Just for fun, I'm going to disagree with everyone idea that the HM2 will be around forever...or at least partially disagree.
Yes, it is a great squirrel round...but IMO it doesn't do much that a .22lr can't do, except shoot a bit flatter. A .22lr that handles hypers well is every bit as capable as the HM2 in it's effective range....but the .22lr will carry more energy down range due to the heavier bullet.
Yes, the HM2 is inherently more accurate...but I have .22's that'll take the heads of squirrels at 50yards easily--and at 1/3 the price per box. The accuracy at 100yds might not be quite as comprable...but with the limited opportunity I've had to play around with the HM2--it has fizzled off enough by 100yds to be less than ideal (IMO) on even prairie dog size game.
I've noticed my local shops carry only 1-2 selections of HM2, while they have nearly to full line-up of HMR's...
I asked my local shop the reason why--his response was that they sell at least 4 HMR's to 1 HM2's.
For the same price in a rifle, the HMR packs a bunch more punch--and most shooters are willing to pay more per box for the extra energy and effective range--myself included.
I can't justify buying an HM2 since I already own the "bigger, better" HMR, and I can't see enough reason to support why the HM2 would be superior than some of my .22lr's.
Now, do I think the HM2 is going to go the way of the 5mm? NO!
But I do feel like it's going to be more difficult to find ammunition for the HM2 than it will for the HMR. You will end up special ordering, or driving all over town to get your desired load...while every single place you go will stock everything you may desire for the HMR...simply because the HM2 will not be as large of a money maker as the HMR...
I think we can blame a lot of this on the way the round was released...
Releasing the HMR first, with the huge hulla-balloo that every one gave it right away, made the HMR a huge success. Many people jumped right out to buy a rifle chambered for the .17 Magnum Rimfire and test after test after test was done trying to prove how effective the round *REALLY* was...as we all know, it was a major success!
Then, almost as an afterthought, they released the HM2. Of course it didn't get the attention it desired at first, as everyone was still focused on the bigger, badder HMR...why settle for less when the HMR is just so darn appealing?
Then at some point, someone realizes "Hey, the HM2 is quite as explosive of small edible game."
Sure this gets a lot of attention for a while. The HM2 is the go-to round for cheap, accurate squirrel hunting as the big brother HMR has a tendency of blowing things apart.
But then?
Out comes the HMR in a FMJ. Now you don't have to worry nearly as much about shot placement, as the FMJ isn't so destructive. Now the HM2 really doesn't have much, if anything over the HMR...except $4-5 per box.
The HMR will shoot longer, faster, and flatter than the HM2.
It's more capable of taking larger size game (raccoon and fox, argueably coyote), it can take any size game at a further distance than the HM2, it's higher velocity makes it less effected by wind than the HM2, in real-world situations it's more accurate than the HM2, and lets be honest...the HMR is a lot more sexy than the HM2 to boot! The HMR is just a lot closer to a "do all" type cartridge than the HM2...
BUT, to re-iterate...I don't think the HM2 is going to disappear as it does still have it's place, and it will always win the arguement when it comes to cost-to-shoot.