TC
I stumbled across this now...
The Rubber bullet mod had the barrel cut down and an adaptor screwed onto it. I’m guessing about 3/4 to 1” in dia. And about a 12” long.
I used one about 25 years ago , courtesy a Copper who was also a gun lover. The projectile is /was a pointed rod of softish, black rubber about the same length as the adapter.
It was muzzle loaded, pointy end facing out. A standard blank was loaded and it was aimed 3/4 of the distance to the target and fired so that the rubber projectile hit the ground and bounced up to about knee height when it reached the target. Direct fire was NOT encouraged. At least that’s what I was told by the cop. We tried out the 3/4 of the length, and various distances , including direct aimed shots at a Fig 11 at 50 m, and even a couple of shots in the air. When aimed to bounce off the ground, it tumbled and went sideways into the Fig. 11
I own a No. 4 and a long Tom in .303 and love the old ladies.
I bought a single shot break action Rossi .410 /.22 switch barrel some years ago and soon learnt to reload as the cost of ammo was very high. An old bloke at the club taught me to anneal and reform .303 cases into very decent .410 loads.
The First step to convert 303 cases for .410 was
Make sure they were boxer primed

thin the rim or they wouldn’t fit into the shotty.
Anneal necks and shoulder
Run them into a bigger FLS die- I used a 9.3x62- makes them pretty straight wall.
Prime with LP primers
Load with 5-6 gr of any pistol powder
Cram in a wad of toilet paper as OP wad
Sooji or cream of wheat till the case mouth
Seal with TP and soap
In a safe place, point it straight up and shoot.
Some cases came out nicely fireformed but others had a coke bottle shape, which opened up in a couple of reloads.
I do all this with no press, just hand tools. And now, use a .38 FLS die to open up the mouth.
These cases never need any resizing, last for ever , very minute of rabbit, and I use everything from shot,3x000 sg, and 150 gr 0.356” wadcutters which group 3-4” offhand at 25m. And have just made a few 150 gr wadcutters with a 12mm cork cylinder glued and screws onto the base to enhance accuracy.
Forgot to mention earlier, I also use black powder ‘square loads’ of 1/2 Oz Ffg and 1/2 Oz no. 7 shot for the .303/.410 shells. Since I taught my son this , he loves letting them off and photographing the blast, smoke and flash at dusk.
Makes cleaning the bbl a PITA. But they’re as Effective as smokeless loads

Good therapy to get through this lockdown