The String Measurement

Recently we discussed some very old marksmanship in connection with Civil War sharpshooters (see this post and this one, and there’s more to come, thanks to expert Fred Ray). And a couple of commenters asked about the “string” measurement of marksmanship precision and accuracy that was used at the time, and well up into the 20th Century, before modern measurements of precision (group size) and accuracy (distance from point of aim and intended point of impact) were developed.

Fortunately, the late Steve Ricciardelli of Steve’s Pages incorporated an explanation in a list of measures of group size and central tendency:

String Measurement

This is an old method still used to determine a shooter’s skill at hitting a target. It assumes the point of aim is always the desired point of impact and is simply the sum of the distances from the point of aim to each bullet hole. Originally a string was used to gather the distances, hence the name. It is still a valid measure of total error relative to the aim point. String Measurements however cannot be used to analyze sight settings because it only measures the magnitude of error, not the direction of error. It is also not a useful measure of group size because a tight group located away from the Bullseye will produce a large String Measurement.

The string measurement is old, but it remains surprisingly useful on a real-world basis, to get a broad idea of the practical accuracy of a specific shooter and firearm combination, or to put shooters in a rough rank order (say, if grouping soldiers for marksmanship training). Now, a marksman seeking to maximize performance (think of, say, a benchrest shooter) would not want to use it, because it is important to him or her to separate the possible causes of misses; you do something different if your windage is off than you do if your group is too large.

Turns out a string is useful for something, even though ATF doesn’t say it’s a machine gun any more. (Yes, they once did. But that is another story!)

15 thoughts on “The String Measurement

  1. SAM

    To be fair to the BATFE (stop laughing) it’s only machine gun if it’s a shoestring with a loop on both ends. String any how – OK; shoestring with loop at one end – OK- shoestring with loop at both end – machine gun GO TO JAIL: Go directly to Jail. Do not pass Go. Do not collect $200. (but if they would let you pay £200 tax you would be OK).

    1. Sommerbiwak

      His did they get to the conclusion that a shoe string with two eyelets is a machine gun? Is there a firearm that can be converted to fully automatic fire with this?

        1. OBob

          You take a semiauto rifle with a reciprocating handle like an M-14 or mini-14 and a shoe lace with a loop ties in each end. Put one loop over the charging handle and loop the lace down behind the trigger guard, over the trigger and out the right side. Grip the gun normally but put the other loop over your trigger finger.

          When you pull the loop the lace trips the trigger, then when the handle moves rearward it slackens the lace letting the trigger reset. When the handle goes forward the slack disappears and the trigger it tripped again.

          In the US this is a felony punishable by 10 years and a $250k fine. Keep in mind that the ATF says a MG is and device that COULD be used to convert a firearm into a MG OR is “readily convertible”. In court they’ve argued that “readily convertible” means with less than 1 day’s work in a machine shop, ignoring that many guns can be createdfrom scratch in that time (I think the champ was the PPSH-43 at something like 2.3 man hours). By their “logic” every shoe store in the country is dealing in unregistered machine guns.

          For the record I’ve never done this and don’t own a rifle with a reciprocating handle, but have seen a video of a mini-14 shooting a respectable 600rpm. The ATF supposedly reversed its ruling when a MG manufacturer afixed a small metal plate to a shoe lace and attempted to register it with the them.

          1. Sommerbiwak

            thank you.

            I get that using a shoe string to turn a firearm full auto is forbidden as manufacture of a machine gun. but treating it as a machine gun is let us say counter intuitive. But then sheet metal pieces are considered “machine guns” that function as auto sears and convert any semi auto firearm these fit into, into a full auto one. And this line of thinking was then consequently applied to other devices like the shoe string. But then a drop in full auto sear is not a machine gun in the dictionary sense either.

            BATF should have gone through and registered the shoestring. It would then be legal to use this shoestring for converting an M1A or Mini-14 into full auto. Logic does apply only peripherically to laws and regulations after all. ;-)

      1. Steve M.

        Skills. The ATF has skills. They can find guns anywhere. ‘Cept when they’re running ’em to the cartels!

        It’s like the EPA. The EPA says that a hazardous waste can be liquid, sludge, gas or solid, but in order for it to be a hazardous waste it must be a solid. Umm, okay…..


    Shoestrings? BATFE? Machineguns? Like Sommerbiwak, this gag is going over my poor foreign head.

    1. Hognose Post author

      Sorry, I thought everybody knew the shoestring story. Come to think of it, one of the places it was written up was, I think, Dean Spiers’s The Gun Zone, which is now tango uniform. I guess I ought to write it up again. But yes, ATF declared a length of string or shoelace with a loop tied in it a machine gun. After much ridicule, they withdrew the ruling.

  3. John Distai

    Off topic, but related to measurement:

    I have a pistol I’d like to remove the finger grooves from. This is my first time trying this, and I don’t want the thing to look like a Bubba hack job when I’m finished. I have fine craftsmanship skills and am patient and meticulous in my work.

    I’ll remove the material very slowly, by hand, using high quality needle files and modeler’s rasps. No Dremel, Foredom, or other power tool use. Work will proceed slowly, and meticulously.

    I would to know the depths of the peaks and troughs of the grip in relation to the inside edge of the magazine well. I don’t want to unnecessarily weaken the structure by removing too much material at the wrong places.

    I would use a standard caliper to take these measures, but the grip flares at the magazine well to accommodate the little finger. This flare prevents me from getting a straight measurement using a standard caliper.

    Any advice on what tools or measurement methods I should use to find those peak and trough depths?


    1. John M.

      Take the pinky flare off first and then measure. If you take too much off, round it off and state authoritatively, “I added that to help if I need to strip the mag.”

      -John M. (who has poor craftsmanship skills)

    2. Hartley

      It shouldn’t be hard – take two straight rods or suitable flat bars, put one inside, along the inside of the mag well, and the other across the tops of the finger ribs. This will establish whether the finger ribs/lumps are uniform or tapered, and should allow the measurement of the thickness including the ribs/lumps. Measuring the depth of the grooves should be easy using the other end of the caliper. Also, take a good look inside to make sure there aren’t any cavities you need to compensate for. :)

  4. John Distai

    Thanks. What you suggest is certainly an option. Clearing that pinky swell is the toughest part. I have some old tuning forks. If the distance between the forks are enough to clear the pinky swell, I can do just what you suggest.

    Otherwise, I could make a wooden version that has enough clearance, take the crest and trough measurements, and then check the difference between them.

    After more searching, I realized that the appropriate micrometer would also work, as well as a “universal bevel protractor”. Many options. I just need to take my time and be careful.

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