Barrel · Twist · Zero · Load
Rifle Setup Optimizer
Pick a chamber and say what you shoot. ZeroForge flies every factory load that chamber can fire and answers the four questions a build comes down to: how long the barrel, how fast the twist, where to sight it in, and what to feed it. Already own the rifle? Enter your barrel and twist and it solves around them instead.
- 1Chamber
- 2The job
- 3The barrel
Which chamber?
The chamber decides what can go in the rifle. Pick it and ZeroForge solves the length and twist that get the most out of everything it will fire — not just out of one favourite load.
What do you shoot?
This sets the target size, the distance that matters, and how hard flatness is traded against energy, wind and recoil.
The build
01
The build
Length and twist are the two things a barrel can never change once it is cut, and the zero is the first thing you set after it is. All three are solved across every load this chamber fires — not for one favourite bullet.
02
What this build does
The selected load, flown from the barrel above. Change the load anywhere on the page and every figure here follows it.
03
Every job, at a glance
The same chamber solved nine times over, once per profile. Read down the columns to see how much the answer really moves with the job — sometimes barely at all, which is worth knowing before you commit a barrel to one of them. Tap any row to load that job.
04
Length and twist, ranked
Every length and twist this chamber is built in, played off against each other. Each cell shows where that pairing places and how far behind the winner it lands, so you can see whether the recommendation is a clear win or a coin toss.
05
Ammunition, ranked
Every factory load this chamber can fire, ranked for the selected job with all of them flown from the same barrel. Each row also carries the barrel, twist and zero that load would have asked for if the rifle had been built for it alone — the gap between the two is what that load gives up to share a barrel with the rest.
06
Twist coverage
Twist is a gate, not a tuning knob: it decides which bullets this chamber can stabilise at all. Here is what each rate offered holds, and what it leaves behind.
07
Drag data
Which drag curve this load is flown against, what the coefficient was scaled by, and where the published number came from. A G7 of 0.289 and a G1 of 0.556 can describe the same bullet, so the model matters as much as the number.
08
Range card
The full solution for the selected load on the barrel, twist and zero above — drop, come-up, wind, speed and time of flight, every hundred yards out. Rows turn red below the speed of sound, where no point-mass model is worth trusting.
09
Trajectory
The top ranked loads in this chamber, each on its own best zero for this job, all flown from the same barrel. Tap a name below any chart to take it off or put it back.
10
Velocity
11
Retained energy
12
Wind drift
13
Velocity against barrel length
What the powder actually gives you, inch by inch, for the top ranked load. The curve bends over where the charge has spent itself — past the marked point another inch is worth under 10 fps, which is weight you carry for nothing.
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How to read the numbers
What a zero distance actually is
Your scope sits above the bore, so the bullet starts life about 1.6 inches below the line of sight and has to climb to meet it. It crosses that line twice: once close in, and once again on the way down. The second crossing is what everyone means by the zero.
Between those two crossings the bullet is above the line of sight. Past the second one it falls away for good. So a zero is not a setting you copy from someone else — it is a decision about where you want the two crossings and how much rise you will tolerate in between.
Point blank range, and why it beats a round number
Pick the vital zone of what you shoot — say 6 inches. Now find the longest zero whose arc never climbs more than half of that (3 inches) above the line of sight. That zero gives you the longest stretch of ground over which you can hold dead centre and still be inside the vitals. That stretch is the point blank corridor, and its far edge is maximum point blank range.
It is almost never 100 or 200 yards. Those are round numbers from a habit of zeroing at whatever the range happens to offer. The corridor diagram on the Barrel tab gives you the real answer for the load in front of you, at the sight height and standard atmosphere this app solves on.
- Small vital zone (varmints) → shorter zero, tighter corridor
- Large vital zone (elk) → longer zero, generous corridor
- Higher scope → later first crossing and more close-range offset
The 36 yard zero, and why near zeros exist
Every zero has two crossings, and you can sight in on either one — they are the same setting. Ranges are short and ammunition is not free, so people sight in on the near crossing and let the far one look after itself. That is the whole story behind the numbers you hear repeated: 36 yard on a 5.56 AR, 50/200, 25/300 on iron-sighted service rifles.
