Predicting arrow penetration often begins with figuring kinetic energy, but that calculation only measures energy at impact. Momentum is a better predictor, because a heavy arrow doesn't slow down as quickly as a lighter arrow. Traditional archers prove this theory consistently by driving heavy arrows at modest speeds right through big animals like moose.
Physics aside, when debating slow/heavy arrows versus fast/light arrows for heavy big game, why not consider fast/heavy arrows? Try shooting some of these beastly arrows and broadheads out of your compound.
Heavy arrows require shafts of 500 grains or more, without a broadhead. You can start with an Easton Axis Full Metal Jacket Dangerous Game shaft (17.2 gpi), Alaska Bowhunting Supply GrizzlyStik Safari (16 gpi), or Carbon Tech Safari shaft (15.5 gpi).
Others that'll give you a good start include the Carbon Express Heritage (12 gpi) or new Pile Driver (10.4 gpi), Gold Tip Big Game 100 (10.6 gpi), or Carbon Force Black Mamba (10.5 gpi).
Shaft weight alone won't get it done, but you can add weight in several other ways, the most conventional being the addition of plastic weight tubes. Three Rivers Archery carries weight tubes in 3, 5, and 8-gpi versions. Installing brass inserts weighing 50 to 100 grains will add more heft to your shaft. And another common technique is inserting one arrow shaft inside another. This greatly increases shaft weight and can maintain necessary spine stiffness, but you'll need to experiment.
Some unusual methods for increasing shaft weight include inserting weed-whacker line, rope, salt, sugar, wood shafts, and even bullets of the correct diameter. Use caution, however, as these methods could lead to problems in shaft-to-shaft spine consistency.
Stabilizing your heavy broadhead/arrow combination requires serious fletching. Rotation is imperative, so make sure your vanes or feathers have maximum helical.
While momentum is a huge factor in penetration, broadhead design is another. Some designs -- notably two-blade, cutting-tip heads -- cut deeper because of what has been called "mechanical advantage." And hunting heavy, big game makes any advantage worth considering.
Strong, ultra-sharp broadheads that will add significant weight to your arrow shaft include the Alaska Bowhunting Supply GrizzlyStik Single Bevel broadheads (up to 310 grs.), German Kinetics Silver Flame (up to 210 grs.), the Steel Force African PhatHead (190 grs.), and Muzzy Phantom SS (200 grs.).
Some final thoughts:
- Remember, you're setting up for heavy, potentially dangerous game. Arrows weighing 600-900 grains really aren't necessary for compound shooters chasing deer and even elk-sized game. However, you can bet these arrow setups will be very effective on game of that size.
- Don't judge penetration by shooting a target. It stops arrows with friction, which differs greatly from skin and tissue, which are lubricated by body fluids and open up when cut. Nothing, not even ballistic gel, simulates a live animal.
- Be prepared for considerable experimentation. Arrow spine is still the most critical factor, and it is altered by the weight of broadheads and other components. You must achieve perfect arrow flight if you expect to maximize penetration.
- For improved flight and penetration, maintain a "forward of center" (FOC) balance of at least 12 to 16 percent on your heavy arrows.
- Your bow will be noticeably quieter when shooting heavy arrows, but a rangefinder is vital to compensate for increased trajectory.
If you plan to pursue the toughest of game, forget formulas and combine an efficient compound bow with ultra-heavy arrows. Why should traditional archers have all the fun?