Disco SSG thread

[Buldawg76]

A good rule of thumb for the transfer port size for best power and efficiency balance is for the transfer port to be 75% of the bore diameter.

A 177 caliber should have a TP port hole of .133" and can go as big as .140" without pellet loading issues.
A 22 caliber should have a TP port hole of .165" and can go as big as .1875" without pellet loading issues.
A 25 caliber Should have a TP port hole of .1875" and can go as big as .200" without pellet loading issues

The above are intended for use with round nose pellets as pointed or wadcutter will have some issue loading in the bigger port diameters but I have found that loading with the barrel pointing downward while gently shaking the gun to allow the pellet to slide past the port on its own works quite well with the max size ports.

Mike

[Dairyboy]

Thanks for the confirmation of its mainly a power thing and not a efficient thing. Also thanks for letting me know bout the tp sizes. Since I have a .177 and the stock TP size of a disco is .140 it looks like I'm goin to leave the gun stock besides making an SSG.

[Buldawg76]

Some clarification on the TP port sizes in that the only issue with pellet loading as far as transfer port hole size is if the barrel port was drilled out to the max sizes. The transfer port itself will not affect pellet loading if it is over the max size unless it is the actual entry hole diameter of barrel port so you can go bigger with the port or tubing or what ever is used as the seal between the tube and barrel.

Just limit the barrel port size to those listed above as it will be the choke point so a bigger port seal tube will only create a choke point in the air flows path. It can at times be somewhat beneficial to reduce the sealing tube/port size to create a venturi effect such as in a carburetor which tends to increase the air flow velocity thru the venturi but the gains may only be marginal if at all.

Test with a smaller port as you can always go bigger easier than go back smaller.

Mike

[Rivers3Plinker]

Thanks for the confirmation of its mainly a power thing and not a efficient thing. Also thanks for letting me know bout the tp sizes. Since I have a .177 and the stock TP size of a disco is .140 it looks like I'm goin to leave the gun stock besides making an SSG.

Mike's information is correct and the same I've seen prior on the TP ports and barrel port with similar concerns for barrel port size working on pumpers. That's where I started with valve mods on air guns. Pumpers are different and I understand that but some of the modifications done to improve airflow are the same. You can also leave the port sizes stock but still pickup some flow and add power. Whenever you do that, you use more air one way or another.

In regards to efficiency, I was talking in terms of efficient air flow and reducing turbulence. I was looking at leaving the TP port and barrel port close to stock and improving the flow through those holes by improving airflow upstream. Hence the angle drilling of the exhaust port and working / blending the sharp turn in the roof of the port and reducing the poppet stem behind the head and/or enlarging the throat.

Like you, I'm going to start out stock and put the SSG on it and go from there. I'm running .22 caliber though, so I have a bit more room for power improvement. I don't however want to go hogging out the passages, making a noisy, air wasting gun.
In your case, with the .177 caliber, I think you are making the right choice, particularly if you don't have goals of making much more power and just want more shots and flatter ES.

[Dairyboy]

Thanks for the confirmation of its mainly a power thing and not a efficient thing. Also thanks for letting me know bout the tp sizes. Since I have a .177 and the stock TP size of a disco is .140 it looks like I'm goin to leave the gun stock besides making an SSG.

Mike's information is correct and the same I've seen prior on the TP ports and barrel port with similar concerns for barrel port size working on pumpers. That's where I started with valve mods on air guns. Pumpers are different and I understand that but some of the modifications done to improve airflow are the same. You can also leave the port sizes stock but still pickup some flow and add power. Whenever you do that, you use more air one way or another.

In regards to efficiency, I was talking in terms of efficient air flow and reducing turbulence. I was looking at leaving the TP port and barrel port close to stock and improving the flow through those holes by improving airflow upstream. Hence the angle drilling of the exhaust port and working / blending the sharp turn in the roof of the port and reducing the poppet stem behind the head and/or enlarging the throat.

