Need tuning advise....1720T

[BigTinBoat]

I recently picked up a 1720T to use for small game hunting and plinking in the yard. I have a 1750XT running off a 17ci bottle at 11FPE with 200+ shots per fill and was looking to set this gun up at 14-15FPE with 8.44JSB's and not really worry about shot count as long as I get 20+ per fill.

Got some great "preliminary" advise from GTA forum members on set-up and here is what I did.
I got a new transfer port and drilled it to .109 and I put a 3.5 BSB (3 rings and 2 thin washers) in front of the hammer.
Set the Hammer throw to 1/4 turn CW and Hammer Spring to 2 turns CW
Filled the gun to 3000 and shot my 7.9 CPHP's (cheaper to use for testing instead of my JSB's but FPE should be similar)

I have an idea of what changes to make to the Hammer throw and spring but would like to see what those here, with quite a bit more experience then me, have to suggest

Shot   FPS   Pressure on Gauge
                          3000
1        833
2        843
3        850
4        856
5        856
6        862
7        874
8        882
9        883
10      888
11      886
12      894
13      899
14      899
15      897       2400
16      900
17      907
18      902
19      895
20      901
21      896
22      894
23      888
24      885      2000
25      890
26      880
27      878
28      871
29      875
30      873
31      865
32      860
I think ending pressure was 1800 (pumped it back up before recording it)

Looks to me like I'm getting 24 shots (# 7-30) within 4% at an average of 889fps and 13.86FPE

What are your suggestions to get my best shot count using 8.44 JSB's and getting 14FPE?

BTW- sound is a tad bit louder, but accuracy appears unchanged.

Please let me know your suggestions. Won't have much free time to change and test until after Sunday so let me hear them...

[FuzzyGrub]

Use some jsb 10.3g and you will be there.  

You are close, so keep throw adjustments within 1/8 T + - of where you are. 

I'd do a full string at 1 Turn in and and another at 1 Turn out on preload to frame where you are at, and go from there. 

[BigTinBoat]

I'd do a full string at 1 Turn in and and another at 1 Turn out on preload to frame where you are at, and go from there.

Will do this. At present setting I guess pumping beyond 2700 is of no use. Hoping an additional turn in will get me use all the way to 3000.

I think the 10.3 would have the energy but I was looking at trying to stay with the 8.44 so the trajectory is flatter

[bstaley]

Just increase hammer spring tension until you get your first usable shots @3000 psi.  This will also increase your FPE a bit.  Once you get that set, then turn the stroke adjustment in very small amounts if necessary to get the power level you want.

[BigTinBoat]

Ok I think I might have it. Not sure if it can get better. I left the Hammer throw alone and turned the spring to 3 turns CW.
3000 fill - CPHP 7.9
1-913
2-919
3-914
4-928
5-923
6-920
7-920
8-927
9-938
10-930
11-934
12-931
13-929
14-927
15-925
16-925
17-916
18-918
19-911
20-909    (2100psi)
21-908
22-902
23-894
24-896
25-885   (1850psi)
26-881
27-876

So my 1st shot is in the 4% range and I have 22 shots with an average of 921fps - 14.88FPE
The 23rd and 24th shots are within 5% and the average is then 919fps and 14.82FPE

I pumped it back up to 2700 and shot 6 of the CPUM (10.5) at an average of 812fps for 15.38FPE

Any ideas to possibly improve the efficiency? or am I there?

Thanks!

[bstaley]

I'd say you have a bit too much spring tension now. If you are looking for 4% back it off until your first shot at 3000 psi is right around 900 fps.  That should get you a couple more shots.

[BigTinBoat]

I'd say you have a bit too much spring tension now. If you are looking for 4% back it off until your first shot at 3000 psi is right around 900 fps.  That should get you a couple more shots.

Now that you mention it, that makes perfect sense to set spring at 3000 so that I am right at the bottom of my 4%, of course I figured if anyone knew how to tune with the mod it would be you. Thanks for the help.

My original thought was to increase the spring tension just a bit and fill to 3100PSI. I did fill to 3100 and left the spring the same but then the first 3 shots were 860, 870 and 890 then it went to 909

[BigTinBoat]

Well I think I'm were I want to be.

All my strings I shot with the JBS 8.44 this time.

Filled to 3000 and backed off the spring tension 1/4 turn. First 2 shots under 4% - Got 23 shots from 889-926 with an average of 909fps for 15.49FPE
Filled to 3000 and increased the spring tension 1/8. First shot just under the 4% - Got 22 shots between 892 - 926 with an average of 912fps for 15.59FPE
Filled to 3000 and increased the spring tension 1/8 (back to where i was with the 7.9) First shot just under the 4% - Got 22 shots between 893-930 with an average of 916fps for 15.73FPE
Filled to just over the 3000 line and increased spring tension 1/4 turn. All shots from 1-22 in the 4% - Got 22 shots between 900-938 with an average of 923fps for 15.97FPE.

I can't over how with this O-Ring Buffer I can keep increasing the FPE and the shot count is not suffering. Thanks BStaley

Oh and from 10yards here are the 100 shots I took....all in one hole, able to be covered by a Mercury Dime

[bstaley]

Nice!  I really need to get one of those.

[BigTinBoat]

Just thought I would see how easy this gun is to tune now.

Kept the same spring tension and then backed the Hammer Throw out fully CCW.
Filled to the top side of the 3000 psi line again. Shot 30 shots (JSB 8.44) from 847 up to 878 and back to 839. (Only using 29 shots though as the last one was out of the 4% by 4fps) Average velocity was 863 with an average FPE of 13.96.

So with just changing the HT by 1/4 turn I can go from 22 shots at 15.97FPE to 29 shots at 13.96FPE using the same fill pressure and same spring tension.

The more I shoot this gun the more I like it. Even fits me so well I'm thinking about giving my Dad the Disco and getting a PRod in .22.......just worried about getting a "bad" barrel.

[wilsonj1018]

Prods are just as sweet and they will all shoot with one pellet or another but not quite as well as the 1720 lw barrels can be had too don't forget that if you do order one get some hn ftt in 5.53 54 and 55 they will shoot one of them I can assure you