AT44-10 Long (.177) Regulated Shot String

[SpiralGroove]

Hey Guys,
Finally got my AT44 Long (.177) ready for a shot string.  With a remaining air cylinder of 185cc (after regulator/20cc plenum), using CPUM and starting at 2900 psi (hammer turned in all the way) -> got the following inefficient shot string:

Shots:
  1-10       ER2, 920, 915, 919, 924, ER2, 930, 922, 924, 924

  11-20     926, 932, 930, 927, ER2, 923, 914, 915, 915, 925
 
  21-30     924, 916, 920, 927, 925, 922, 926, 923, 923, 927

  31-40     925, 921, 918, 916, 912, 910, 894, 887, 893, 878

It looks as if the gun dropped off the regulator at about shot 36; the set-point was about 1400 and the pressure gauge read about 100 bar, so that seems reasonable.  The first 36 shots went from 200 bar down to 100 bar giving me an efficiency of about .63 .
All ports were opened up to 9/64" or .1406, all valve holes have been plugged except 1 (.1406) , used JB weld on the valve and added a sail too.

Looking to get 70 shots at 920 fps going from 200 bar to 100 bar, I believe I need to try the following because I'm wasting too much air:
1) Use a stronger valve spring
2) Make a Nylon Hammer
3) possibly reduce the size of the brass transfer port

Happy Turkey Day,
Kirk
 

[PIPERMAN]

4)Reduse the regulator working pressure 

[Mod90]

I'd say as a test to back off the hammer spring a little &  fill to 3100-3200 psi & see what happens, should gain quite a bit more regulated shots.
Right now that string is just a flat line & then starts to taper off after going off reg.

[SpiralGroove]

Hey PIPERMAN,
I could reduce the regulator's set point a few hundred psi, but the gun is just shooting air out the barrel and I believe mostly needs to have reduced hammer weight.  A stronger valve spring would also keep the valve from staying open too long.  I may want to check my debouncer to as it may not be working right.

Mod90, the hammer is already turned in (to reduce hammer strike) as much as possible.  With 4 turns out, the gun originally shot at 980 fps at 200 bar.  I also don't want to overfill - I had my fill of Air Bombs of late

Any thoughts are appreciated

[Mod90]

Are any of the valve holes plugged? That might increase the efficiency a little.

[SpiralGroove]

Mod90,
Per my first post, that's done too 

[Motorhead]

Easiest to counter the hammer weight ... MUCH stiffer poppet spring

[Mod90]

Sorry, I missed that 

I doubt the debouncer might be the cause, they don't usually fail.

A stronger valve spring & slightly lighter hammer would help a good bit, but I don't see those 2 mods alone getting you to 70 shots from 36. They might get you up to 50-55. TP seems a little large, seems the air can start to expand a little  before it has a chance to reach the barrel to do its job of moving the pellet.

Start by swapping out the spring first, that see where it goes.

[SpiralGroove]

Thanks Motorhead,
I'll put in a stronger valve spring (on poppet) first.

Hey Mod90, you're right, 200 bar down to 100 bar with 185cc of air is dismal!
However, if you look at Lloyd's calculator, 70 shots using 100 bar of air should be doable using CPUM at 920 fps.
This assumes I can get efficiency up to 1.22%; so it just means tweaking the Hammer/Springs/air flow to get there.
Who knows, maybe the Hatsan AT44 platform cannot be made as efficient as the BAM B50/B51

[PIPERMAN]

A valve spring will do nothing,
it's there to close the valve when tube is empty so you can put air in.
The air behind it is closing the valve,not the spring

[SpiralGroove]

Hey PIPERMAN,
The spring inside the valve (on the valve stem/poppet) keeps the valve from opening excessively when hit by the Hammer/striker.
The problem I'm having (IMHO), is the valve is staying open too long, allowing unneeded air (for the shot) to be blown out the barrel, thus lowering the guns efficiency.  I need to get efficiency up to about 1.22% from .63%.  If I use a stronger valve spring or a lighter hammer, the valve will not knocked open in excess of what is need for the shot.

I agree, lowering the regulator's set-point may help too, but my velocity is already about at my goal.  After all, when the regulator dropped off, shot velocity did not go up    

[PIPERMAN]

Well,i had lots of valve springs change (trying to see what's going on)
but never saw a difference,softer,harder,nothing...
I'm talking about this spring

[Ribbonstone]

Doesn't seem to make much difference until either (a) the spring is super stiff or (b) the coil bound length is long enough to limit the valve stem travel.

Lighter striker seems to work better to speed up the valve cycle, but I'd like to try both next time I have the Hatsan apart.

[Rocko]

I would shoot it awhile & let that new reg. settle in before I started messing with anything.