GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on June 24, 2016, 04:39:26 PM
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With the new Cothran valves, and other high performance valves, being available for guns like the Disco and MRod, I thought it might be a good idea to look at what are the largest transfer ports that can be fitted into various diameter barrels.... Consider this drawing for barrels of 7/16", 1/2" and 9/16" OD.... For reference, the red circle is .30 cal, smaller circles are .22 or .25 cal, and larger ones are .357 and .408 cal....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/Barrel%20Ports_zps26dplkrd.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/Barrel%20Ports_zps26dplkrd.jpg.html)
The outer vertical lines represent the OD of the transfer port, and the inner vertical lines the ID, in all cases leaving a 0.047" (3/64") wall thickness, which I find is about the minimum with a Teflon transfer port.... You could go thinner with brass, but then you won't have room for O-rings to seal it (even if you go to 1mm Metric).... The horizontal line joining the outside vertical lines is the depth of the recess required to seat the TP fully into the barrel OD.... This is what limits the caliber, because as you increase the barrel ID (caliber), the wall thickness between the bore and the flat for the transfer port eventually disappears.... You then have two choices, go to a smaller OD transfer port (which limits the ID, just what you don't want on a large caliber).... or reduce the depth of the recess in the barrel, which will thin out the area for sealing the TP to the barrel, and it can then leak....
You will note that the largest calibers I drew in each barrel OD have similar wall thickness (over 1/16"), and I stopped where I would consider going larger to make the barrel either unsafe, or too easily damaged.... and besides any larger the TP recess would cut into the bore.... That gives a practical maximum of .30 cal in a 7/16" OD barrel, .357 cal in a 1/2", and .408 cal in a 9/16".... You could go to .458 cal in a 5/8" OD barrel, although I didn't show that.... If you go to those maximum calibers, you have to be VERY careful when machining the recess for the TP that you don't machine too deep, or you will break into the bore, and lose the sealing surface at the front and rear of the TP....
Within these limitations, there ends up being a maximum TP diameter, relative to the maximum caliber, for each barrel OD.... This puts an additional limit on the FPE you will be able to develop.... For pellets, a TP diameter of 75-80% of the caliber is all you really need.... but for the maximum FPE using bullets, the ideal situation would be to have bore-size porting, which means the TP would be the caliber on the ID, and the barrel port would be oblong, blending from the TP diameter down to a narrower, longer port, to prevent loading problems.... Here are the relative percent sizes of the ID of the TP to the caliber, for the combinations above....
7/16" OD barrel, maximum 0.219" transfer port ID....
.22 cal = bore size ports
.25 cal = 88% of caliber
.30 cal = 73% of caliber
1/2" OD barrel, maximum 0.250" transfer port ID....
.25 cal = bore size ports
.308 cal = 81% of caliber
.357 cal = 70% of caliber
9/16" OD barrel, maximum 0.281" transfer port ID....
.308 cal = 91% of caliber
.357 cal = 79% of caliber
.408 cal = 69% of caliber
Based on this, I would suggest that if you plan on building a bullet shooter, you look at the larger OD barrels, so that you can fit larger transfer ports, to get the maximum possible FPE.... Trying to get too large a bore in a skinny barrel will really make your job difficult, when going for maximum airflow.... In addition, the thin barrel wall will increase the flexibility of the barrel, introducing more vibration during the shot cycle.... These are all things to consider when converting any rifle to a big bore.... and this post is timely, considering the new high-flow valves coming available....
Bob
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Thanks for the post. I have always been curious about coverting a disco 22 cal to shoot cast lead bullets.
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I have a Disco Double that shoots either .25 cal cast (~ 50 gr.) or .30 cal pellets (about the same weight).... I find it difficult to get enough power to shoot .30 cal cast any heavier than about 70 gr., because of the limitations of the exhaust port diameter in my valve, which is 0.219".... It does a decent job with .357 cal pellets, but not enough airflow for bullets.... Having said that, I can get 3 shots at 170 FPE with 127 gr. bullets in .357 cal.... Here is how it compares in the various calibers....
(http://i378.photobucket.com/albums/oo221/rsterne/30%20cal%20Disco%20Double/DiscoDoubleTunes_zpsef65df88.jpg) (http://s378.photobucket.com/user/rsterne/media/30%20cal%20Disco%20Double/DiscoDoubleTunes_zpsef65df88.jpg.html)
My gun uses 1/2" OD barrels in a custom breech.... The TP is 5/16" OD x 7/32" ID.... Valve is secured with three 10-32 high-tensile screws to allow 3000 psi fills....
