Observations on the Barnes 70 Grain TSX
The Barnes 70 grain TSX is a very long bullet for its weight and caliber. At a nominal length of 1.037 it is longer than the heavier 77 grain Sierra MatchKing.
Besides being a long bullet, the 70 grain TSX has a specific gravity much lower than a copper jacketed/lead core bullet due to its sold copper construction. For example, the Hornady 75 grain BTHP has a specific gravity of approximately 10.3, while the Barnes 70 grain TSX has a specific gravity of only 8.9.
The Barnes 70 grain TSX will be unstable from a 1:9 twist barrel with a gyroscopic stability factor of less than 1 at all velocities. The 70 grain TSX will only be marginally stable in a 1:8 twist barrel with a G.S.F. of 1.05 at 2500 fps. A 1:7 twist barrel will produce acceptable stability with the 70 grain TSX with a G.S.F. of 1.37 at 2500 fps.
Can you tell the difference between the Barnes 70 grain M/LE TAC-X Tactical Bullet and the Barnes 70 grain TSX Triple-Shock X bullet?
Silver State Armory's 70 grain TSX load (pre-Nosler takeover) easily matched the reported muzzle velocity of the 5.56mm Optimized load. From a 16" Colt barrel with a NATO chamber, chrome lining and a 1:7" twist, the SSA load had a muzzle velocity of 2788 FPS with a standard deviation of 15 FPS. From a 20" Colt barrel it had a muzzle velocity of 2899 FPS with a standard deviation of 21 FPS.
As I previously posted, the Silver State Armory 70 grain TSX load had a muzzle velocity of 2899 fps from a 20" Colt barrel. This is a hot load. Factory loads using equivalent weight bullets, for example the Federal Gold Medal 69 grain Sierra MatchKing, generally run around 2750 fps from a 20 Colt barrel.
Since the SSA 70 grain TSX load is such a hot load, I decided not to use my Krieger barrel, with its match chamber, to evaluate the accuracy of the SSA load. Instead, I used my 20 Noveske HBAR which has a chamber designed to fire the military pressure MK262 load safely, on full-auto, in hot environments. This barrel has a 1:7 twist. Using match grade hand-loads, this barrel turned in a 10-shot group at 100 yards that had an extreme spread of 0.726.
Previous accuracy testing using hand-loads of the 70 grain TSX bullet proved the bullet itself to be capable of some fine accuracy. A 10-shot group fired from my Krieger barreled AR-15 at 100 yards had an extreme spread of 1.12.
The SSA 70 grain TSX load was tested shooting from a bench-rest at a distance of 100 yards. A Leupold Competition scope with a magnification of 45X was used for sighting. Wind conditions were monitored using a Wind Probe.
Just for kicks, I did something a little different this time. I fired two 5-shot groups! Those groups had extreme spreads of 1.22 and 1.91. I then fired a traditional 10-shot group, which had an extreme spread of 1.93 and a mean radius of 0.61. I over-layed the two 5-shot groups on each other using RSI Shooting Lab to form a 10-shot composite group. The 10-shot composite group had an extreme spread of 1.93 and a mean radius of 0.63.
As I posted previously in this thread, a barrel with a 1:9 twist theoretically will not stabilize the 70 grain TSX. I put the theory to test using a Colt 16 HBAR with a 1:9 twist.
I fired a 10-shot group of the 70 grain TSX round at a distance of 100 yards using the 1:9 twist barrel. Accuracy was terrible; the group had an extreme spread of over 3 inches. Several of the bullet holes were slightly oblong in shape and a coulple of the bullet holes were clearly key-holed demonstrating that a 1:9 twist barrel will indeed not stabilize the 70 grain TSX.
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