I sometimes use projectiles made of PVC pipe backed by bore-sized 3/4-inch thick plywood disks, as I recall this is what I show in the video I linked above. I don't bother with anything like concrete since I find it unnecessarily labor-ingtensive. I tape the plywood plug onto the back of the pvc pipe then fill the pipe with sand, then tape over the front of it to keep sand out of the bore. I've shot that load in rifled breechloading BP cannons where the PVC pipe is correct size to be engraved by the rifling, and at 100 yards the projectile is quite accurate. Of course it always breaks up on impact, but sked 40 PVC pipe isn't that expensive. We've also used the thicker sked 80 pipe that usually has to be bought at specialty plumbing suppliers, and it is more expensive, I have to think twice "do I really need that thickness" before buying a hunk of it.
I cut the plywood disks with the closest size of hole saw from a cheap Harbor Freight set of multiple-size hole saws. I use a drill press to mount the hole saw, as a hand drill is very hard to control.
So a question I have is why use concrete if sand would work? However note that sand is not rigid and in acceleration probably could swell the pipe somewhat and exert some additional force on the bore, so I only use it in very strong guns. If there's any doubt concrete is better since it takes away that extra uncertainty.
On the topic of things falling apart in the bore, if the PVC fits against the bore fairly close, it is worth a try I think. The plywood disk is being pressed onto the end of the PVC pipe with a great deal of force during inbore acceleration, and as long as the rear of the pvc was cut square, I don't think any significant amount of gas is going to get inside the projectile at launch, I've never seen this happen.
If you are worried about things coming apart, check out the schedule 80 pvc pipe, it is something like 1/4 in. thick and incredibly strong stuff.
Here's the table of dimensions, both OD and ID, top table is the cheaper white sked 40, bottom is the thick, more costly gray stuff. Go by the OD size to get something close to your bore, ignore the column on left as that is just a name rather than a true dimension. I have machined PVC pipe on a lathe to get a "bit too large" size to fit, make a simple mandrel to support the inside of the pipe and center it as it is too squishy to swing in a lathe without some form of mandrel. PVC cuts very cleanly and easily on a lathe. Take a copy of the table along with your calipers when shopping:
http://www.harvel.com/pipepvc-sch40-80-dim.aspI've only used this system in very strong orignial steel and bronze guns in good condition, I don't recommend it for any old cast-iron unlined guns, part of the success I think depends on a very smooth bore (or rifled) so as not to hang up or break projectile. I would also use very moderate loadings while trying this out,if you do so. I am only recommending this for "advanced gunners" who have a lot of experience and "know the piece" they are firing, I would not suggest doing this unless you have fired a good deal with traditional projectiles first.
