CAUTION: The following post includes loading data not covered by currently published sources of tested data for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
Grant 14,
It's probably because the chamber in the derringer isn't a smooth bore over the full length of the cartridge as it is in a revolver cylinder. You'll have to make a chamber cast or impression to be sure, but if I were designing a derringer, I wouldn't want to use up that much of the limited barrel length on freebore, so I would put the throat where there is a more modest amount of jump from a conventionally shaped bullet bearing surface to the lands.
I have some of the Speer shot capsules and could not get the square end of one to fit into the rifling of either of the Dan Wesson 357 barrels I had on the bench, nor into the muzzle of my K-38. They measure about 0.350" at that squared-off tip and about 0.354" at the open back end. That difference is a small draft angle to make them easy to pop out of their injection mold. The SAAMI bore minimum is 0.346" and maximum is 0.350", with most guns falling in the middle at 0.348" bore and 0.357" groove. So these capsules just aren't going to slip into an average rifled portion of a .38 Special barrel. So I'm guessing you have the shorter leade I described and that would certainly account for what you experience trying to chamber one.
See my potential pressure warning below.
Rodfac,
You need to put
the required off-book load warning at the top of a post when it isn't a published load technique. In this instance, I have put it in for you. Why it is important is outlined below.
If I understand your loading technique correctly, you are loading 3.5 grains of Bullseye with zero powder space. With a 111 grain projectile, QuickLOAD estimates the pressure at 27,000 psi assuming the start pressure is like having a saboted bullet. That assumes a chamber like the Derringer's, with no vent. It also assumes the diameter of the throat will be fully occupied and allow no gas bypass. It also assumes the projectile isn't unseated by the primer before the powder can get its burn up to pace. It also assumes the shot doesn't make space by compacting under pressure. Quite possibly, none of those conditions will hold. So actual pressure can only be determined by measuring. It is probably OK, based on your experience with it, but if that depends on the barrel/cylinder gap of a revolver being present to help with gas bypass or not, I can't say. If it does, peak pressure could be higher in the Derringer than your gun is experiencing. It's a bit of an unknown.