When you look at the Benjamin Nosler ammo, you'll see that they went for 3 thin driving bands, and the first two are fully engaged in the rifling.
GuysHow about making a chamber cast with Cerrosafe metal, that way you can have a good solid cast to take measurements from etc.https://www.ebay.com/itm/301891304226Very easy to take a cast of a chamber.HTH'sDon
a bit off topic, but with the original fx smooth twist where only the last few inches of barrel was grooved, how does “jump” before engaging grooves work out in this case?
The CAD models below show how deep the breech cone is for a 2 degree included angle, and for a standard taper pin reamer included angle of 1.194 degrees. The other factor that directly affects the length of the cone is the rifling depth. In this case, the land height (or groove depth) is 0.004". This is typical for calibers in the .357 to .45 range. For PB and airguns.As such, the fact that the model depicts a groove diameter of .452" does not detract from a discussion about a .357 caliber airgun barrel. The cylindrical section behind the cone is depicted as 0.001" larger than groove diameter. The TP diameter is show as 70% of the groove diameter, with the TP placement somewhat arbitrary, based on the assumed length of the projectile and loading depth.I happened to have a barrel design in .45 that I then modified to show the breech cone depth. As a reference point, the breech cone spec for the .45 ACP pistol caliber has a 4.7 degree included angle at the start of the lands. Very abrupt, compared to the typical 2 degrees, popular for PCP air rifles on this forum. But then the pistol bullet has some jump, from the case to the lands. Too shallow and angle would result in the bullet skidding excessively in the rifling. (Slight skidding can normally be observed in fired bullets, just at the leading edge of rifling impressions of gently captured round nose bullets). While common with PCPs, loading bullets partially or fully engraved into the rifling would eliminate skidding, it would not be practical with PBs for a number of reasons (with few exceptions, such as schutzen rifles: https://www.pyramydair.com/blog/2015/06/ruger-model-3-32-40-schuetzen-rifle-part-1/ ).