Sumatra 2500 carbine tune.

[Froggy.]

We all know Sumatra rifles have way to much hammer spring preload.
I decided to cut my spring to remove most of the preload... removed about 1inch.
I also made a brass TP... 3.5mm hole...factory one is 5mm.
I don't have a shot string with the full length spring and 5mm tp but with the power weel at max it started at 1050fps with 18. 13gr jsb's and dropped into the 900's by shot 4 gulping an incredible 10 bar air per shot.
With shorter spring and 3.5mm tp.
190bar. 998...991...980...976...964.  160bar.
Shot string with power weel on the green dot 3 clicks from the lowest power setting.
192bar
1...919
2...934
3...926
4...935
5...932
6...922
7...922
8...918
9...920
10.910
11.909
12.905
13.906
14.899 advance power weel 1 click. 165bar
15.931
16.929
17.922
18.922
19.914
20.905 +1 click 150bar
21.918
22.908
23.905
24.903
25.890
26.889
With the factory settings and the power weel on the same position velocity would start at 950 and drop about 6fps per shot... I had to click up the power wheel every 6 shots. And only got  20 shots within 40 fps.
Even after cutting the spring, it's still way to stiff for the rifle to shoot a bell curve string.
The standard spring is 1.4 mm wire...I found a correct size spring with a 1.2mm wire diameter that felt way to soft... I cut it to the max length where the rifle will still cock just before coil bind.
Also polished trigger parts and replaced the trigger shear spring with a lighter unit I wound myself.

Shot string with softer hammer spring and 3.5mm tp.
185bar
1.. 822
2.. 830
3.. 848
4.. 850
5.. 854
6.. 853
7.. 850 175 bar
8.. 852
9.. 859
10.859
11.857
12.860
13.870 165 bar
14.865
15.873
16.874
17.880
18.873
19.887
20.885
21.888
22.890
23.890
24.891
25.890 135 bar
26.890
27.887
28.884
29.876
30.871
31.860 120bar
32.858
33.853
34.838

Now I just need a slightly stiffer hammer spring and a smaller tp to increase shot count and up the velocity.
With the lighter hammer and shear springs the trigger pull is now only 9.9oz and cocking effort only 4lb!

[Rescue35]

Nice results. Keep us updated.

[Froggy.]

Went back to the factory TP. I now get 18 shots within 3% between 905 and 930 fps with only a 155bar fill down to 105 bar.
I'm happy with this setup because I have a 200bar cylinder = a lot more fills.
5 shot groups at 44yards 18gr jsb's ...I experimented with different holds...causing some of the groups to wander a bit.


That 5.8mm. 228 inch group is my best ever at 44yards.

[mista meener]

that is great froggy you got that rifle shooting long strings per fill into tiny groups. the best outcome. good work

[topwaterjim]

Love the info,I gotta bookmark this for reference.

[ancient1one]

Excellent work and write up.. I have wanted a Sumatra Carbine in 25 caliber for over a year. After seeing your post its time for a Carbine 25 caliber.

[Nutcracker]

Excellent work and write up.. I have wanted a Sumatra Carbine in 25 caliber for over a year. After seeing your post its time for a Carbine 25 caliber.

Agree, I have been following all the .25 carbine post for the last few months.  Could not take it any longer.  Just ordered one from PA. Out of stock, but they should have some by first of Month.  I also ordered a MTC Connect for something else, but may put it on the Carbine just to see how well I am going to like the Connect.  If that doesn't work out well on the Carbine, then I may put my  Ultra Dot on it for awhile.  I also have a 3-9 Leupold EFR coming back from Leupolds shop. The AO ring just spins. I am thinking the EFR or the Ultra Dot, would be a better match for it.

Is the distance between the rear toe of the stock & front of forestock, to close for a sling to carry comfortably?

 

[Nutcracker]

Went back to the factory TP. I now get 18 shots within 3% between 905 and 930 fps with only a 155bar fill down to 105 bar.
I'm happy with this setup because I have a 200bar cylinder = a lot more fills.
5 shot groups at 44yards 18gr jsb's ...I experimented with different holds...causing some of the groups to wander a bit.


That 5.8mm. 228 inch group is my best ever at 44yards.

Am I following you correctly, you obtained these results by  cutting 1" from the factory spring?

[Froggy.]

Rifle with custom LDC.... Noise reduced from 108db to 84db...very back yard friendly.


One piece scope mount milled to fit Hawke 6-24 x 44 sf...the front clamp was milled leaving one set of clamping screws.


Shot these groups at 44yards.. 18 gr jsb's.. From a 145bar fill down to 95 bar. Groups were shot in order 1234...1 and 4 are 6 shots 2 and 3 5shots.
Placed the coins in the middle of the aim points.
The last group was shot just to see how much the pellet would drop as pressure lowered below 105 bar.

[Froggy.]

Went back to the factory TP. I now get 18 shots within 3% between 905 and 930 fps with only a 155bar fill down to 105 bar.
I'm happy with this setup because I have a 200bar cylinder = a lot more fills.
5 shot groups at 44yards 18gr jsb's ...I experimented with different holds...causing some of the groups to wander a bit.


That 5.8mm. 228 inch group is my best ever at 44yards.

Am I following you correctly, you obtained these results by  cutting 1" from the factory spring?

No...it's with the factory TP and the lighter hammer spring.

[Nutcracker]

OK, I see. TY

[Froggy.]

Used the power wheel to reduce power and flatten the bell curve for 16gr jsb's.
 180bar.
1...860
2...869
3...877
4...870
5...876
6...880
7...881
8...881
9...877
10.880
11.883
12.882
13.881 160bar.
14.879
15.870
16.878
17.879
18.877
19.880 150bar.
20.880
21.877
22.878
23.870
24.868 140 bar.
25.865
26.873
27.860
28.862
29.861
30.864
31.860
32.859 125bar.
32 shots within 3% 24 fps.

[Froggy.]

44 yard groups with 16gr jsb's. I lost the plot on the last group...pulled 2shots.

[Thunnus 72]

Please help. Where does the the trivger assembly? aTook it apart sear spring go? Took it apart to polish sear and didnt see where sear spring went and am now stuck.
Thanks