The 6mm GT is a centerfire rifle cartridge standardised by SAAMI in ANSI/SAAMI Z299.4. SAAMI sets its maximum average pressure at 62,000 psi, and measures its reference velocities in a 24.000 in standard test barrel rifled 1 turn in 7.50 in. This guide sets out SAAMI’s published reference velocity for each bullet weight, computes the muzzle energy each one carries, and compares the 6mm GT with the 243 Winchester on the same basis.
6mm GT ballistics chart: SAAMI reference data
SAAMI publishes one nominal mean instrumental velocity for each reference bullet weight, measured at 15 feet from the standard test barrel. It does not publish energy, so the ft-lb column is computed. Where SAAMI lists more than one velocity for a weight, both ends of the span are shown.
Scroll the table sideways for the remaining columns.
| Bullet weight | Reference velocity | Computed muzzle energy |
|---|---|---|
| 109 gr | 2,910 fps | 2,049 ft-lb |
What the numbers mean for a shot
SAAMI lists a single reference bullet weight for the 6mm GT: 109 grains, listed at 2,910 fps, which computes to 2,049 ft-lb. SAAMI publishes no other weight for it, so the table has one row. For any other bullet weight, use the figures the maker publishes for that item.
None of this is a downrange figure. SAAMI measures at 15 feet, so retained velocity, energy and drop depend on the bullet’s ballistic coefficient, which SAAMI does not publish. Each product in our 6mm GT ammo range lists whatever its maker publishes for that item.
Twist rate, chamber and pressure
SAAMI’s standard test barrel for the 6mm GT is 24.000 in long and rifled 1 turn in 7.50 in, with a .243 in groove diameter. That is the barrel every reference velocity above was measured in, not a statement about your rifle: a shorter production barrel will usually give lower velocity, and a twist slower than a bullet needs will not stabilise it. The maximum average pressure is 62,000 psi. Fire 6mm GT ammunition only in a firearm chambered for it. Several unrelated cartridges share a number, so match the full designation on the box to the marking on your barrel.
6mm GT vs 243: the SAAMI comparison
A figure from one maker’s catalogue cannot be laid beside a figure from another’s: each picks its own test barrel, zero and bullet, and many publish no test barrel at all. SAAMI is the shared basis, because it fixes one standard test barrel and one way of measuring velocity for every cartridge it lists.
Scroll the table sideways for the remaining columns.
| SAAMI specification | 6mm GT | 243 Winchester |
|---|---|---|
| Maximum average pressure | 62,000 psi | 60,000 psi |
| Standard test barrel length | 24.000 in | 24.000 in |
| Test barrel twist rate | 1 turn in 7.50 in | 1 turn in 10.00 in |
| Test barrel groove diameter | .243 in | .243 in |
| Lightest reference load | 109 gr — 2,910 fps / 2,049 ft-lb | 55 gr — 3,880 fps / 1,838 ft-lb |
| Heaviest reference load | 109 gr — 2,910 fps / 2,049 ft-lb | 100 gr — 2,850–3,080 fps / 1,803–2,106 ft-lb |
SAAMI standardises the 6mm GT at 62,000 psi and the 243 Winchester at 60,000 psi, so the 6mm GT runs 2,000 psi higher. SAAMI lists no bullet weight in common for the two, so there is no like-for-like velocity row to compare. Their standard test barrels differ (24.000 in, 1 turn in 7.50 in for the 6mm GT; 24.000 in, 1 turn in 10.00 in for the 243 Winchester), so compare the reference rows with that in mind.
What SAAMI cannot settle is anything past the muzzle. It publishes no ballistic coefficients and no downrange data, so it cannot say which cartridge shoots flatter at distance; that depends on the bullet. We stock the other side of this comparison too: 243 ammo.
6mm GT ballistics questions
What is the maximum pressure of the 6mm GT?
ANSI/SAAMI Z299.4 sets the maximum average pressure for the 6mm GT at 62,000 psi.
What twist rate and barrel length does SAAMI use for the 6mm GT?
SAAMI’s standard test barrel for the 6mm GT is 24.000 in long and rifled 1 turn in 7.50 in. Your own rifle may differ.
Is the 6mm GT more powerful than the 243 Winchester?
SAAMI lists no bullet weight in common for the two, so there is no like-for-like row. Its highest computed reference energy is 2,049 ft-lb for the 6mm GT (109 grains) and 2,134 ft-lb for the 243 Winchester.
What is the effective range of the 6mm GT?
Effective range is not a SAAMI specification and it is not a measured figure. SAAMI measures velocity at 15 feet, so it says nothing about performance at distance; that depends on the bullet and the shooter.
How much does the 6mm GT drop at 300 yards?
SAAMI publishes muzzle velocity only, so it cannot give a drop figure. Drop depends on the bullet’s ballistic coefficient and on the zero range. A drop figure is unusable without its zero, so use the figure a maker prints for the specific load.
Where to buy these loads
Hilltop Armory carries 6mm GT ammunition from Hornady. Browse the full 6mm GT ammo range.
