Like I said, curiosity. Many, many years ago I hunted deer up close in heavy cover with a 12 Ga. and slugs. But I don't recall them being as heavy as the ones you are proposing. If I remember correctly it was Winchester that offered the fastest slugs.
A standard slug is 437.5 grains and is booking along at about 1,600 fps. Mine weigh in a 775 grains and are designed like bullets with lubrication and lube grooves. I am only looking to get about 1,100-1,200 fps out of them but we will see where we land. I have started with the slowest pistol powders I have and may ultimately end up going with a rifle powder like Reloder 7.
The loads for the brass cases are 20 grains of blue dot as a starting load. The most conservative Max charge listed by Alliant for 2oz shot charge (875 grains) in a 3" shell was 33 grains. Owing to the fact that a shot charge doesn't need to press into rifling, I dramatically reduced my charges. ( I may only get like 500 fps with these loads but it's a good starting point.) A shot charge also has the powder pressed under the wad under some compression and in a plastic hull. My shells are tremendously more voluminous than a plastic shell due to the very thin brass walls. Even so... the walls taper a little so the 11 gauge over powder wad becomes wedged in about 1/8" above where the powder column ends. So there is an air gap there. You can actually hear the powder in my brass shells so they aren't compressed at all. The only other load was that plastic shell which was 30 grains of Alliant 300 MP. 300 MP is significantly slower than Blue Dot (even slower than H-110) as it is designed for maximum charges in heavy magnum revolvers. Even so... it is about 1.75x the charge used in a .410 shotshell. Given that a 12 gauge brass shell has an average of 3.5-4x the volume of a .410, I figured that a charge in the safe range for blue dot with a similar weight loads, should be more than safe for a significantly slower powder.
I had to use data for 3" hulls because it is so hard to cram 2 oz of shot into a 2.75" shell. However a 3" shell is still rated at the same pressures as a 2.75" hull. It just has larger shot capacity. This is why they are usually marginally slower than 2.75" shells. So, powder charge for shot loads should be similar to a slug charge given that most slugs have no need to be squished into rifling... Mine however does, so I significantly reduced the charge to start with. Because all the space in a shotshell is taken up by either wads or shot, case capacity has almost no effect on pressure for these shells (except for marginal differences in internal dimensions from different manufacturers). So a 3" shell should have the same space for combustion as a 2.75" shell of the same make, it just has more space for shot. In fact, there are 1-3/8 oz, 2.75" loads that have HIGHER max charges than 1-3/8 oz 3" charge weights... all just based on plastic or paper hull designs. So, to my way of thinking, you could effectively interchange the data between 2 3/4" hulls and 3" hulls and give them a margin for error.
I am not trying to make excuses here, I am just trying to effectively explain my reasoning for choosing the weights of powder that I did, and the reasons I believe they will be well within pressure maximums for a 12 gauge barrel and action.
The original paradox cordite load was a 750 grain bullet at 1,200 fps from a thin walled 12 bore paradox gun so that was what I was shooting for. (no pun intended.)
I am a nervous Nelly when it comes to making sure I don't blow guns up in my face and that is why I gave myself so much margin for safety with these initial loads.
I hope that makes sense...
Now... if you ran the data in quickload and tell me that I am going to be running 30,000psi... I will definitely rethink my approach. I do appreciate your concern.
. I am waiting for a nice day to get to the range.
