I've tried three different primers so far: Lg. Pistol, Sm. rifle, and Lg. Rifle. At the very worst, the sound levels are close to that of a good-size cap-gun. Don't really have to worry about the noise levels that much, unless you are firing in a very small enclosed area (try maybe a 8'x8' room). But, overall, it is nothing compared to shooting a pistol round, or an unchecked primer alone.
I don't completely fill up the brass on my 45 Colts. I made my own mold using a block of wood, and a drill bit just over the size of a resized Colt casing. One thing to remember when making something like this: A drill press is a great asset, and ALWAYS cut the block in half FIRST before you drill the holes; otherwise, you will have some funny looking plugs. Plus, I measured out how deep I wanted the bit to go, by measuring with calipers the length of a FMJ 230gr RN. Once the bullet was measured, I transferred that length to the drill bit, and used a black marker to mark how deep to go. Another thing I found out that will help keep the brass cases cleaner after firing, is to push the plug as far as it could go into the case with simple hand pressure. This helps keep the residue from coming out too readily.
Barrel length is not too much an issue. The pistol I used for my wax shooting sports a 7 1/2" bbl. Short terms, I used my SRH 454. Works pretty good.
Be careful where you shoot them though; they do go fast enough to hurt. I clocked a few plugs with the different primers, and they did make a dif. The plugs I made average 16gr or so in weight. With a Lg. Pistol primer, the velocity was around 390FPs. The Sm. Rifle primer, for which I had to use a 454 case, was a bit slower, running at just under 300 FPs. Now the Lg. Rifle primer, using a 45 Colt case for this as well, zipped out the bore at 461 FPs. But the Lg. Rifle primer is probably at the limit for my plugs; I used paraffin alone, which IMO is too hard and brittle. With the Lg. Rifle, I had one come apart and turned it into a small wax spread gun. Have fun, stay safe, and remember to use eye protection especially. Plus, don't forget good ventilation; you need it to help keep residual lead exposure to a zilch or at least a small minimum, and remove the primer smoke. Kind of like those indoor ranges, make sure the stuff is drawn
away from you.
-Patriot