Internal Ballistics — Pressure, Powder & Recoil

Everything the bullet does downrange is set in the millisecond and a half it spends in the barrel. Muzzle velocity, its shot-to-shot consistency, the recoil the shooter absorbs, and even the small vertical scatter that opens a group at 1000 yards all trace back to the pressure history inside the chamber. This volume is the inside of the gun, up to the crown.

2.1 The Pressure-Time Curve

Ignition lights the primer, which lights the powder, and chamber pressure climbs steeply. It peaks early — for many rifle cartridges within the first couple of inches of bullet travel — and then falls, because once the bullet is moving the bore volume behind it grows faster than the burning powder can generate gas.¹ The bullet keeps accelerating the whole time it is in the bore (velocity rises monotonically to muzzle exit) even though pressure is dropping through most of that travel, because there is still net forward force on the base right up to the muzzle. The area under the pressure-versus-position curve is, to first order, the work done on the bullet.

Burn rate is governed by powder chemistry, grain geometry and coating, and — critically — by the pressure the burning surface is exposed to: hotter, denser gas burns the propellant faster, so burn rate and pressure feed back on each other.¹

2.2 Progressivity

A progressive-burning powder is shaped and coated so its burning surface area increases as the grain is consumed — perforated cylinders and flake geometries expose more area as they erode.² That rising surface area keeps gas generation up even as the bullet accelerates and the bore volume expands, partly compensating for the pressure drop a single-perforation or degressive powder would suffer. The practical result is that a progressive powder holds its peak pressure a little further down the bore and delivers more velocity for a given peak pressure — which is why slow, progressive powders dominate large-capacity rifle cases.

2.3 Chamber Pressures — Read These as Medium Confidence

The SAAMI maximum average pressures below are useful for orientation, but they were assembled in the source research from search-aggregated tables that cite SAAMI, not from a directly fetched SAAMI standards document. Treat every figure here as medium confidence and verify against a live SAAMI PDF before using any of them as a hard design number.³

Table 1 — Chamber Pressures — Read These as Medium Confidence

Cartridge	SAAMI MAP (approx.)
.223 Remington	55,000 psi
.308 Winchester	62,000 psi
6.5 Creedmoor	62,000 psi
.30-06 Springfield	60,000 psi
9mm Luger	35,000 psi (38,500 psi +P)

The 5.56×45 NATO versus .223 Remington comparison deserves its own caveat, because two true facts are tangled together. NATO 5.56 is loaded to a higher pressure than commercial .223 (commonly cited around 62,000+ psi), and the NATO measurement method is not SAAMI’s — NATO uses a case-mouth transducer positioned differently from SAAMI’s, so the numbers are not an apples-to-apples comparison. Part of the roughly 7,000 psi gap is a real pressure difference and part is transducer methodology. Both are true simultaneously; do not collapse them into a single clean number.⁴

2.4 The Energy Budget

Only a modest slice of the powder’s chemical energy reaches the bullet as muzzle kinetic energy — the field converges on roughly 30–36%. One peer-reviewed interior-ballistics optimisation study reports the bullet-KE fraction rising from 31.13% to 33.05% after charge and geometry optimisation, with subsonic, heavy-for-caliber loads in long barrels reaching up to about 36%.⁵ The rest is lost to barrel and projectile heating, thermal energy retained in the gas, the kinetic energy of the ejected gas itself, and recoil.

Longer barrels are more efficient because barrel dwell time is the limiting resource — only a fraction of the propellant’s energy-release window overlaps with the bullet still being in the bore to receive work.⁶ This is one figure to treat as medium confidence (it models a specific gun system, not a universal constant), but the order of magnitude — “about a third” — is robust.

2.5 Barrel Time and Free Recoil

Typical rifle barrel time, from ignition to muzzle exit, is on the order of 1–2 ms; a QuickLOAD-modelled .308 at 2500 fps from a 26-inch barrel comes out near 1.47 ms.⁷ Handgun barrel times are shorter.

Free recoil is pure conservation of momentum: the forward momentum of the bullet plus the ejected gas is matched by equal-and-opposite rearward momentum of the gun.

gun recoil velocity = (m_bullet * v_bullet + m_gas * v_gas) / m_gun
free recoil energy  = 0.5 * m_gun * v_gun^2

Gun mass is the most effective lever — doubling gun weight halves free recoil energy for a given load. SAAMI publishes a formal “Gun Recoil Formulae” reference.⁸ Note that felt recoil is not free recoil: felt recoil is subjective and depends on stock geometry, recoil pad, stance, and muzzle devices. A muzzle brake reduces free recoil by redirecting gas; a pad or a well-fitted stock only changes the perception.⁸

2.6 Barrel Harmonics, OCW, and “Positive Compensation” — Treated Critically

This is a real phenomenon with a contested predictive theory, and it should be written that way. The muzzle is not still when the bullet exits — it is whipping through a harmonic oscillation excited by the pressure pulse and the bullet’s transit, so different charge weights (different velocities, different barrel times) release the bullet at slightly different muzzle angles.⁹ “Positive compensation” is the claim that a charge-weight window exists where slower shots exit with the muzzle pointed slightly higher and faster shots exit with it pointed slightly lower, partly cancelling the vertical spread that muzzle-velocity variation would otherwise cause. OCW (Optimum Charge Weight), Audette’s ladder test, Chris Long’s OBT, and the Satterlee velocity method are all empirical protocols for finding this harmonic “node.”