The 36 is not folklore. Put an M193 55 grain load through a flat-top AR — sight line 2.6 inches over the bore, because that is where the rail sits — and ask for the zero that keeps the bullet inside four inches of the line of sight for as long as possible. This app solves it at about 37 yards near, 268 yards far, with the bullet never more than 4 inches high or 4 inches low from the muzzle out past 300. Hold centre on a torso anywhere in that stretch and you hit it.
Sight height is what moves the answer. The same load under a low-mounted scope at 1.6 inches wants roughly a 25 yard near zero instead, because the bullet has less climbing to do. Copying a number without copying the mount height is how people end up shooting high all afternoon.
Every build sheet in this app quotes both crossings for exactly this reason — sight in at whichever your range allows.
Flattest path is a different question
Point blank range asks: how far can I hold centre? Flattest path asks: what zero makes the hold-over you need at the far end and the hold-under you need in the middle come out equal? The two give different zeros, and the Flattest Path profile solves the second one directly.
One trap worth knowing. Balance on its own is not enough, because hold-over and hold-under equalise at any distance you pick — stretch the window far enough and they come out equal at twenty inches each, which is arithmetically balanced and useless in the field. What makes a 36 yard 5.56 zero genuinely flat is that the two stay equal and stay inside about four inches the whole way. So this app balances them within the point blank corridor for the vital zone you chose, never beyond it.
If someone tells you a cartridge “shoots flat”, the honest number is the total swing on that card: how far above the line it gets plus how far below it falls by the end of the window. Nothing about your rifle changes that number except velocity and BC.
Barrel length: what you really lose
Every cartridge has its own velocity cost per inch, and it is not a small spread — a .22 LR gives up around 10 fps an inch while a .338 Lapua gives up over 40. ZeroForge corrects each load from the barrel the factory chronographed it in, which is why the numbers here will not match a box label taken from a 24 inch test barrel if you are shooting a 16.
The catalogue figure is not the figure at your barrel. Feet-per-inch is measured by cutting a barrel down and dividing, so it is an average across a span. On a saturating curve the local slope at the long end is well below that average — 5.56 is quoted at 25 fps an inch but is giving up under 14 by 20″, and .223 Rem under 16 by 24″. The build card reports the local number as next inch worth, because that is the one that answers whether to cut another inch. The powder spent at figure beside it is where that local gain falls under 10 fps, and it is what caps the barrel window.
Velocity is not the only cost. A short barrel is louder, flashier and burns more powder outside the muzzle. But it also handles better, and past the point where the extra range stops mattering for your job, the inches are dead weight. Each mission card picks the shortest barrel that still does that job.
Twist rate and the stability factor
Twist has to spin the bullet fast enough to keep it point-first. The Miller rule gives a stability factor, Sg. Below 1.0 the bullet tumbles. Below 1.4 it is marginal and accuracy suffers, especially in cold dense air. Around 1.5 to 2.0 is where you want to live.
Long bullets need faster twist, and length — not weight — is what actually drives it. A long monolithic copper bullet can need more twist than a heavier lead-core bullet of the same weight. Cold, high pressure air needs more twist than a hot day at altitude, which is why the Sg on your card moves when you change the conditions.
What the model assumes
Two things are held fixed so that cartridges stay comparable to each other.
Line of sight sits 1.60 inches above the bore for every cartridge. It has to sit somewhere — the bullet starts below the sight line and climbs to meet it, which is what gives a trajectory its near-field shape. Letting it follow the platform meant an AR-chambered cartridge was solved at 2.6 inches while a bolt-gun cartridge was solved at 1.6, so chambers were quietly being compared on two different geometries. Held constant it becomes a control variable, and the barrel and twist that come out are the cartridge's doing rather than the mount's. Sweeping it from 1.2 to 3.0 inches moves the average score by well under a point, so the recommendation does not hinge on it.
