Many years ago, I learned that using a drop tube helps to settle, sort of compress, a charge in a cartridge case, especially rifle cases. Yesterday, I started loading some cartridges where a drop tube would come in real handy.
I had previously worked up a top end load using IMR 7383 under a 147 grain NATO ball in 7.62 X 51 cases. The charge worked out to 42 grains.
This is what 42 grains of 7383 looks like in a 7.62 case by simply dumping the charge in the cases with nothing more than a charging funnel.
Using a drop tube reduces the height of the charge column significantly. This provides room for the bullet to be seated, eliminates any chance of unexpected pressure spikes and promotes consistency in the burn speed. In the photo below, the top of the charge column is below the top of the shoulder.
Building the drop tube was simple. I salvaged the plastic tube that protects the stem of a pocket thermometer and fastened it in the mouth of a 7.62 case with the head cut off. The total length of the drop tube is only six inches long, but makes a big difference in the height of the charge column.
The diameter of the tube fits the case mouth perfectly to the point that I have to gently push it in to get it to fit. The case is fastened in a Forster case holder, the drop tube installed and a funnel placed over the open case on the top of the tube.
When dumping the charge in the funnel using the drop tube, I trickle the charge in slowly. The results can be seen in comparing the first two photos.