.375 is a normal 'cap and ball pistol' size sold by Lyman, Lee, RCBS, Saeco, etc and would indicated using the swage or drawing tool provided.

The next smaller 'standard' size is .350", although I think Lee might make a .360", which would be undersize as well, but perfect for .358's or 9mm's.

It is an easy system to make for yourself, but a lathe would be handy. You don't want the firing pin to hit the hard metal - only the rim of the inserted blank.

In smaller sized cases, I've heard of boring out the primer pocket of normal brass and using shotshell primers. Shotshell primers can be bought in several different 'strengths'. .358 Winchester or the 57mm cases come to mind.

Read the primer tests on page 18 of Ed Huble's 12 Gauge From Hell thread,(near the bottom of pag. 18) for strength in shotshell primer. They'd be the easiest to set up and would probably do for close range. You could have 3 or more in a pocket for grouse. If mor power is needed, a grain or two of AA's 5756,5746, 5677 5766, 5767??? or whatever that number actually is, would help. It's AA's fast burning cast bullet propellent that doesn't need fillers is the one I'm referring to. Look up the correct powder number before trying this. Hanloaders know which powder I'm referring to. Red dot shotgun powder might also work - without fillers. Only a gr, or 2 is necessary - just to boost speeds a bit. In that situation, I'd use normal primed brass only(easier), not modified, but it really wouldn't matter. This system would also work in larger cases, like the 9.3x74 with round blls or short lighter cast bullets.

My brother has a grouse load for his .356 Winchester using 110gr. 9mm bulk cast bullets & a fast pistol powder. At 25 yards, they shoot on the top of the post of his 'plex' scope and into a 3/8" group, normally - sometimes an odd flyer - maybe a bad bullet. I had a similar grouse load for my .458 using 230gr. Speer .45Auto bullets. They'd shoot into bug holes at 25 yards and still hold 3" at 100. At 1,340fps, they had about the same power as a .44 mag 6" Smith with descent loads but only 'popped' when fired - very quiet.