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....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 caliber1/2" OD barrel, maximum 0.250" transfer port ID.....25 cal = bore size ports.308 cal = 81% of caliber.357 cal = 70% of caliber9/16" OD barrel, maximum 0.281" transfer port ID.....308 cal = 91% of caliber.357 cal = 79% of caliber.408 cal = 69% of caliberBased 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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