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I designed the NOE Pellet Moulds, and the skirt diameter was taken as the average of the three pellets that JSB make, the 44.8 gr. the 50.2 gr. and the Predator Polymag…. I just rechecked several tins of pellets I have of all three types, and got these average skirt diameters (the heads all averaged 0.300″)….
JSB 44.8 gr…. 0.306″
JSB 50.2 gr…. 0.307″
Predator Polymag…. 0.309″
I have both NOE moulds, and checked them as well….
Hunter 45.7-49.6 gr…. 0.307″
Hunter Magnum 59.2-65.7 gr…. 0.308″
As I am sure you are aware, the skirt diameter on pellets is significantly larger than the head diameter, to insure proper sealing…. The skirt of a pellet is very thin walled, and designed to collapse on loading, to allow pellets to fit a wide range of barrel and chamber diameters….There are lots of the NOE moulds out there, and I have never heard of anyone having a chambering issue…. The head diameter on the NOE pellets measures, on average, 0.300″….
You are correct, .308 caliber slugs or ball will not chamber easily (if at all) in any .30 cal airgun barrel…. as the bearing surfaces are solid, and not designed to collapse…. If you wish to shoot slugs in an airgun barrel, you normally have to increase the length of the chamber, and add a tapered leade, as they are intended to be used with pellets where the head diameter barely engraves the rifling and the skirt collapses to comply…. In addition, slugs must be sized to the groove dimeter of your barrel, and the manufacturers designation of the Daystate as a .303 (to take advantage of the popularity/recognition of that caliber in the Enfield rifles) may or may not have anything to do with the actual diameter…. The Lothar Walther .30 cal airgun barrel, for example, is 0.298″ land and 0.306″ groove diameter…. If you try and chamber a .308″ slug in that barrel, you will also find it too tight…. My NOE Bob’s Boattail slugs intended for .30 cal airguns have a nose diameter of 0.300″ and a driving band of 0.302″ so that they can be chambered properly, and even then, if the chamber is too short they will require some force to engage into the rifling…. Pellets, however, are not an issue, and chamber and function properly in any airgun barrel they have been tried in….
Bob