Basicly, there is an airspace/cavity in the Bullet just behind the jacket nose and it will expand as the nose is driven back into the cavity and thus expanding it. It actually provides more stable expansion when shot thru obstructions into test media.
That is total opposite from everything that I've heard. I am curious where you have heard this?
Expansion is very reliable as it has no choice but to expand. The basic bullet design is a hollow point with a ball of rubber or silicone placed in the hollow. This is then covered over with the bullet jacket giving it the same profile and look as a FMJ round. When the bullet impacts something the rubber ball forces the hollow point to expand. With a conventional hollow point the hole will sometimes plug up and not expand as the design requires a fluid (like flesh etc) to expand well. With an EFMJ it doesn't require any outside substance. There is a picture of an EFMJ bullet (45acp) showing that it has expanded after going through just 1" of 10% ballistic gelatin (bullet is still in flight). There is also another pic showing expansion after 3/4" plywood.
If you don't have expansion you WILL have penetraion. Its one of or the other usually and sometimes both.
The only drawback I've heard of as far as expansion is that you won't get as much as you would with a conventional hollow point. But if you have a gun that won't feed hollow points or live somewhere that you can't have them this load is an option. From what I've heard they are the issue load (in 40 S&W) for Detroit.
The bullet was designed by Tom Burczynski who also designed the Hydra-shok, Starfire and the Quik-shok bullets.
My information is from the book Stopping Power by Evan Marshall and there is a chapter (number 14) on the EFMJ.
I'm using Federal EFMJ in my Keltec PF9 and from what I've read, the mods are correct - it has no choice to expand, and penetration remains optimal. Will it expand to .50 like a Gold Dot? maybe not....but I'm real sure it will not get plugged up in denim or some other type of cloth when passing into the body.....because there is no hollow point to get clogged up.
Having said that, I'm using them in my Keltec mainly because they are lower recoil rounds, at least for the 9mm. I wanted something that would stop somebody, but wouldn't kick like a mule if I had to fire it under duress.
Forgot about that Slicone Insert in the Bullet Nose, in my description! My bad!
But yes, that bullet design provides more reliable expansion than other designs and would be in amongst my current inventory, were it more available in my area. I've only been able to get one box of 9mm's of it, and I shot that up testing it for functionality. I am looking to buy more of it, in 9mm as well as .45 ACP.
I've been carrying EFMJ for years. Before my retirement, I was on our Major Crime Team, which is a fancy term for the multi-agency, muli-discipline homicide team. I've investigated a number of real world shootings, where the person shot was hit with hollow-point bullets in a variety of calibers, including police Ranger ammo. Those bullets, when passed through heavy cloth or leather, would clog and perform as a FMJ, with no expansion. I discovered the EFMJ and after research and my own testing, I'm sold on them. They expand each and every time, no matter what kind of barrier I put them through. They don't peel back razor sharp petals as some HP's do, but they do expand each and every time.
Some ammo makers now claim that their hollow-points will expand after going through barriers, but I haven't tested them.....yet. In the mean time, the only way I could order them online was a minimum purchase of 250 rounds, so I have a bunch and will continue to carry them.
I have them in .40 S&W, 165 grain, and .45 acp, 200 gr. +P. They are expensive, so I don't plink with them, but those are my carry rounds for my auto-loaders.