If you talk to a lot of gun enthusiasts, many opinions will be shared and they often don't agree. Each person has their own background, reasoning, and justification for what they believe is the best. However, nearly everyone will agree on the following saying:
The bottom line is you want to hit well, hit deep enough, make a big hole and to not over-penetrate - in that order.
Obviously, that means you need to know your firearms and be able to use them to accurately hit a target. Practice. Remember to be safe, use eye and ear protection, make sure you follow the Ten Commandments of Firearm Safety.
Hit Deep Enough
You know if you are hitting deep enough by doing some sort of penetration testing. You need to be able to hit through the target to get into vital organs. If you don't do that, you might as well not shoot.
Make a Big Hole
Again, more penetration testing. This time, you are looking for the size of the track wound that the bullet produces. More is better.
It's also not just about the size of the hole. You're also looking for the size of the cellular disruption that would be caused by absorbing all the bullet's energy.
It's more critical for cops than for other people, but ideally you wouldn't want to shoot an intruder, have it go through your house and into a neighbor's house, through their television, into another room, and into your friendly neighbor.
It also means that a .50 cal Desert Eagle is probably too much gun if you are only concerned with home security. But again, it is the last concern. The .50 will certainly hit deep enough and make a big hole if you can wield it properly and accurately. The likelihood of actually hitting neighbors is pretty slim, but the odds of damaging property is certainly stacked against you.
The typical method that agencies around the world employ is to use ballistic gel. The FBI has typically set the standard for this information and they like to use this gel a lot. This gel is made by Gelita, once known as Kind & Knox. They call it Pharmagel or "250 Type A Ordinance Gelatin". Apparently you can also purchase Vyse gelatin (see their email to me though I would never do so). One kind reader notified me that the Massachusetts State Forensics Laboratory uses Knox 250 A and told me about some problems with Vyse gelatin.
Unfortunately, the authentic stuff is quite expensive for hobbyists. The FBI uses 6x6x16 or 8x6x16 inch blocks. Each block is composed from 1 kilogram of powder and that's roughly $22-$30 to make a single block. I could not find any source that sold any less than 25 pounds of powder, which would make about 3.6 cubic feet of gel.
Mix 1 kilogram of power and 9 liters of very hot distilled water. Pout into a mold and let it cool to room temperature in an area where it will not be contaminated. Chill 48 hours in a sterile environment to a temperature of 4° C (39.2° F). The gel must remain at 5° C to be calibrated. You now have a 22 lb brick of expensive, tasteless gelatin. It is a 10% mixture (90% water, 10% gelatin powder).
A double batch can make a 6x10x18 brick, and a triple batch can make a 10x10x18 brick.
The advantage of using this mixture is that you have the closest resemblance to human flesh that the FBI has found (without actually shooting someone). Also, when you shoot it, you can see the soft tissue damage as well as the track of the bullet. The downside is that it is expensive and you only get one shot per block. Yes, only one shot because the stresses involved with catching the bullet often tears or damages the gel.
I also have extremely detailed instructions available.
You can use standard gelatin from your local grocery store as ballistics gel. Normally it is used for jellies. Using 100% pure USP gelatin powder from Knox or Kroger is extremely similar; the only difference is in the chemical tolerances between the consumer and the scientific market. Just head to the Jello and pudding section and find unflavored gelatin by Knox or Kroger.
Get 6 boxes (about 36 oz / 2.25 lb / 1 kg) of gelatin and 3 gallons / 9 liters of distilled water per block you want to create. Then just follow the exact same instructions as you would for the official gelatin.
You might want to coat your molds with some cooking spray so they pop out. If you forget, you could run hot water over the outside of the mold until it is able to slide out.
The gelatin will spoil, so use it within a week. A few drops of wintergreen oil added in the beginning will help keep it fresh.
Never freeze the gelatin.
The acceptable measure of a block of gelatin is a .177-caliber ground steel BB to penetrate 3.34 inches (8.5 cm) when launched at 590 fps.
