Reader’s Digest Version tonite.
Will go into more detail tomorrow.
Fast powders don’t detonate or explode when case space exists. But do figure if there is room to accidentally add too much fast powder, like a double charge or more, that is going to happen unless one is vigilant. One accident is mistaken for the other. I use fast powders with vastly more airspace than discussed here....like Bullseye or WST in a 308 to shoot a 89 grain 32 wadcutter 950-1050. If case space was gonna cause high pressure surely my chronograph would have caught it.
Nothing other than normal velocities and low pressure. I have too many other examples of small charges in large cases to list. Nearly all have been chronographed. Nothing out of the ordinary. Ever.
If it is believed a powder is not position sensitive, hang with me awhile. ALL of them are. The conditions under which the powder is sensitive clearly have not been tested or statements like “XXX Powder is not position sensitive” would not be bandied about as often as they are.
And yeah, Titegroup and Unique can be position sensitive. None of them get a pass.
38 Special is worse than 9mm in this regard due to the two contributing factors, which are pressure, or a relative lack of it, and unused case volume. With standard pressure loads, 17000 odd psi, in 38 Special Unique shows 100 fps velocity variation with powder shifted front to back. Position sensitive? Yep. If you want low velocity variation in 38 use wadcutters and take up some unused case space. Titegroup does decently as long as standard bulet weights are used. Light bullets and low pressure and it doesn’t do so well.
Filler is properly used when the firing event does not alter the physical characteristics of the filler...does not change it from a solid into a fluid, for instance. Don’t try to compress an incompressible material into a smaller space under a pressure event like firing a cartridge. Avoid airspace when using filler or the purpose is defeated. Fillers don’t intermix with powder, said by a guy who has thrown and bashed cartridges about to see if they do.
Not the way I do it.
If by polyfil you mean Dacron, or 100 percent polyester fiberfill, your usage with 4198 meets all the criteria of effectiveness so long as all space between powder and bullet is well occupied with Dacron. Why would I say such a thing? Because minimum loads of 4198 without filler have miserable ballistic consistency when powder wanders around.
Either do not let powder wander around or use filler. Your choice.
Do not use fast powders with Dacron fiber or fillers like Grex or shotshell buffer. Rule of thumb is to use no powder faster than 2400 with filler. Fast powder fuses Dacron and plastic fillers. Fillered loads are, of necessity, well down in the pressure range.
Fast powders like Bullseye, Red Dot or Unique are great fun in the 45-70 and give mild recoil and report but do not trust old data for these powders published 85 year ago. The amounts used and the pressures given are in error. They are applicable, but not for heavy bullets or high velocities.