Making Changes To Your Rifle That Can Shift The Point Of Impact

BY Herschel Smith
2 months ago

American Rifleman.

Sometimes a POI shift is simply due to a parts change, usually toward the front of a rifle. Changing muzzle devices always requires a re-zero, but even removing and reinstalling the same flash hider or brake can shift POI. Changing a semi-automatic’s gas block or a barrel nut to support a different handguard will also affect shot placement on target. Any alteration of things that touch a barrel, especially in terms of their position, weight or pressure, will alter point-of-impact. So long as the parts change is intentional and everything is mounted and torqued securely, a simple re-zero is all that is required to deal with this shift.

Items that attach to rifle fore-ends are the root cause for most problems I see. The typical offenders are the screw ends for M-Lok, KeyMod and backer-plate accessories. At least one company includes screws that are about twice as long as needed to secure its M-Lok accessories. Thus, it is very easy to wind up impinging on a barrel or gas block, resulting in POI shifts and accuracy-related problems. The heavy-barrel contours typical of chamber areas increase the chances of this problem as you move rearward along a handguard.

I said the same thing about the affect of attaching lights directly to the barrel four years ago, and was [incorrectly] torched in the comments.

But I was right then and this author is right now.  I didn’t make it up four years ago, and this author isn’t making it up now.  Machine harmonics matters.

I would add that not only can you make changes to your rifle that affect the POI, you can make them in a way that yields reduced accuracy even if you re-zero the gun (due to additional harmonics introduced by the component, making for an unstable system).  So this would mean larger groups.

I love floated barrels, and I don’t attach anything to them.  And as all hunters know, if you have a wood stock and it gets wet during a rain, if the wood swells and you can’t take that piece of paper and slide between the stock and barrel (you know what I’m talking about), your zero is off.


Comments

  1. On September 20, 2020 at 4:52 pm, Ned said:

    I watched two guys at the rifle range a couple of weeks ago with high dollar 300 Win Mags shooting groups at 200 yards.

    One guy was resting his barrel on a rifle rest and obviously having one heck of a time.

    My advice regarding the situation was not requested.

  2. On September 20, 2020 at 5:58 pm, John said:

    It is always interesting to watch a slow motion video of rifle barrel movement during firing.
    This makes it very eady to understand why you should try to keep the barrel free floating
    and unencumbered.

  3. On September 20, 2020 at 7:02 pm, Ned2 said:

    Great post and right on the money.
    It’s why I come here!
    Thank you.

  4. On September 20, 2020 at 7:04 pm, Jkr said:

    I’ve ground off a number of those M-Lok screws for being too long.

  5. On September 20, 2020 at 8:15 pm, George said:

    Jkr: yep had varying points of impact and couldn’t figger out what was going on with a rifle that had been shooting good. Attached a bipod mount and the screw was touching the barrel. As it heated up the point of impact changed and groups were ugly.
    An older and wiser friend splained it to me.
    Sigh.

  6. On September 20, 2020 at 10:23 pm, Steady Steve said:

    I bought a Mossberg Patriot rifle, used, from an old vet. He had upgraded the scope but attached a bipod that clamped around the barrel. Group size was not very good (3-4″ at 100 yards). Removed the bipod and bought one that attached to the forend and group size went down to 1″ with the same ammo. Haven’t had any problems with M-lok stuff. What brand was it with the too long screws?

  7. On September 20, 2020 at 11:13 pm, Georgiaboy61 said:

    @ Herschel

    Right you are, sir! Barrel harmonics are a thing, and they matter! Which is why SOP for a very long time, going back to World War One, was that when the GI or Marine clamped his bayonet onto his M1903 bolt-action or M-1 Garand, he needed to learn and understand how that affected his zero and point-of-aim/point-of-impact at normal combat ranges.

    And when those Army/Marine troops experienced their first ‘Banzai’ charges by massed Japanese troops, on places like Guadalcanal and Bougainville, they learned the wisdom of that advice, since – for many units – SOP on the line in the jungle or anytime close-contact was expected, was for weapons hot with bayonets fixed.

    Bipods and shooting rests, even when attached properly, can affect POI. Primarily through altered barrel harmonics, but also through shock wave propagation. If the user fires a rifle with bipod or rife upon a rigid shooting rest, the shock waves produced will travel down the barrel/fore-end, down the rest of bipod, and onto the hard surface of the bench or ground, which will reflect those vibrations back, as well as some from the surface itself. Thus, it is old shooter’s wisdom that use of a bipod or solid rest will usually make one shoot high at a given distance in comparison to off-hand shooting, for example.

    Which is why the old salts typically say that your zero varies by shooting position and whether or not you use a rest of some kind.

    Use of effective padding or dampening can reduce these effects dramatically, so the only way to know for sure what your zero is going to be – is to test it empirically in the field.

  8. On September 21, 2020 at 6:18 am, Nosmo said:

    Which is why barrel bedding tools, aka “barrel channel rasps and files” are such useful tools; it’s surprising how often they get used on synthetic stocks to ensure clearance.

    There is also an accuracy enhancement method that involves establishing deliberate contact between the foreend and the barrel to better control barrel harmonics through dampening, but it’s not a technique for any but the incredibly patient and rarely as successful as simply full-floating the barrel.

    An advantage of full-floating often overlooked is it removes sling tension from the accuracy equation; foreend-to-barrel contact of any type or amount, in conjunction with a loop sling, can easily change where, and how much, pressure is applied to a barrel which will affect its harmonics. Some AR-15/M16 handguards contact a forward handguard mounting flange (or sometimes, will flex enough to contact the gas block) and “slinging up” snugly can have a negative impact on accuracy; were one able to regulate sling tension consistently enough to impart exactly the same amount of deflection in exactly the same direction each time, the POI change can be adjusted for, but that’s a rarety, nor does it account for the sling tension changes imparted by gun movement and/or body shift during recoil, including tissue compression. It should be remembered that projectile barrel departure, while rapid, is not instantaneous with action of the lockwork, “barrel dwell time” is a real thing.

  9. On September 21, 2020 at 11:09 am, billrla said:

    Next up: One day, we will shoot laser beams rather than bullets, and will not need to worry about barrel harmonics.

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This article is filed under the category(s) Firearms,Guns and was published September 20th, 2020 by Herschel Smith.

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