.310 Cadet Reloading, Case Length
Trooperdan;
That sounds to be a feasable reason for the different case lengths. Here in Australia there was a company (SUPER) which used to sell loaded ammunition for this Rifle. They used a swaged lead Projectile, (sample of this type in photo within the original post in this thread), these were not inside lubed boolits but dipped in lube after loading. They had no real lube grooves at all and relied upon what stuck to the outside of the boolit. It has been quite a while since I have seen let alone shot any of these type but I do have 200 of then here in their original box for inclusion in the trail.
Yesterday I took the Bertram Brass cases and trimmed them back to 1.080" loaded these cases with a Winchester Small Rifle Primer, 3.8 grains of Red Dot and seated 135 grain Cast from the H. T. BUGG mold. I tested these rounds in the chambers of two spare barrels I have; one a W. W. Greener and the other a BSA. The rounds were a drop fit in one barrel and a 'push home' effort in the other. As the 135 gr boolit has a longer groove diameter section it would be inclined to engaged the rifling sooner than a lighter projectile.
With different case lengths available; I would adjust the length of the case to suit the individual chamber of your barrels (There appears to be quite a bit of variation in chamber sizes also).
In closing; I would summarize the variation in case length by saying Longer cases for inside lubed projectiles and shorter cases for heel type projectiles and trim cases to suit the individual chamber.
:drinks: cheers for now,
John