33:03 AM by mcoulter »


  • Central New York

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: If You Give a Mouse a Cookie...
« Reply #1 on: December 10, 2018, 08:45:55 AM »
Judging by the shot string, and assuming the gun is unregulated, it appears that you are starting just before the top of the bell curve, or somewhere along the top of it, and seeing that, plus the declining velocity on the low pressure side.... The low pressure side of a bell curve does, indeed, use more air than the high pressure (rising) side, which is missing in that string.... That being the case, clipping a coil off the spring should help, and since you can replace that length by screwing in the adjusting plug, you should have no danger of going too far.... You would get lower velocity and find the missing front part of the bell curve, and all those extra shots....

It looks like the rear tank is only "extra air".... Can you block off the line to the rear tank and just operate on the front one during testing?.... That may allow you to work on the gun safely without degassing it, no?.... Just a thought, please make sure that is safe before proceeding, as I am just going by the photos.... If this is possible, I would order another HP line, which would give you one spare "good" end to turn into a blanking plug....

Bob
  • Coalmont, BC

mcoulter

  • Jr. Member
  • **
  • Posts: 92
Re: If You Give a Mouse a Cookie...
« Reply #2 on: December 10, 2018, 09:32:14 AM »
Thanks for your input on this Bob.  I would love to mirror the back side of curve on the front side.  I can currently get 60 shots (5 mags) with a 20fps drop so I'd be pleased if I could get a string that went from 865 to 885 to 865 across 120 shots.   ::)

And yes, the rifle is unregulated and I was pondering options for temporarily making it a one-tank gun for testing... 

Here is the string from the 1/2 turn reduction in HS spring.  This is where I was seeing 60 shots being about right:

https://photos.app.goo.gl/vzE3TjrMC5kP3Vb16

« Last Edit: December 10, 2018, 09:51:49 AM by mcoulter »
  • Central New York

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: If You Give a Mouse a Cookie...
« Reply #3 on: December 10, 2018, 09:43:56 AM »
If your velocity is still too high after you get a nice bell-curve (eg. a 4% ES)…. then you will have to either restrict the transfer port slightly.... or drop the fill pressure a bit and compensate by reducing the hammer strike a bit further....

Bob
  • Coalmont, BC

mcoulter

  • Jr. Member
  • **
  • Posts: 92
Re: If You Give a Mouse a Cookie...
« Reply #4 on: December 11, 2018, 05:28:37 PM »
Thank you for your advise Bob! 

I took off about 2mm off the spring.  I was absolutely amazed how difficult it was to snip the spring!  I was barely able to make a dent in the wire with my snippers.  I had to use needle nose pliers to take that crease and bend it back and forth to finish the cut.  Next I tried to file the break point down a bit.

I probably won't be able to test until the weekend  :-(
  • Central New York

caniborrowsomeammo

  • Administrator
  • Hero Member
  • *****
  • Posts: 766
Re: If You Give a Mouse a Cookie...
« Reply #5 on: December 12, 2018, 12:55:46 PM »
Thank you for your advise Bob! 

I took off about 2mm off the spring.  I was absolutely amazed how difficult it was to snip the spring!  I was barely able to make a dent in the wire with my snippers.  I had to use needle nose pliers to take that crease and bend it back and forth to finish the cut.  Next I tried to file the break point down a bit.

I probably won't be able to test until the weekend  :-(

Spring steel, whodathunk? Get yoself an air or electric cutoff tool, cheap at Harbor Freight. Zips them springs short right now and also can be used to grind flat the end of the spring.
Buncha BB guns that I don't get to shoot as much as I like. Grinding out a living on the farm.

steveoh

  • Administrator
  • Hero Member
  • *****
  • Posts: 1688
    • Airgun FPE Calculator
Re: If You Give a Mouse a Cookie...
« Reply #6 on: December 12, 2018, 02:09:46 PM »
Quote
Remove the rear air coupling (yes, it came buggered up like this...)

Looks like a plumber who did work on my house, worked on your rifle at some point.  Apparently the dude's favorite tool was a set of Channel Lock pliers.
  • Benicia, California
Quackenbush .58 Outlaw
Shooting Chairs
Vallejo Ferry Schedule
DAQ .458 LA Outlaw Rifle
Sam Yang 909s .45
Sam Yang Sumatra .25
RWS Diana 350Magnum Compact Pro .22
QB-79 .177
Crosman 1322
Crosman 1377 - HoRodded 10 FPE
Diana Model 27 (childhood airgun)
Tolman Skiff
Airgun Calculator

mcoulter

  • Jr. Member
  • **
  • Posts: 92
Re: If You Give a Mouse a Cookie...
« Reply #7 on: December 16, 2018, 11:21:30 AM »
I snipped about 3mm from the hammer spring and got a 90-shots string with an extreme spread of 19 fps.  Average velocity dropped to 871 fps (30.5 fpe) from 889 (31.8 fpe) with JSB 18s.  This is right about where I want to be.  I still don't see the "front" section of the power curve though. 

It occurred to me that what I thought was a 200 bar fill is more like a 190 bar fill.  I had asked a couple of places where I had been getting my tank filled to compare my gauge to their gauge and each was off by the same amount.  I'll probably repeat this again with a 210 bar fill and see what happens. 

Oh, I almost forgot to mention that after snipping the hammer spring I am still installing it with the minimal amount of preload.  I have the adjuster backed as far out as possible.

https://photos.app.goo.gl/KJidLuzdRdUJos6aA



Perhaps the best news is that this string produced the most consistent groups that I have seen with this rifle.  Here are nine 10-shot groups from the chrony testing.  The distance is 53 yards and the pellets were jsb 18s.   The last (9th) group did open up a bit.






Close-up of group #4: 
« Last Edit: December 17, 2018, 07:34:04 AM by mcoulter »
  • Central New York