Thanks to all that responded with floral foam info. Continuing with the 38-55..........  when breech seated, is there generally a gap between the mouth of the case and the base of the bullet or does the case mouth actually fit over the bullet some amount? Guess this actually a general question, unless it is different for different cartridges.

I don't think is is a matter of different cartridges as much as it is individual rifles/bullets/loads. Tinkering with seating depth adjustment on the seating tool is just another one of the fine tuning variables.  Some guys seat their bullets a fair distance ahead of the case mouth and flare the case mouth so it gets a scrape/seal in the chamber throat. you'll have to experiment and figure out what produces best results with your specific rifle/bullet/load combinations.  seems like I was advised to start with the bullet base 1/10th of an inch ahead of the case-mouth when I was breach seating.
With my new rifles I intend to breach-seat I'll probably start like that, tune for best accuracy with powder then adjust using the seating depth as the variable with the top couple powders/loads

I adjust my seating depth so that the first shot out of cold clean barrel starts forming the group. The means and methods for accomplishing this task was published in the ASSRA Journal but I don't recall the year or number.

JLouis

what about breach-seating though

My 38/56 was throated by CPA to suit the Paul Jones Spitzer bullet that I have. I seat the bullet .075" ahead of the case mouth. If I push the bullet out it shows the rifling marks on all the driving bands and halfway through the base bend. I was told that this was a good starting point. This setting has produced my best group so far .590" for five shots at 100 meters. I have not tried it at any other distance. So far the best group I have had at 100 meters with fixed ammunition is 1.120". All groups were five shots using black powder, the rifle was equipped with a 24x scope for all load testing. I can provide the load information if you require it.
Mike.

Here is how I set up for my first shots with a new rifle. 
I set my breech seater to .065" and breech-seat a bullet then I knock it out and check the rifling should stop somewhere just shy of the base of the bullet. Next Breech seat another bullet take a case that you have flared the case mouth on so you get a good seal. Seat a primer put it in the rifle and pull the trigger. Now put your hand over the action and eject the case. There should be enough pressure for the case to pop back out of the chamber and into your hand. Now you have a good seal. 
  Next take it to the range and shoot it until you find the speed where you get the smallest groups. When you have this you can do a final adjust to your seating depth. Moving .005” at a time see if your groups get any smaller or larger.

40 Rod