Re: Penatration & Expansion test.
When you shoot at subsonic velocities (1,040 to 1,080 fps depending on altitude), your barrel will need a little more twist to stabilize a bullet. Normally, rifle bullets are used - but this presents a problem in that most rifle bullets require a minimum of 1,600 to 1,800 fps in order to have some expansion. I own a 300 Whisper/Blackout that is designed to shoot both sub and supersonic rifle bullets. I have spoken with most major manufacturers of bullets and I have yet to encounter a normal copper-clad target or hunting bullet that will expand at subsonic velocities. They usually end up just penciling through.
My go-to subsonic round in my 300 Whisper/Blackout is a 240 gr. SMK over 7.9 gr. of Lil' Gun powder shot out of an 11 inch barrel with a one-in-eight twist. The round is devastating at close range. At 50 yards, the bullet has already begun to destabilize, with half the rounds beginning to "key-hole" on the paper target.
However, this "key-holing" can be a good thing. If you can find a long, heavy-for-caliber bullet that your rifle barrel will barely stabilize at subsonic velocities - you will find that it will begin to tumble once it penetrates the target. They saw this in Viet Nam when they first tried to shoot the longer 68-70 gr. bullet FMJ in an M-16 barrel with a one-in-nine twist. They would not stabilize much beyond a couple hundred yards. But it sure wreaked havoc on whatever it hit.
Subsonic loads do better with the faster burning powders typically associated with shotgun or pistol loads. Hodgdon 110, Trailboss, and Lil' Gun seem to work well. It can be tricky to try too much experimenting on your own as pressures vary a lot depending on how much empty space you have in the case.
You can learn a lot by reading on some of the forums dedicated to rounds primarily developed for subsonic use, such as the 300 Whisper, the 458 SOCOM, the 50 Beowolf, etc. As more people become involved with this aspect of shooting, more sources seem to surface. For example, Outlaw Bullets is a custom bullet makers that has designed a bullet that reputedly will open up at subsonic velocities. Another bullet maker (whose name currently escapes me) has a bullet with an extra large hollow point into which the reloader can insert an .17 cal varmint bullet to enhance it's expansion capabilities.
There is a lot of info on these other sites. They contain load data, and also a lot of info on expansion or bullet performance. A lot of these experiences are based on shooting into ballistic gelatin, which many believe to be most similar to human tissue.
Good luck on your quest. Be sure to post back with your findings.