Like you, I'm going to start out stock and put the SSG on it and go from there. I'm running .22 caliber though, so I have a bit more room for power improvement. I don't however want to go hogging out the passages, making a noisy, air wasting gun.
In your case, with the .177 caliber, I think you are making the right choice, particularly if you don't have goals of making much more power and just want more shots and flatter ES.

Yeah my disco is tuned shooting AA 10.34 around 20fpe for 16 shots with an ES of 2% which is untouched from mac1. If anything I would want to dial it back to 880-900fps instead of around 935-940 but am wanting to wait to tune until i get an SSG figured out. If I want more power I have a .22 and .25 marauder both with plenty of mods for power so I don't need more power out of my disco.

[Buldawg76]

Dillon
Just an FYI on the JSB/AA 10.34s shooting them at the 935/940 fps you state you are getting now is to fast for the best accuracy from them, if you indeed slow them down to 890/900 fps you will a see a marked improvement in accuracy with them.

I shoot field target with a 177 Mrod and used to shoot them at 920 fps until one of the far more experienced club members told me if I slowed them down to 890/900 fps I would improve my score by a good bit and that's what I did and scored my best match since the year and a half I had been shooting FT.

I also got the added benefit of 10 more shots per fill so I believe if you slow them down to the 890/900 you are wanting to get you will likely get 20 plus shot per fill and increase your accuracy and groups as well. It worked for me and you can always turn it back up. The SSG will just improve the tune you already have in it.

Mike

[David Mccann]

hello here, hope everyone is well. we all have the crud here. did muster courage to shoot a string today. diff ammo this time check this out. remember, only 104cc res. i did delete 3 errors that fired but werent read.
 Created: 03-15-2016 06:14:06 PM
Description:  3000psi jsb 18.3 jumbo heavy end 900psi   reggd disco
Notes 1:
Notes 2:
Distance to Chrono (FT): 1.00
Ballistic Coefficient: 1.000
Bullet Weight (gr): 18.130
Temp: N/A
BP: N/A
Shots
# FPS FT-LBS PF
79 669 18.02 12.13
78 671 18.13 12.17
77 681 18.67 12.35
76 681 18.67 12.35
75 676 18.40 12.26
74 680 18.62 12.33
73 679 18.56 12.31
72 676 18.40 12.26
71 681 18.67 12.35
70 684 18.84 12.40
69 682 18.73 12.36
68 678 18.51 12.29
67 690 19.17 12.51
66 683 18.78 12.38
65 687 19.00 12.46
64 680 18.62 12.33
63 688 19.06 12.47
62 ERROR 3
61 681 18.67 12.35
60 685 18.89 12.42
59 690 19.17 12.51
58 688 19.06 12.47
57 691 19.23 12.53
56 685 18.89 12.42
55 690 19.17 12.51
54 685 18.89 12.42
53 679 18.56 12.31
52 680 18.62 12.33
51 683 18.78 12.38
50 687 19.00 12.46
49 689 19.11 12.49
48 689 19.11 12.49
47 696 19.50 12.62
46 689 19.11 12.49
45 689 19.11 12.49
44 692 19.28 12.55
43 688 19.06 12.47
42 686 18.95 12.44
41 689 19.11 12.49
40 690 19.17 12.51
39 690 19.17 12.51
38 693 19.34 12.56
37 695 19.45 12.60
36 694 19.39 12.58
35 692 19.28 12.55
34 685 18.89 12.42
33 687 19.00 12.46
32 686 18.95 12.44
31 685 18.89 12.42
30 685 18.89 12.42
29 687 19.00 12.46
28 687 19.00 12.46
27 687 19.00 12.46
26 696 19.50 12.62
25 690 19.17 12.51
24 690 19.17 12.51
23 689 19.11 12.49
22 687 19.00 12.46
21 694 19.39 12.58
20 690 19.17 12.51
19 694 19.39 12.58
18 694 19.39 12.58
17 683 18.78 12.38
16 686 18.95 12.44
15 688 19.06 12.47
14 682 18.73 12.36
13 691 19.23 12.53
12 683 18.78 12.38
11 682 18.73 12.36
10 685 18.89 12.42
9 686 18.95 12.44
8 682 18.73 12.36
7 684 18.84 12.40
6 678 18.51 12.29
5 681 18.67 12.35
4 674 18.29 12.22
3 678 18.51 12.29
2 682 18.73 12.36
1 685 18.89 12.42
Average: 685.56
StdDev: 5.60
Min: 669
Max: 696
Spread: 27
True MV: 685.64
Shots/sec: 0.03
Group Size (IN): 0.00