If you don't care about 100% laboratory-grade perfection, you can reheat the gel and reform it into a block after you shoot it. This is easily achieved by microwaving the gel and then reforming it into a block with the normal methods.
It is suggested that you do not shoot naked gel. Use cloth or some dense hard wood. Shooting bare gelatin will not produce enough differentiation with your different loads. To see the difference in ammunition, some intermediate barrier is needed. The cloth covering is much more demanding than what you might guess.
Gel may be expensive. You can use several alternate methods if you want to reduce your costs.
Jugs of Water
You want large containers and keep the tops open. The water will end up shooting out of the container. Using high speed rounds will cause them to fragment and disintegrate. Slower rounds can be captured mostly intact. Please make sure to have way too much water as the rounds may surprise you and go much further than anticipated.
Get some large wastebaskets to hold the water and newspaper. Two in a row will work for pistols, three should stop rifles. When you save up newspaper, stack it by alternating the major fold so that it lies square. Take out all inserts, glossy paper, and non-newsprint.
Stack the dry paper on edge in the wastebasket. Try to get the edges as square as possible for consistency. Center the paper in the basket and leave space around all sides for expansion. Fill the wastebasket with water. Keep it filled to the brim and let it soak for at least 2 hours. As the paper fills with water, the level may drop. Keep it filled.
To transport, pour out excess water and put the containers in plastic bags. Do not remove the paper from the wastebaskets until you are ready to fire. Stack the "bales" of paper on edge and shoot into the flat surface of the bale. You can make about five good shots to get accurate data before the paper disintegrates too much.
Guar Gum Slime
Stir together 48 parts warm water for 1 part borax (1 cup of warm water and 1 teaspoon borax). Set aside.
Stir together 192 parts cold water and 1 part guar gum (that translates to 1 cup of cold water and ¼ teaspoon guar gum). While stirring constantly, heat the mixture until it boils. Remove from heat and stir in 8 parts (2 teaspoons) of the borax mixture. Continue to stir until the slime forms. Do not play with it until it is cool enough to handle comfortably.
Store in an airtight container. 1 lb of guar gum (6 teaspoons per oz) will make about 24 gallons, which is about 3 cubic feet.
Pour ½ cup of water (3 parts water) into a small bowl. Add 6 tablespoons borax (1 part borax) and stir until dissolved. Set aside.
Pour ½ cup of water (3 parts water) into a medium-sized glass or metal bowl. Add ½ cup of white school glue (3 parts glue - a 4 oz bottle) of white school glue. Stir until combined. Optionally add a few drops of food coloring to change the color.
Pour the borax mixture into the glue mixture. Knead with hands until most of the water is absorbed and you can form a ball of slime in your hands. Lift the slime from the bowl and discard any water mixture remaining in the bowl. Return the slime to the bowl and allow it to sit uncovered for 15-20 minutes or until fairly firm.
Well, we are looking for a human or animal analogue. Red meat gives you the same results as shooting the gel. When using rib bones, it will be difficult to reproduce your results and thus make the experiment less statistically viable. I suppose you could do this if you have vast amounts of money or meat lying around. It is key that one can reproduce a study in order for it to be meaningful.
Shooting Holes in Wounding Theories: The Mechanics of Terminal Ballistics - A thorough and in-depth site that explains a lot of what happens with various sizes of ammunition. Very well written.
American Handgunner - Article: Penetration Testing Big Bore Handguns - "A handgun isn't all that far behind an Elephant rifle"
Bullet Penetration Testing - Compares newspaper, water, and actual results from a deer. By viewing many images, you can see the different rounds before and after they were fired into various mediums.
rec.guns FAQ - It's always a good idea to know where one should go for answers.
Simplified 10% Ordnance Gelatin Preparation and Test Procedure - Very organized instructions with pictures to help out with the steps.
Ballistic Gelatin Recipe - From the rec.guns FAQ.
Bullet tests report - PDF formatted report with images and tables for penetration tests.
FBI bullet penetration tests, in PHP, sortable - It's pretty nice to be able to look at the information in this manner.