Bob
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I have always been curious, but never took the time to look up the 10-32 screw mod as i didnt have a disco. To do the 10-32 screw mod all you have to do is drill and then retap the valve in the disco?
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I had this same very problem when I converted my 22synrod to .357. Since the receiver fit a barrel diameter of 7/16" or I measured .433", and the wall thickness of the barrel was only "approximately" .039" i had no chance of seating the transfer port into the barrel.
At first, I made my transfer port flush on both sides like a normal transfer port and had leakage. I couldn't get past the 100 fpe mark. It wasn't until I was watching a tube buggy getting built "on TV" that got me thinking on how to solve the problem.
So, I made a plastic core to fit in the transfer port, made a sleeve that the transfer port and core can fit into, and using a drill bit the same size of the barrel (.433") drilled the transfer port so that the top of the transfer port would have an arch the same as the barrel.
Press fit and sealed. Leak gone and the gun hit 128 FPE.
Here is the drawing.
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Good solution, shorty.... It would actually make it easier to seal the TP to the barrel with a larger port, and never any worry about hitting the bore with a flat.... I would think a 3/8" OD transfer port could be made to work just fine on a 1/2" OD barrel.... Drill the breech straight through at 3/8", it will even hold the side of the plastic TP tight against the sides of the barrel.... That would be the perfect solution for the 3/8" TP recess in the Cothran valve.... 8)
Bob
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With the new Cothran valves, and other high performance valves, being available for guns like the Disco and MRod, I thought it might be a good idea to look at what are the largest transfer ports that can be fitted into various diameter barrels.... Consider this drawing for barrels of 7/16", 1/2" and 9/16" OD.... For reference, the red circle is .30 cal, smaller circles are .22 or .25 cal, and larger ones are .357 and .408 cal....
(http://i378.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/Barrel%20Ports_zps26dplkrd.jpg) (http://s378.photobucket.com/user/rsterne/media/Parts%20for%20Sale/Barrel%20Ports_zps26dplkrd.jpg.html)
The outer vertical lines represent the OD of the transfer port, and the inner vertical lines the ID, in all cases leaving a 0.047" (3/64") wall thickness, which I find is about the minimum with a Teflon transfer port.... You could go thinner with brass, but then you won't have room for O-rings to seal it (even if you go to 1mm Metric).... The horizontal line joining the outside vertical lines is the depth of the recess required to seat the TP fully into the barrel OD.... This is what limits the caliber, because as you increase the barrel ID (caliber), the wall thickness between the bore and the flat for the transfer port eventually disappears.... You then have two choices, go to a smaller OD transfer port (which limits the ID, just what you don't want on a large caliber).... or reduce the depth of the recess in the barrel, which will thin out the area for sealing the TP to the barrel, and it can then leak....
You will note that the largest calibers I drew in each barrel OD have similar wall thickness (over 1/16"), and I stopped where I would consider going larger to make the barrel either unsafe, or too easily damaged.... and besides any larger the TP recess would cut into the bore.... That gives a practical maximum of .30 cal in a 7/16" OD barrel, .357 cal in a 1/2", and .408 cal in a 9/16".... You could go to .458 cal in a 5/8" OD barrel, although I didn't show that.... If you go to those maximum calibers, you have to be VERY careful when machining the recess for the TP that you don't machine too deep, or you will break into the bore, and lose the sealing surface at the front and rear of the TP....
Within these limitations, there ends up being a maximum TP diameter, relative to the maximum caliber, for each barrel OD.... This puts an additional limit on the FPE you will be able to develop.... For pellets, a TP diameter of 75-80% of the caliber is all you really need.... but for the maximum FPE using bullets, the ideal situation would be to have bore-size porting, which means the TP would be the caliber on the ID, and the barrel port would be oblong, blending from the TP diameter down to a narrower, longer port, to prevent loading problems.... Here are the relative percent sizes of the ID of the TP to the caliber, for the combinations above....
7/16" OD barrel, maximum 0.219" transfer port ID....
.22 cal = bore size ports
.25 cal = 88% of caliber
.30 cal = 73% of caliber
1/2" OD barrel, maximum 0.250" transfer port ID....
