There are some who claim that increasing the COAL (Cartridge Over All Length) in an AR from the standard 2.260 to 2.300 will net an increase in velocity. Technically, it's true. No less a ballistics authority than Bryan Litz would agree — up to a point.
In Litz’s article on the Berger Bullets website he states, “In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.”
This is all well-and-good as we seek “free” velocity gains, but the amount of difference in COAL in two otherwise equal loadings is key. For example, the Accurate Powder website FAQ #23 states: "However, [1.0mm] in the case of a rifle caliber, which is much longer, will have a very small to insignificant effect.”
In the case of an AR, the difference between a COAL of 2.260 and 2.300 is, yes, 1.0mm.
Now, what I found really interesting in a recent announcement about the ABM line of ammunition developed by Litz himself is the minimal gain in velocity from two otherwise identical loads having differing COALs.
Litz has two new .308 Win. loadings, one at 2.950 and the other at 2.80 — difference of 0.150 or 3.8mm.
How much velocity does he gain from loading the one almost 4mm longer than the other?
The longer gets 2628 fps from a 24” barrel and the shorter gets 2608 — difference of 20 fps.
What to conclude? If you can load long within your magazine constraints in the hope of gaining “free” velocity, do so — every little bit helps — but don’t knock yourself out. Unless 20 fps is meaningful to you, you’re largely wasting your time buying expensive magazines that allow for longer loading or cutting the fronts out of magazines. Those seeking to turn a mediocre-performing cartridge into a superb performer by increasing the COAL by 1.0mm are quite possibly barking up the wrong tree.
Discussion? Any real-world data on how much the 6.5 Grendel can expect to gain loading 2.300 vs. 2.260?
In Litz’s article on the Berger Bullets website he states, “In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.”
This is all well-and-good as we seek “free” velocity gains, but the amount of difference in COAL in two otherwise equal loadings is key. For example, the Accurate Powder website FAQ #23 states: "However, [1.0mm] in the case of a rifle caliber, which is much longer, will have a very small to insignificant effect.”
In the case of an AR, the difference between a COAL of 2.260 and 2.300 is, yes, 1.0mm.
Now, what I found really interesting in a recent announcement about the ABM line of ammunition developed by Litz himself is the minimal gain in velocity from two otherwise identical loads having differing COALs.
Litz has two new .308 Win. loadings, one at 2.950 and the other at 2.80 — difference of 0.150 or 3.8mm.
How much velocity does he gain from loading the one almost 4mm longer than the other?
The longer gets 2628 fps from a 24” barrel and the shorter gets 2608 — difference of 20 fps.
What to conclude? If you can load long within your magazine constraints in the hope of gaining “free” velocity, do so — every little bit helps — but don’t knock yourself out. Unless 20 fps is meaningful to you, you’re largely wasting your time buying expensive magazines that allow for longer loading or cutting the fronts out of magazines. Those seeking to turn a mediocre-performing cartridge into a superb performer by increasing the COAL by 1.0mm are quite possibly barking up the wrong tree.
Discussion? Any real-world data on how much the 6.5 Grendel can expect to gain loading 2.300 vs. 2.260?
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