Our .38-55 dies are manufactured to load to SAAMI specs for the cartridge. This includes the use of bullets not larger than .377 however, our seating die will accommodate bullets to .379.
We utilize a 375 Winchester seating die in the .38-55 die set because it allows us to keep production costs lower and in turn, our die prices more reasonable. The use of this seater is acceptable just as using a 38 Special seating die works for the .357 Magnum. The difference is cases between the 375 Winchester and the 38-55 is approximately .060.
Our sizing die for the .38-55 is meant to size the case inward enough for customers who want to use .375 bullets which is common in some rifles. If larger bullets than .377 are used, it is probable that you will see a significant case bulge that represents the bottom of the bullet near the top of the case. While the use of larger bullets than .377 is common for the 38-55, we have decided not to open the sizing die up because it would then fail to hold a .375 bullet.
Normally, using a softer lead alloy allows a .376 or .377 bullet to fully obturate in a larger bore reducing leading and providing increased accuracy. Another option is to purchase an extra long decapper at a very reasonable cost to replace the one in the sizing die. The result is a decapping pin that protrudes below the bottom of the die. This allows the user to deprime without having to size the entire case to accomplish it. To use the modified die, turn it into the press until the pin goes down to the priming hole in the shell holder when the ram is at the top.