clayman49 said:
I have trialed card board cut out cups (these were in the shape of the German Iron Cross) but I gave up on these as I found them tedious and time consuming to load.
I have a unchoked 16ga double with rough bores, and barrels too thin at the muzzle to hone them out completely, so I've worked with these and a variety of other DIY biodegradable shot protectors and have come back to two variations of a simpler design that's reasonably easy to use.
It's a 2-petal shotcup similar to the old Alcan "shot protector". If you haven't seen these, they were made from a strip of flat plastic, with a circle (the base) in the middle of two rectangles (the petals), with a fairly narrow connection between the base and each petal. The circle was, obviously, bore diameter, and rectangles had a width of 1/2 the bore circumference and a length of the height of the shot charge. You folded the petals up and roll them to insert them into the shot shell on top of whatever wads you were using for a seal and cushion.
The DIY ones can be made from either thick paper or light cardstock. The basic shape is a rectangle, with a width of 1/2 the bore circumference, or just under, and a length of the bore diameter plus twice the height of the shot charge. If the material is not too stiff, the shotcup can be made from the initial rectangle with no further cutting: wrap it around the end of the "former", align and form into a cylinder, then fold in the "corners" on the base under. With stiffer material, one can cut away excess around the base like the Alcan ones, but I've never found it necessary and just make cuts at the bottom of the petals, angled slightly into the base (to give a slightly wider and stronger attachment), and fold the excess around the base up into short reinforcements, either inside or over the outside of the bottom of the petals. It is often easier to pre-form the shot protector at home, getting the petals even, creasing the edges of the base, etc., reopen it flat for compact storage and transportation, then reform it quickly on loading.
They are reasonably easily laid out using hand-drawing, spreadsheet cells (without full base details), or a CAD program. With manual or CAD drawing, one can add details of the shotcup's base, which helps alignment when you're loading.
I found an interesting variation on them while working on steel-shot loads (haven't finished development yet). If you make the petals ~1/4" wider than half-bore-circumference, the overlap you get when you load them makes them much less likely to to allow a gap if they slip a bit out of alignment. This also makes them less fussy to load through a choke (one of my buddies has a choked 12ga double), and possibly suitable for use inside paper cartridges.
I also found a very interesting free material - the smooth-on-one-or-both-sides card stock of old telephone book covers. You find similar material in other paperback book covers, some flyers/junk mail, and color/photo laser printer/copier card stock (but NOT inkjet photo stock). They range from 6-10 mil (.006-.010") thickness and the clay finish (not the plastic finish of inkjet stock) makes them really slick. This is thin enough to be reasonably easy to work, but gives surprisingly good protection for the thickness, and I think the slickness is critical to this - naturally, they go slick-side out. In my limited testing so far with some 8-mil phone-book covers, one thickness was sufficient to prevent scrub-through with a 1oz/3.1/4dr load of Fe#5 in my 16ga. I was getting scrub-through with much thicker multiple-thicknesses of uncoated card stock. I still need to test more to verify that one thickenss is enough for my fowler with 1.1/4oz 3.1/2 or 3.3/4dr Fe#3 loads, and I suspect one might need 2 thicknesses (2 protectors nested, probably at right angles) when shooting steel through a choke, or possibly even 3 when going through the forcing cone of a suppository gun. The lumpiness of heavi-shot might require greater thickness than steel or bismuth.
Joel