Lightweight hammer improving accuracy.

[Froggy.]

Might just be me, but I could never get my Sumatra. 22 carbine to shoot accurately off a bipod. It was very hold sensitive..69g hammer 25lb spring 1.4 Inch hammer travel... Felt like shooting a low power springer!
I went through many stages of reducing hammer weight and valve mods to improve this... Even with the 17g hammer and 12lb hammer spring I was having issues when using a bipod.
Got the nylon hammer down to 9 grams and spring 8lb 1.4inch stroke... Valve throat 4.2mm... Stem 2.8mm and 6mm poppet.
Success at last... Shooting 18gr jsb's at +-860fps. Still not moa at 100 yards but getting closer.
Wind was about 3 mph right to left.
First groups checking zero at 50 yards, was shoot 1/2 mil right... Aimed 1/2 mil left for bottom target.

Following groups shot at a lasered 100 yards.. Prone... Bipod.
Screwed up the centre target group... Taped the holes and shot the other groups from a single fill doping for wind.
 

[WesBob]

Wow! That's a huge hammer reduction! Nice shooting!
How is your shot string now, did it effect the overall es in the same amount of shots? Nice work!

[Froggy.]

Thanks!

[WesBob]

Wow, nice shot count and string! Looks like a regulated string, must be very well balanced!

[skorec]

YES – fainter/less   hammer strike do fainter/less  gun push. 
If you use balanced valve or hammer less  valve ( Huben K1 ) you can do it also without   less power.
Also more gun weight  or more  barrel centric hammer  strike ( as at AirForce Condor  )  can help.

Notice
Nice shooting at 100 yd

[Froggy.]

The Sumatra carbine used to max out at just over 44fpe with
18gr jsb's with 69 hammer... I now get 37 fpe with 15 gram hammer and half the hammer spring force.

[skorec]

YES –very good result .
You have excellent efficiency , accuracy and speed for short barrel Sumatra

Whit the long barrel Sumatra will be probably better to shot Jumbo Monster REDESIGNED at about 950 FPS. I thing that also at this case lighter hammer and spring may help. 

Notice.
Personally I prefer  shorter not less power spring  because the gap between the hammer and  valve stem may increase efficiency too.

[Froggy.]

YES –very good result .
You have excellent efficiency , accuracy and speed for short barrel Sumatra

Whit the long barrel Sumatra will be probably better to shot Jumbo Monster REDESIGNED at about 950 FPS. I thing that also at this case lighter hammer and spring may help. 

Notice.
Personally I prefer  shorter not less power spring  because the gap between the hammer and  valve stem may increase efficiency too.

The sumatra has a very long hammer stroke, increasing lock time. I prefer this because I can get a very light trigger pull, only 7oz, with the long softer hammer spring and very light hammer.
The full length rifle makes 45 fpe with 17g hammer shooting 18gr pellets... Should be close to 50fpe with monsters.

[skorec]

I can not believe - The stock  Sumatra has 70+ FPE
http://gunroom.com.au/s/item/details/eun-jin-sumatra-2500-500cc-22-cal/

[Froggy.]

I can not believe - The stock  Sumatra has 70+ FPE
http://gunroom.com.au/s/item/details/eun-jin-sumatra-2500-500cc-22-cal/

Yes the stock sumatra makes 70fpe easily with 32gr EunJin pellets at 1000+ fps with standard 4.3mm barrel port.
When I modify the valve I restrict the valve throat from 8mm to between 5 and 4.2 mm.  This makes it possible to Make a smaller poppet... 6 mm 127lbs to open in my case... Factory poppet is 12mm 508 lbs to open at 2900psi.
I'm limited in max power by the amount of air I can flow through the restrictor and how thin I can make the valve stem.
With my 2.8mm valve stem I get a air flow area similar to a 3.3mm tp...reducing stem to 2mm will give 3.7mm.
I haven't tried for max power with the 4.2mm restrictor, but I should be able to get 50fpe with a 2mm stem in the long rifle .

[Motorhead]

Results don't surprise me at all ... lower the cyclic vibration / oscillation the guns wiggle prior to the actual shot is reduced.

[Froggy.]

Tested a few pellets at 40 yards, no wind.
Bipod prone position.. 5 shot groups.
Very impressed with the 14.35gr jsb's shooting at +-960 fps.
Placed a pellet in the 2 smaller groups for size perspective.
Baracudas were useless.

[ranchibi]

Froggy, excellent work on that gorgeous Sumatra, nice groups! Like you, I cannot find a better pellet than what JSB makes. You live in a beautiful country

[Froggy.]

Thanks ranchibi!
Made a temporary camera mount to fit the scope... It's a bit shaky and looks like it's filmed with a potato, 30x optical zoom up to 120x digital, but it gives an idea of the noise, shot cycle and ease of accuracy.
40 yards.. Bipod.. Prone.. 18gr jsb's.

[moorepower]

Looks like you probably have enough air capacity on board! 13ci tank on lower tube?

[Froggy.]

Looks like you probably have enough air capacity on board! 13ci tank on lower tube?

9.5ci.

[skorec]

Froggy,

Rally nice shooting.
Did you taste also 70+ yd ?