the 124gr EFMJ Expanding Full Metal Jacket (... 7.62 millimeter FULL METAL JACKET...)
It isn't known to be a high penetrator at all. In fact everything I've read says it penetrates a little less than the average 124gr Hollow Point (Gold Dot, Golden Saber, Hydrashock etc.)
It offers 2 advantages: almost guaranteed expansion (Hollowpoints don't always expand things get in the way and clog them up), and expansion through dry materials. So it SHOULD penetrate less through things like dry wall, and wood than a HP, and certainly less than a FMJ 9mm. Though I haven't seen any studies confirming this.
It works basically like a super soft point, i.e. a soft point that is very soft. Expands to about .53 every time in most of the tests I've seen. Penetration in bare gel... I've seen as high as 13" as low as 11"
Apparently it doesn't handle glass or steel very well (unlike the Gold Dot).
It was designed by Tom Burcynski (spelling?) of Quick Shock, and Hydrashock fame.
If you do a search on this forum for EFMJ, you should find the post that Tom himself contributed to. There are many others.
Most people conclude it is a great thing for people in areas that disallow Hollow Points (Europe, NJ), but may not be quite as good as regular Hollowpoints.
Personally, I like'em, a lot.
You can buy them REAL cheap over at:
www.ammoman.com
the owner Eric, is a real nice guy. He also has a page with a LOT of information on the EFMJ on his site (which is a LITTLE hard to navigate).
The tip of the bullet has a form of silicone in it that is covered with the jacket. When the bullet strikes the target, it expands. It has been tested, and expanded fine when it was cold, and hot. It shouldn't have any trouble doing so if it is very old either.
The one downside: it's a very long bullet for its weight, hence, in order to get regular 124gr 9mm velocity, it must be driven to +P pressures.
That's all I can recall at the moment, I've spent a good deal of time reading about it, and I keep my eyes open for more.
Here's another review of it:
http://greent.com/40Page/general/EFMJ.htm
Here's the direct link to the page with the data on ammoman.com:
http://www.ammoman.com/webstore_9mm.htm
http://www.ammoman.com/40_EP_DESC.htm
e-mail me (it's in my profile) for all of the expansion data (from various sources) I've gathered on it.
If it actually penetrated 13.5" like it says on ammoman's sight I'd carry it without hesitation, but most other tests I've seen say it doesn't quite penetrate that far.
Why was this post so long? Cause you're a newbie, and from Michigan, and I'm a Michigander. That and I talk a lot.
-Morgan