One fixed atmosphere, and why. Every figure here is computed on the standard day that published ballistics tables are computed on: 59 degrees Fahrenheit, 29.92 inches of mercury, sea level, with a 10 mph full-value crosswind. That is deliberate. It makes these numbers directly comparable with a factory chart or any other calculator, instead of being right only for the weather at one moment in one place. The barrel and twist this app recommends are decisions you make once, and they do not change with the forecast.
Humidity is carried at 50 per cent and is very nearly irrelevant: across the entire range from bone dry to saturated, drop at a thousand yards moves under two inches in three hundred — about half a per cent. It is in the model because the physics has it, not because it changes an answer. Latitude is held at 45 degrees, the usual reference for published Coriolis figures.
What this does not do is predict your day. Cold dense air, altitude, and a chronograph that disagrees with the box will all move the trajectory more than anything on this page, and no input box fixes that — only shooting it does. Use these figures to choose a barrel; use paper to build your dope.
The vital zone follows the load, not just the job. A job sets a target size, but the job alone is not the whole story: a .22 LR and a .308 both asked for general purpose field were being judged against the same six inch deer vital, which flatters the rimfire badly. What a cartridge is honestly used on follows from what it still carries at the distance in question, so retained energy sets a ceiling on the zone and the job sets the other — the smaller wins. It can only ever shrink the zone, never grow it, because no amount of energy makes an animal's vitals bigger. In practice .22 LR is judged against 2″, 5.56 against 5″, and .308 against the full 6″.
Bore angle. The barrel does not point where you are looking. To put the bullet back on the sight line at the zero, the bore is tilted up by a fraction of a degree — about 4.8 MOA for 5.56 at a 257 yard zero, 6.9 MOA for .308 at 219, and 10.5 MOA for .22 LR at 72, because a slow bullet needs far more of it. That angle is exactly the elevation your scope is already holding when the turret reads zero, and it is why a rifle whose bore is pointed straight at the target shoots low at every distance.
Two levels of precision, on purpose. Ranking a chamber means flying several hundred trajectories, so the sweep integrates at a coarse step — at that resolution the drop at 500 yards sits within about a seventh of an inch of the fully converged answer, which cannot change an ordering. Everything actually drawn or tabulated is then re-solved at a much finer step. A convergence run from dt 0.0016 down to 0.00005 shows the answer settled by 0.00025, so that is the step used for the corridor diagram, the range card and the charts. The corridor curve itself is sampled at better than one point per screen pixel, so what you see is the solution rather than a polyline coarse enough to notice.
What the solver actually does. Each shot is flown by fourth-order Runge-Kutta integration of a point-mass model against a Mach-indexed drag curve — G7 where the load publishes one, which is 206 of the 332 loads on file, G1 for flat-base and rimfire bullets where G7 would be the wrong reference shape. Air density follows the ideal gas law with humidity taken out through the Magnus saturation vapour pressure; the speed of sound is corrected for that vapour too, because it sets every Mach number in the drag lookup. Wind uses the lag-time formulation, spin drift Litz's approximation, and Coriolis is computed from first principles for both the horizontal component and the direction-dependent Eotvos term.
Twist changes the trajectory, not just a pass mark. A barely stable bullet flies with a small persistent yaw, and yaw costs drag. Below a stability factor of about 1.5 the effective ballistic coefficient falls away, reaching roughly a tenth below nominal near Sg 1.0. So an under-twisted barrel correctly comes out slower, lower and more wind-sensitive rather than merely flagged. In .223 with a 77 grain match bullet at 600 yards, going from 1:8 to 1:12 costs 7.4 inches of drop and 6.8 inches of wind — the twist is not a footnote to the trajectory, it is part of it.