The serious benchrest and F-Class community treats the existence of a node as plausible and repeatedly observed, but the predictive theory is not settled — contributors openly disagree on the mechanism. This is folk-engineering with genuine anecdotal support, not a peer-reviewed law. Use node-finding as an empirical tuning method; do not treat any specific predictive formula as established physics.⁹

2.7 Powder Temperature Sensitivity

Muzzle velocity drifts with propellant temperature, and the amount varies enormously by powder. Across a 0–125 °F swing, cited total spreads run from about 4 fps for H4350 and 8 fps for Varget up to roughly 50 fps for a temperature-sensitive powder like Reloader 15.¹⁰ Per-degree figures for the flattest powders are around 0.14 fps/°F, though other testing shows Varget ranging 0.15–0.37 fps/°F depending on the load. Sensitivity is load- and cartridge-dependent — seating depth and case fill change it — not a fixed powder constant. These figures are medium confidence.

2.8 Muzzle-Velocity SD Becomes Vertical Dispersion

Velocity consistency is not an abstract virtue; it becomes vertical scatter at distance, because a faster bullet drops less over the same flight time. As a worked example, a 7mm 168 gr VLD with an extreme spread of 30 fps shows about 5 inches of vertical dispersion at 1000 yards.¹¹ For a .308 near 2600–2660 fps, a 60 fps MV difference produces roughly 21.9 inches of point-of-impact shift at 1000 yards — very roughly 5–6 inches of vertical shift per 50 fps of MV change. A useful cross-comparison: an MV standard deviation of 20 fps has a long-range vertical effect comparable to a 2.0% SD in ballistic coefficient.¹¹ Velocity consistency and BC consistency both matter, and they can be traded off against each other in exactly this way. These magnitudes are medium confidence and load-dependent, but the lesson is not: SD you can measure at the muzzle is dispersion you will measure on paper.

2.9 Bibliography

Footnotes

1. “Propellant characteristics: shape, burning rate and pressure.” https://forensicreader.com/propellant-characteristics-shape-burning-rate-and-pressure/ (confidence: medium). ↩ ↩²

2. Progressive-burning surface-area behaviour, aggregated from the ForensicReader source above and https://www.frfrogspad.com/intballi.htm (confidence: medium). ↩

3. SAAMI MAP figures — search-aggregated from SAAMI-citing catalogs (theballisticassistant.com and similar); NOT verified against a live SAAMI Z299 document in the source research. Re-verify at https://saami.org/technical-information/ansi-saami-standards/ before using as hard numbers (confidence: medium). ↩

4. 5.56 NATO vs .223 pressure and measurement-method distinction. https://www.mygundeal.com/blog/2026-04-11-reloading-reloading-223-remington-vs-5-56-nato-chamber-pressure-diffe (confidence: medium). ↩

5. “Numerical Calculation and Uncertain Optimization of Energy Conversion in Interior Ballistics Stage,” MDPI Energies 13(21):5824. https://www.mdpi.com/1996-1073/13/21/5824 (confidence: medium-high; models a specific system). ↩

6. David Bookstaber, “Physics of Gun Energy, Recoil, and Range.” https://david.bookstaber.com/Interests/2013/02/physics-of-gun-energy-recoil-and-range/ (confidence: medium). ↩

7. Barrel-time figures, search-aggregated incl. QuickLOAD-cited value and https://www.rimfirecentral.com/threads/barrel-time.577207/ (confidence: medium). ↩

8. SAAMI, “Gun Recoil Formulae.” https://saami.org/wp-content/uploads/2025/03/Gun-Recoil-Formulae-2018-07-9.pdf ; recoil calculator https://agentcalc.com/firearm-recoil-calculator (confidence: high for the SAAMI formula’s existence and the free-vs-felt distinction; medium for the paraphrased long-form equation). ↩ ↩²

9. Barrel harmonics / OCW / positive compensation, treated as contested. https://forum.accurateshooter.com/threads/accuracy-node-vs-barrel-harmonics ; https://www.longrangeonly.com/forum/threads/optimal-charge-weight-ocw-load-work-up-by-ryan-furman.8183/ (confidence: medium for the phenomenon; low for any specific predictive mechanism). ↩ ↩²

10. PrecisionRifleBlog, “Powder Temperature Sensitivity.” https://precisionrifleblog.com/2025/03/08/powder-temperature-sensitivity/ ; https://www.68forums.com/threads/powder-temperature-sensitivity.53653/ (confidence: medium). ↩

11. PrecisionRifleBlog, “How Much Does SD Matter?” https://precisionrifleblog.com/2015/04/18/how-much-does-sd-matter/ (confidence: medium). ↩ ↩²