Right now, there is no shortage of information, both good and bad, on the Internet, at the gun shops, and on the ranges regarding the ‘Best’ bullets to use with your AR-15 carbine. Due to nationwide shortages from the “panic,” you may be stuck with whatever is left over for the next few months. Regardless, here are few things to think about before you buy your next batch of ammunition.
Almost all bullet manufacturers have a very “high end” bullet that will cost you a fortune. However, depending on your barrel’s ‘Twist rate’ (number of turns the bullet makes over a given length), that high end bullet may not give you the performance you thought you were paying for. There are many factors to consider when choosing which ammunition to fire through your rifle. Each rifle has a fixed barrel length and “twist rate” measuring how many times the projectile will revolve as it travels down the barrel. The combination of barrel length and “twist rate” will significantly affect the performance of different bullet weights and shapes.
The first M-16 A-1s came with a 1:14 (one full revolution in 14 inches) twist and later changed to 1:12 to match the 20 inch barrel length. The standard modern-day military M-4 comes with a 1:7 twist rate to stabilize ball ammunition in a 10.5 inch barrel. A 1:9 twist rate seems to be popular these days with a multitude of barrel lengths. With bullets weights ranging from 35gr to 90gr, and without going into an entire terminal ballistics class, the short version is that the heavier bullets need a higher twist rate to stabilize the projectile over distance.
The physics are pretty simple, a heavier bullet spinning faster will have more stability than the same bullet spinning slower over the same distance. A great example of this is the fact that the 77 gr. Black Hills ammo works very well with a 1:7 mil-spec barrel but it does not work very well with a 1:9 barrel. For weapons with a 1:9 or 1:10 barrel, the 55gr or 62gr ammo works much better over the same distance.
Additionally, high twist rates mean more barrel wear, which for most people is insignificant – but if you shoot a lot, 55Gr bullets are generally cheaper then 62Gr or 77Gr bullets – and if your shooting a lot of it, that 1:7 barrel shooting 77Gr is going to get chewed up faster than a 1:9 shooting 62Gr. If you shoot a lot, add up the cost of ammo, wear & tear, etc… before you buy – and don’t shoot expensive bullets through your low twist barrel because you will not get enough spin to stabilize them.
Special Mission Units in the military have been using the Black Hills 77gr ammo for well over a decade with very good results in accuracy and combat effectiveness in all kinds of locations. That said, they are still using 7.62 for most distance shots… So before you go and buy a 1:7 barrel and a pile of 77Gr. bullets because you want to shoot your Ar15 at 600 meters – Do the math. A .308 Remington 700 with iron sights will cost you $700 and be more accurate at 500 – 800 yards than the best scope you can put on your new 5.56 gun regardless of twist and bullet weight!
~ Chris White
#ballistics, #Barreltwist, #Bulletweight, #carbineballistics, #firearmstraining