.25 cal = bore size ports
.308 cal = 81% of caliber
.357 cal = 70% of caliber
9/16" OD barrel, maximum 0.281" transfer port ID....
.308 cal = 91% of caliber
.357 cal = 79% of caliber
.408 cal = 69% of caliber
Based on this, I would suggest that if you plan on building a bullet shooter, you look at the larger OD barrels, so that you can fit larger transfer ports, to get the maximum possible FPE.... Trying to get too large a bore in a skinny barrel will really make your job difficult, when going for maximum airflow.... In addition, the thin barrel wall will increase the flexibility of the barrel, introducing more vibration during the shot cycle.... These are all things to consider when converting any rifle to a big bore.... and this post is timely, considering the new high-flow valves coming available....
Bob
Pretty good stuff Bob.
I went a litle different way with the port in a .25 synrod. I had already machined a .375" bore in my receiver and milled a .187" x .309" slot in the barrel because this slot equals the volumetric area of the bore. When the Cothran valve came out, I was tickled that it already fit the .375" opening in my air reservoir so the most excellent valve was basically a drop in being I had already embarked on some mods and just so happened to be pretty much on the same page with the dimensions as Don when he engineered his valves.
Using brass rod, I machined a tport with an o-ring groove on the bottom that would compress the o-ring that shipped installed in the valve, and for a seal in the receiver it fit tight enough that a little acrylic silicone seal took care of mounting it into the receiver and also sealing it against leaks. The ID at the valve is .250" tapering to the .187" x .309" slot which matched up with the slot in the barrel.
I don't have chrony numbers yet, but it is surely throwing some lead, recoil is quite noticeable and it is right loud. Can't wait to chrony the Exact King Heavy 33.95gr Diabolo. I was trying for 80fpe minimum out of this project, and would like to get 100 if it is possible. The Exact King Heavy has a head diameter of .250" and a skirt of .261" and my barrel has a groove diameter of .256"~.257" and a bore diameter of .250" so far these have the best fitment of any pellets I have found, the head would be a bore rider and the skirt should seal nicely.
You are much better with numbers and math, how would my slotted port compare to your figures you posted?
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This thread was made to explain what is the maximum OD transfer port that can be used with a given OD of barrel, without breaking through into the bore with the flat that the TP seals against.... The OD of the TP then dictates the maximum ID that can be used.... It has nothing to do with what size you WANT to use for a given bore, it explains the limits of that you CAN do with a given OD.... and why you should use a larger OD barrel for the larger calibers....
I assume your SynRod receiver is 1/2" ID, intended for a 1/2" OD barrel.... I didn't make a drawing like that, but there is room for a 3/8" OD transfer port flat in a 1/2" OD barrel PROVIDED the bore isn't too large, .30 cal MAX.... If you try and machine a 3/8" flat in a 1/2" barrel in .357 cal, I can pretty much guarantee you will hit the bore before you manage to get a complete 3/8" wide flat.... I just installed a .357 with a 1/2" OD in a custom receiver with a Cothran valve, with a 3/8" transfer port, and the only way I could do it was to stop the depth of the barrel TP flat before it was a complete circle.... in fact it is only 5/16" wide.... When you look up through the receiver, there are gaps right where you would want an O-ring to seal, but I used a Teflon transfer port, and that worked just fine.... With a .250 cal barrel that is 1/2" OD, you can machine a 3/8" wide flat without hitting the bore, so you should be OK....
Your solution of using a 3/8" OD brass transfer port, in a 3/8" hole in the receiver, and then sealing it up to the receiver and barrel with Silicone Sealer may work, or not, or may fail over time.... There is a lot of pressure trying to blow that Silicone out of there.... I would simply use a piece of 3/8" Teflon rod that is 0.010" too long for the distance between the barrel flat and the flat on the valve, and let it crush to seal.... As far as using an oblong barrel port to achieve bore-size area without having the port too wide and damaging the pellet, that is something we have been doing for quite a few years now, and it works perfectly.... The barrel port on my .257 Hayabusa is 0.190" x 0.325" at the boreline, tapering to a 0.257" ID transfer and valve port.... At 3000 psi, I have seen over 160 FPE with an 88 gr. bullet.... My Disco Double, with a Cothran valve, got 116 FPE at 2900 psi with a 50.6 gr. bullet in .25 cal.... by using an oblong barrel port similar to yours....
Bob