Validated against published data, not against itself. Hornady 140 grain ELD Match in 6.5 Creedmoor at 2,710 fps, standard atmosphere, checked against independent G7 reference tables and Hornady's own factory figures:
| Quantity at 1000 yd | This app | Published | Difference |
|---|---|---|---|
| Retained velocity | 1481 fps | 1478 fps | 0.2% |
| Drop, 100 yd zero | 319″ | 318″ | 0.4% |
| Wind, 10 mph full value | 68.6″ | 68″ | 0.9% |
| Supersonic reach | 1376 yd | 1382 yd | 0.4% |
Against Hornady's own published path at 300, 400 and 500 yards the agreement is within 1.7 per cent, and muzzle velocity is reproduced exactly for all 332 loads. Where the app will diverge from measurement is deep in the transonic band, below about Mach 1.2, where no point-mass model with a single drag curve tracks reality well — which is why the charts change colour there rather than pretending otherwise.
Barrel length is solved from the powder, not extrapolated. Velocity against barrel length is not a straight line. Powder burns, pressure falls as the gas expands, and the velocity gained per inch shrinks with it until bore friction cancels it out entirely. The model used here has velocity approach a ceiling and then turn over, and it is anchored to measured data: the curve is made to pass through each load's published velocity at its own test barrel and to have the published feet-per-inch as its slope there. Across all 332 loads it reproduces published velocity to within a rounding error, and simply behaves correctly away from those points.
The length reported as the limit is where a further inch is worth under 10 feet per second — not the theoretical peak, which for a small-bore centrefire sits past 32 inches and is of no use to anyone. That gives 18″ for .22 LR, about 17″ for .300 Blackout, 28″ for .308 Winchester and 35″ for .338 Lapua, which is where those cartridges really stop paying for steel.
Where a cartridge lives in one or two rifles, only those lengths are offered. 5.7×28 is the clear case: it is the P90 at 10.4″ and the PS90 at 16″, plus their AR-pattern derivatives, and offering an 18 or a 22 inch barrel would describe a rifle nobody has ever built. Those two lengths are kept whatever the job window would otherwise prefer, because narrowing to one would offer a choice that is not a choice.
Feet-per-inch is a secant, not a derivative. That figure is produced by cutting a barrel down and dividing the velocity lost by the inches removed, so it is an average across a span rather than the slope at one end of it. Treating it as an instantaneous slope broke badly for 5.7×28, whose loads are catalogued from a 4.8 inch pistol: the curve saturated almost immediately and the model claimed a PS90 was 110 fps slower than a P90. Where a load's test barrel and the cartridge reference are far apart, the curve is now made to pass through both points and the published figure is honoured as the average between them, which is exactly what it measures. All 332 loads still reproduce their published velocity exactly.
Only lengths that are actually manufactured. Sweeping every inch produced answers like 21 and 23 inches — real numbers, not real barrels. Candidates are now drawn from what factories cut: self-loaders at 10.5, 11.5, 12.5, 14.5, 16, 18, 20, 22 and 24 inches, manual actions at 16 through 30 in even inches, rimfires including 16.5, and lever guns at 18.5 where that is as common as 18. A recommendation you cannot buy is not a recommendation.
Two things are flagged rather than hidden. Sixteen chamberings here are not commonly built as self-loaders — there is no semi-automatic .45-70, and offering a 24 inch one would be a confident answer to an impossible question — so switching the action says so instead of inventing a catalogue. And anything under 16 inches is a short-barrelled rifle in the United States and needs a stamp before it is built; those lengths stay on the list because they are manufactured, with the paperwork noted.
Bolt action or semi-auto changes two things and nothing else: a self-loader gives up about 15 fps to the gas port, and it is built between 10 and 24 inches where a bolt gun runs 16 to 30. That is why long-range jobs come back 28″ on a bolt gun and 24″ on a self-loader.