[Dairyboy]

Thanks Mike that's what I was hoping for not that it isn't accurate now but alittle improvement is always welcome. I've noticed my jsbs 18.13 for the Mrod prefer under 900 as well even though there still very accurate above.

David Mccann that is an amazing shot string very impressive.

[David Mccann]

thank you sir, lucky i guess. i sure as heck am no genius. id rather be lucky than good. if your good, people expect more consistency from you. all the time. lol cheers

[Buldawg76]

Thanks Mike that's what I was hoping for not that it isn't accurate now but alittle improvement is always welcome. I've noticed my jsbs 18.13 for the Mrod prefer under 900 as well even though there still very accurate above.

David Mccann that is an amazing shot string very impressive.

Dillon
The 10.34s and 18.13s are two different shape pellets with different BCs so they will respond somewhat different at different velocities. The 10,34s are a dumbbell shape and therefore have slightly less drag in flight and the higher velocity will tend to destabilize them more so than the Diabolo shape of the 18.13s since those have more drag on the flared skirt which help to stabilize them at the higher velocities. It does not really make a significant difference until you get up in the transonic regions of 960/970 fps plus ranges but it does have some effect and more so on the 10.34s than 18.13s.

Mike

[Dairyboy]

David lucky or not either way that gun you built is pretty great and makes others like me want to do stuff like you have to have so many shots from that small of a res it's very impressive.

Mike thanks for clarifying I never really thought that the different shapes would make much of a difference but I'm very appreciative for that knowledge. So I will definitely be shooting for a tune around the 17-18 fpe range with them.

[Buldawg76]

Dillon
My Mrod FT gun is tuned for 18.6 fpe or right at 890 to 900 fps with the 10.34s and is a 1/2" group gun at 50 yards if I do my part as it is far more accurate them I am.

I prefer 15,89s in my 22s but right now it because I have 5000 of them to shoot up and then will be buying 18.13s or even the 25 grainers for my Hatsan AT44 that shoots the 25 grainer at 850 fps for a whopping 41 fpe at 35 full power shots in factory stock form.

Mike

[mobilemail]

