17 cal

The .17 HMR is pretty much a necked down .22 mag round. I think they started selling the ammo and rifles for it in 2001 or 2002 so not too long ago.

As far as the plastic tips go this link could help explain it.
http://www.gunsandammomag.com/ammunition/plastic_fantastic/

I didn't read most of it so hopefully it helps

They came out in 2002 as my wife was one of the first ones. You did not say which one you had.

The first rounds were Hornady, and they use a 17gr bullet that is modeled after their varmit bullets. When they hit something they rapidly expand and fragment inflicting a considerable amount of damage. The tip also helps with the BC. Of course there are others that make them now, and some are HP types. My wife still shoots the regular Hornady load. Find one that you like, and your rifle likes, and once sighted in get a bunch. They all shoot slightly different, and if you want to make those 200yd+ shots, you need to be using the same bullet as you are sighted in for.

We like it better than the .22 mag since it shoots farther, flatter, has a better bullet, and isn't as prone to ricochet.

-Bidah

I have a Marlin 917V, with Leupold VXI, 3-9x40. It is a tack driver, no modifications needed. I've put .17 HMR, hollow points and ballistic tips, in milk jugs filled with water at several ranges out to 200 yards, and the bullets fragment every time.