The chamber is the unit of analysis. Not the bore, and not the cartridge. A barrel blank has a bore and a twist, but until the reamer goes in it is not anything — and once it has, it fires one specific set of ammunition and nothing else. Almost every chamber here takes exactly one cartridge. The one real exception is 5.56 NATO, whose longer throat and higher pressure ceiling let it fire .223 Remington safely as well; .223 Wylde does the same, so the two are listed as one entry, because they differ only in freebore geometry and this app has nothing that would tell them apart. A .223 Remington chamber is listed separately and takes .223 only — 5.56 in a .223 chamber is a genuine pressure problem, not a technicality.
Ammunition is ranked, not scored. The underlying numbers are only meaningful relative to the other loads in the same chamber, so printing them would invite comparisons they cannot support. Rank 1 is the best use of that barrel for that job.
Nothing about the rest of the rifle is assumed. Chamber throat, gas system, muzzle device, stock and optic all vary too much between bolt guns, semi-autos and everything else to be modelled honestly from published data, so they are left out rather than guessed at. Barrel length and twist are what a barrel is permanently committed to when it is cut, and they are what this app answers.
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Model & sources
The solver
The shot is integrated with fourth-order Runge-Kutta on a Mach-dependent drag curve — G7 where the load publishes one, G1 for flat-base and rimfire bullets where G1 is the correct reference shape. Drag is not a constant: it roughly doubles through the transonic band, which is exactly why long-range trajectories curve harder than a simple drop formula predicts.
Air density comes from your temperature, station pressure and humidity. Speed of sound comes from temperature alone. Propellant temperature moves muzzle velocity by each cartridge's own sensitivity, referenced to 70F where the correction vanishes.
The bullet is flown once along the bore line to build a drop table; each candidate zero is then a tilt of the sight line over that same table. That is exact for the sub-one-degree launch angles a rifle uses, and it is what lets the app solve 332 factory loads without stalling.
What the wind numbers mean
Crosswind drift is computed from lag time — the difference between the bullet's real time of flight and the time it would have taken in a vacuum. That is the standard treatment and it is why a slower, higher-BC bullet often beats a faster, lower-BC one in wind.
All drift figures are full value: a 90 degree crosswind. A wind quartering at 45 degrees is worth roughly 70 percent, and a straight head or tail wind is worth almost nothing sideways. Spin drift is separate — it is the bullet walking in the direction of its own rotation, and for right-hand twist it always goes right.
Energy floors and honest limits
The 1,000 and 1,500 ft-lb figures used by the game profiles are long-standing rules of thumb, not law, and they vary by jurisdiction and by who you ask. They are in here as a consistent yardstick for comparing cartridges, not as permission.
The other limit is transonic. Around Mach 1.2 the drag curve gets steep and slightly unpredictable, and groups usually open up as the bullet passes through. Where your load crosses that line is a more honest accuracy ceiling than any energy number.
What this tool is not
Ballistic coefficients are published figures, and manufacturers are optimistic. Muzzle velocities are catalogue numbers corrected for your barrel, not your rifle's. Recoil uses an estimated propellant charge. Every one of those is a source of error that a chronograph and a session on paper will fix.
Treat everything here as a starting point that gets you close, then true it on the range. Confirm your zero, confirm your come-ups at distance, and adjust. No solver replaces that.
Sources
Standard drag functions G1 through GI from the McCoy tables as published by JBM. Stability from the Miller twist rule with the velocity and atmospheric corrections. Spin drift from the Litz approximation. G7 coefficients are manufacturer-published where available, Applied Ballistics measured values where noted, and derived from published G1 elsewhere — the source is shown against each load.
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Units & sight height
Angular units
Distance
Velocity
Read this first. ZeroForge is a planning and comparison tool for lawful hunting, target and sport shooting. Everything it prints is a starting point, not a substitute for confirming your own rifle on paper at known distances. Published ballistic coefficients and catalogue velocities both carry real error. Follow all applicable firearms laws and safe handling practice, and confirm game energy and calibre requirements with your own state or country's regulations before hunting.