For over 50 years I have hunted squirrels with a rifle. My all time favorite guns were a rifle in .22 LR, a Thompson Center Contender Carbine in .22 Hornet, and the same gun in 6mm TCU. Recently I tried the new .17 MACH II and the details of what makes, or breaks, a good Squirrel gun were brought painfully home.
One of the most important, and overlooked, factors in a squirrel rifle is the effect of air movement, or cross wind, on the bullet's flight. The .17 M2 has a published muzzle velocity of 2,010 feet per second. The standard 17 grain bullet has a very poor ballistic coefficient (BC) of about .125. In only a 10 mile per hour cross wind this little bullet will be deflected some 3.5 inches laterally from your point of aim at 75 yards.
Because of its higher muzzle velocity the .17 HMR is little better, being moved laterally about 2.5 inches at the same range. Any time you shoot outdoors there will always be some air movement and both of these new cartridges are noticeably deflected by even the slightest puff of air.
The popular .22 Long Rifle and .22 Winchester Magnum shoot bullets that have even poorer ballistic coefficients and velocity's and are even more affected by air movement.
The effect of air movement on a bullet is directly proportional to the velocity and BC of the bullet. At the same velocity, a bullet with a twice the BC will be deflected half as much by the same air movement. Don't expect too much from any of these little guns when shooting at ranges much beyond about 40 yards outdoors.
Another important detail for a good squirrel rifle that is commonly overlooked is the sight height above the bore center line. These days everyone wants a scope on his or her gun. About the lowest you can mount a scope is 1 3/8 inches above the bore. And 2" inches above the bore is not uncommon. With a scope and almost any cartridge you are going to shoot low at short ranges of less than about 25 yards. The higher your sights are above the bore the worse this problem becomes. If you practice and become good with old fashioned, low mounted open sights you can eliminate this second problem and be right where you need to be from 15 yards out to over 75 yards.
No matter how good you and your equipment are it becomes very difficult to consistently hit a squirrel at ranges beyond 75 yards. If your gun and sight set up will keep you within plus or minus 1/4 inch from 25 yards out to about 75 yards you have achieved the ideal set up.
The perfect cartridge for such a squirrel rifle may be the tried and true .22 Hornet, shooting a 55 grain Spire Point Boat Tail Bullet. At a MV of 1925 feet per second muzzle velocity, the 10 mph cross wind deflection will drop to about 1 7/8 inches. Load the same bullet to a MV of 2300 feet per second and the wind drift will decrease to less than 1 1/2 inches. The larger and extremely accurate .221 Remington Fireball Case will also work if at similar velocities.
If you want to get really precise, move up to the likes of the 6mm TCU cartridge. Load for 1909 feet per second muzzle velocity and your 75 yard 10 mph wind drift will drop to only 3/4 inch. At a maximum muzzle velocity of 2600 fps the 75 yard wind drift is reduced to less than 1/2 inch.
Some of you are going to say this guy is crazy shooting Squirrels with an 85 grain 243 bullet. But you will actually do more damage to a squirrel by hitting it with the explosive 17 grain HMR or Mach II bullets than you will with the tougher .243 projectile at moderate velocity.
One must be extremely cautious where rifle bullets go if they miss or pass through the target, especially when shooting at things up in the trees! The last example here uses a relatively tough 85 grain 243 varmint bullet. At these slower velocity's it may not come apart when on a squirrel or a twig, and could carry a very long distance.
The upshot of all of this is to be aware of the effect sight height above the bore center line and the effect of air movement have on where your bullet strikes the target. For me there are significant disadvantages to using a scope for squirrel hunting--weight and bulk, for example. The additional bulk and weight of a telescopic sight and mount negatively affect any rifle's handling. And for some shooters target acquisition is slower than with iron sights. Plus, good quality scopes and mounts cost money. My experience is that, if you practice and have 20/20 vision (uncorrected), you can actually bring home just as many squirrels using iron sights.
I think the issue of air movement and its dramatic effect on slow and/or low ballistic coefficient bullets is not widely appreciated. The introduction of the .17 M2 and .17 HMR cartridges makes this a timely subject as, despite their relatively high velocity they are very susceptible to wind deflection.
The mechanics of shooting game are much different than target shooting from a bench rest. I guess my point is that very few shooters seem to appreciate that anymore.