What cause accuracy and inaccuracy in a pistol round?

Consistency of ammunition and dwell time are the factors affected by hand loading.

These are what are mainly affected with your round construction and components.

What is physically happening to cause the two guns to shoot the bullet differently?

Two major things.

Most important is the quality of the barrel. Groove diameter has to be a few ten-thousandths inch smaller than bullet diameter for best accuracy. That's why custom 30 caliber match grade barrels have groove diameters of about .3075" and match bullets are about .3082" to .3084" diameter. Uniform bore and groove dimensions have to be there too; .0001" or less spread in both. And a uniform rifling twist; no quick changes in rate. Naturally, chamber has to be well centered on bore axis.

Another is parts fit; barrel to receiver is most critical; receiver face needs to be square with the barrel tenon thread axis. And how the barreled action fits the stock; has to be very repeatable from shot to shot and receiver in full and even contact with stock that doesn't change with recoil and a totally free floated barrel. The resonant frequency and its harmonics of a given barreled action that make it whip and wiggle in different directions are very repeatable from shot to shot - if the receiver's well fit to the stock and the bolt face is square with the chamber axis.

It's been proved many times that several rifles of different makes and models built with the right parts and assembled correctly will shoot the same lot of good ammo equally accurate. Even if their barrels have different lengths and shapes.

It's been proved many times that several rifles of different makes and models built with the right parts and assembled correctly will shoot the same lot of good ammo equally accurate.

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If there was a competition between them you are saying there would be no second place?

jmorris, you might have to wear out barrels across all of them to see the difference. Any subtle differences will typically be lost in human variiables. Of course there'll be a second or lesser place. But if there's less than a 10% spread across all, is that important? There's a 10% spread in any rifle's accuracy if you shoot several dozen 30-shot test groups.

I've seen dozens of rebuilt M1 and M14 rifles shoot the same load of .308 Win handloads with accuracy equal enough across all at about two-thirds MOA at 600 yards.

Same with the first handloads for Sierra Bullets' first 30 caliber 155-gr HPMK bullet that shot about half MOA at 600 yards in several rifles from around the world all with different barrel profiles and chamber/bore/groove dimensions.

I'm not basing my opinion on a couple of few shot groups. Dozens or hundreds of rounds shot through them very accurate is a good indicator.

Great discussion on contributing factors to accuracy! Josh, what many members are posting is reloading/competition match accuracy GOLD! The knowledge base and experience of THR members are truly amazing.

Joshboyfutre said:

Something is happening as we increase from min to max and the groups tighten then open back up.

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pistol ... 9mm ... load three different loads using the same bullet weight ... One slow one average and one hot, usually one load is gonna be noticably more accurate than the other two.

Taking all outside influences out of play, some loads will be better than others. Why?

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While there are many reloading variables that affect accuracy (which other members explained in detail), if we take all outside influences out of play and focus on powder charges, shot groups getting smaller as we conduct powder work up could be from powder burn efficiency and consistency of chamber pressure build.

At start/low charges, powder may not burn efficiently (especially with slower than Unique burn rate powders and oversized factory barrels with longer leade/freebore) to expand the case neck/mouth to seal with the chamber and leaking gas around the case/bullet will produce less consistent chamber pressures resulting in larger shot groups. This is why faster burn rate than W231/HP-38 powders are better suited for lighter target loads.

As powder charge is increased, more efficient powder burn will better seal the case neck/mouth with the chamber and less gas leakage will contribute to more consistent chamber pressure build which will result in smaller shot groups. Because of this, barrels with longer leade will benefit from longer OAL/COL and barrels with larger chamber/groove diameter will benefit from larger sized bullets.

from min to max and the groups tighten then open back up

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I have seen this and many will claim you often do not get smallest shot groups with max load data. Keep in mind that max chamber pressure listed on the published load data is "average" tested near/at SAAMI max pressures. Depending on the powder, how chamber pressure build as powder burns and expanding gas push on the bullet base is not linear. For smaller shot groups, you need more consistent chamber pressure build and sustained push behind the bullet base for more consistent muzzle velocities and lower SD numbers, which may not come from near max/max load data. (But as many like Walkalong posted, this is where quality of bullet come into play as better quality bullets will contribute to greater accuracy, especially at higher velocities)

This is why we conduct full powder work up to first identify charge that starts to reliably cycle the slide and extract/eject spent cases then use .2/.1 gr increments of powder charge to identify the most accurate load for the bullet being used.

Factory loads are run on tolerances. If you buy a box of cartridges, disassemble and weigh the bullet and powder separately for each one in the box, you'll probably not find two completely identical. Better manufacturing = better tolerances = higher costs. How accurate do you need to be?


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Hornady also offers a factory loading of the 125g 9mm HAP projectile that is intermittently available. It's much more reasonably priced.

Hornady Steel Match Ammunition 9mm Luger 125 Grain Jacketed Hollow Point HAP Steel Case Box of 50

If one wants extreme accuracy, follow Ely's lead. On their match cartridges, they visually inspect each round and weigh each round so that all the cartridges in a box are the same. Of course, price what that box will cost you vs off the shelve plinking ammo.


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Use of same headstamp, once fired and new brass can reduce flyers as expressed by many bullseye match shooters - http://www.bullseyepistol.com/bullseye.htm

But the OP specificied "Taking all outside influences out of play ... as we increase from min to max and the groups tighten then open back up" what causes the shot groups to change in size?

Any subtle differences will typically be lost in human variiables. Of course there'll be a second or lesser place. But if there's less than a 10% spread across all, is that important?

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Not important to me and consistent with my thinking. Just trying to help him with the "Why" part. We can manufacture to close tolerances but that is different than identical.

That said even with excellent manufacturing we still have to catch the anomalies that don't provide acceptable results or we wouldn't need quality control departments.

The fact that some manufacturers seem to always produce fine products while others put out lemons is evidence that some have more effective QC than others. You can make bad products and still have a good reputation as long as you don't let them out the door.

I have found that bullets that are not square with the bore axis all the way around at the exact point when exiting the barrel can really affect accuracy. If the crown is not square the rifle will not shoot true to the axis either. Bullet quality as in air pockets in the core or jackets that are not drawn to equal thickness around the diameter will be inaccurate as well.

What cause accuracy and inaccuracy in a pistol round?

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I've found honing the nut behind the trigger with meaningful practice to be effective.

A good shot will still be able to shoot a pistol with accuracy problems better than the guy who shoots a box each summer.

More than once I've had people tell me, "This rifle/pistol can't hit anthing." I ask if I can fire it, and almost always find the gun shoots fine.......