I've been reloading the the 12 GA SPW for probably close to 20 years now and yes you can reload a 1X fired hull but it is difficult and sometimes just now worth the results. When I do use a 1X hull, I will use a hull that is longer than what I need so I can trim it down which then leaves a nice clean mouth for roll crimping. I also use one of these from Ballistic Products which is called a Spin Doctor. You put in drill and spin it in the mouth of the spent hull to reset the mouth. The heat from spinning along with some downward pressure can sometimes reset the mouth of the hull so it almost like new.
Over the years, I have just bought the already primed hulls from SlugsRUs https://www.slugsrus.com/
so my finished round is always the same. Here are some notes from my experience that I posted up on GBO sometime ago that may help.
The bottom line on really good slug accuracy is no different than metallic reloading. You need to be extremely consistent in what you do; everything from wad pressure, roll crimp pressure, total OAL, and so on. This is why I decided to use primed hulls only and the unskived Cheddites seemed to work the best for consistent accuracy.
The accuracy can be exceptional with the SPW. This is a 100 yard target that I sighted in couple years ago with the 12 Ga. SPW. I had put a new scope on slug gun so I needed to sight in. The first shot was high and right, I over compensated to right by going wrong dang way with scope in brain cramp and then over compensated to the left. The correction to center resulted in 3 rounds in almost same hole at 100 yards. I am shooting a 870 with thumbhole stock, Hastings bull barrel and I put a Timney trigger job into it which brought trigger down to 2 1/2 lbs. The reason I am showing this target is to show the consistency of the load
. I can actually walk a load into the bull even at 100 yards with scope adjustment which is normally tough to do with a slug due to inconsistency that can be with them.
I switched over to Alliant Pro Reach several years ago when SR4756 became extinct and glad I did. I am shooting the 12GA 1 1/4 at 1450fps and it shoots thru same hole at 50. Pro Reach does have some 20 GA loads that may be worth looking at since you get higher velocity with less powder so you can use the shorter hulls. This then lets you use a longer hull cut down if you choose to do so. The 20GA SPW weighs 350 gr which is not quite 7/8 ounce so you need to decide if it can work. I would suggest calling Alliant to see if they have any thoughts. They have been great to work with in the past.
Here is a buck I killed this year with the SPW and went maybe 40 yards. He was toast the second I hit him and was running downhill if you know what I mean when a buck is hammered hard. I have killed a lot of deer with this slug and will not switch out to another since they have performed so well.