hello here, hope everyone is well. we all have the crud here. did muster courage to shoot a string today. diff ammo this time check this out. remember, only 104cc res. i did delete 3 errors that fired but werent read.
 Created: 03-15-2016 06:14:06 PM
Description:  3000psi jsb 18.3 jumbo heavy end 900psi   reggd disco
Notes 1:
Notes 2:
Distance to Chrono (FT): 1.00
Ballistic Coefficient: 1.000
Bullet Weight (gr): 18.130
Temp: N/A
BP: N/A
Shots
# FPS FT-LBS PF
79 669 18.02 12.13
78 671 18.13 12.17
77 681 18.67 12.35
76 681 18.67 12.35
75 676 18.40 12.26
74 680 18.62 12.33
73 679 18.56 12.31
72 676 18.40 12.26
71 681 18.67 12.35
70 684 18.84 12.40
69 682 18.73 12.36
68 678 18.51 12.29
67 690 19.17 12.51
66 683 18.78 12.38
65 687 19.00 12.46
64 680 18.62 12.33
63 688 19.06 12.47
62 ERROR 3
61 681 18.67 12.35
60 685 18.89 12.42
59 690 19.17 12.51
58 688 19.06 12.47
57 691 19.23 12.53
56 685 18.89 12.42
55 690 19.17 12.51
54 685 18.89 12.42
53 679 18.56 12.31
52 680 18.62 12.33
51 683 18.78 12.38
50 687 19.00 12.46
49 689 19.11 12.49
48 689 19.11 12.49
47 696 19.50 12.62
46 689 19.11 12.49
45 689 19.11 12.49
44 692 19.28 12.55
43 688 19.06 12.47
42 686 18.95 12.44
41 689 19.11 12.49
40 690 19.17 12.51
39 690 19.17 12.51
38 693 19.34 12.56
37 695 19.45 12.60
36 694 19.39 12.58
35 692 19.28 12.55
34 685 18.89 12.42
33 687 19.00 12.46
32 686 18.95 12.44
31 685 18.89 12.42
30 685 18.89 12.42
29 687 19.00 12.46
28 687 19.00 12.46
27 687 19.00 12.46
26 696 19.50 12.62
25 690 19.17 12.51
24 690 19.17 12.51
23 689 19.11 12.49
22 687 19.00 12.46
21 694 19.39 12.58
20 690 19.17 12.51
19 694 19.39 12.58
18 694 19.39 12.58
17 683 18.78 12.38
16 686 18.95 12.44
15 688 19.06 12.47
14 682 18.73 12.36
13 691 19.23 12.53
12 683 18.78 12.38
11 682 18.73 12.36
10 685 18.89 12.42
9 686 18.95 12.44
8 682 18.73 12.36
7 684 18.84 12.40
6 678 18.51 12.29
5 681 18.67 12.35
4 674 18.29 12.22
3 678 18.51 12.29
2 682 18.73 12.36
1 685 18.89 12.42
Average: 685.56
StdDev: 5.60
Min: 669
Max: 696
Spread: 27
True MV: 685.64
Shots/sec: 0.03
Group Size (IN): 0.00

That bust be a modded disco to fill to 3000psi, or you are braver than me.

[David Mccann]

yes sir modded screws

[Buldawg76]

David
You are using grade 8 10-32 screw in the valve correct to be safe to 3000 psi. I am going the same route with my modded disco tubed, prod trigger, challenger hammer and RVA setup with a daisy Avanti 20 inch LW barrel 177 caliber and 1399 stocked FT gun I am working one now.

Hoping for 60 plus shot at 900 fps with JSB 10.34s to be able to shoot an entire match on one fill.

Mike

[David Mccann]

18-8 stainless steel 8-32

[Rivers3Plinker]

Disregard

[mooseslayer]

I found the link to rsterne recommended 10-32 bolts from Mcmaster Carr ..

http://www.mcmaster.com/#91306a352/=vhazyb

I measured the depth of my existing hole set the stop on my drill press to only drill the same depth as the existing valve screw hole. I could actually see where the tip of the drill just scored the bottom of the hole. I then took another drill I ground flat to finish the last bit. I also used the drill press to start the tap perfectly straight and then finished by hand with a bottoming tap. You can feel when you hit bottom. i think I got 5 full turns on the screw. I had the 3 screws given to me from another member because they wouldn't ship to Canada.

[ped]

on my 3000psi fill disco I used m5 valve screws and also drilled the breech ,tube and rear of the valve in the 22xx forward breech position with a m4
the next one I do i'll make a new valve front so the trigger frame front screw screws into the valve like a 13xx and will also fit a grubscrew through the tube from the top in the same location extra to the std fixing locations

[Buldawg76]

18-8 stainless steel 8-32

David
Do those screws have the .3125" head and are they button head, low profile or allen head screws. Got a link to the source and specs of the screws.

Mike