It’s generally understood that weapons that fire from an open bolt are less accurate than those that fire from a closed bolt. It’s certainly logical that both the weight of the moving bolt, plus the inevitably longer locktime, will increase dispersion. An interesting controlled experiment the US Army conducted in 1978 confirmed the existence of, and tried to quantify, this increase in dispersion. Additional findings were that:
- Not just the dispersion, but also the precision of the shots suffered. That is to say, not only their separation one from the next in the same group, but their separation from the intended point of aim.
- Comparing prone unsupported to the less accurate and consistent offhand position, the penalty for using an open bolt was greater in the offhand (this might be predicted logically).
- Right-handed shooters were consistently off target to the right, and this increased when firing from an open bolt. Lefties erred in the opposite direction.
Unfortunately the only usable data come from the dry-firing tests, in which a laser and receptor were used. This methodology could not be validated by live-fire testing for an interesting reason: the innate accuracy of the weapon they used, the XM19 SPIW, was so poor.
For the purpose of the test they had a single XM19 and a quantity of XM645 flechette ammunition. The test weapon was Serial Number 6 and was lent to the Ballistics Research Laboratory by the manufacturer, Aircraft Armaments Incorporated of Maryland.
THE XM19 was AAI’s entrant in the Special Purpose Individual Weapon contest in the 1960s, and the follow-on Future Rifle Program of the 1970s. It fired flechettes at a very high cyclic rate and was intended to mate with a grenade launcher (the M203 was an outgrowth of the first stage AAI’s GL development; a second-stage repeating launcher was not a success). It had extreme reliability problems; while then-current service weapons, like the M14 and M16, could fire thousands of rounds without a stoppage, the XM19 never managed more than dozens. (In a profile of AAI founder/designer Win Barr for Small Arms Review, George Kontis wrote that bad single-shot accuracy at range was what killed the AAI flechette rifle).
The XM19 was also badly designed ergonomically, awkward, overweight, and muzzle-heavy even without the grenade launcher (which was not installed during the 1978 tests). Why this test was not done with a more conventional weapon is unclear. It may have simply been opportunistic, in that both open- and closed-bolt lockwork for the XM19 was readily at hand.
These facts suggest a follow-on experiment. It would be worthwhile to repeat the test but with a more modern alternative. For example, if you got 20 marksmen to fire 10-shot groups each with an AR clone rigged to fire from open and closed bolt.
That’s a problem with the ATF, which defines any open-bolt semiauto as a machine gun, no matter how robust the disconnector is. But the work-around is to use a lower with no magazine well at all. A single-shot weapon can’t be a machine gun, no matter how intently the ATF tries to find a technical violation to fry somebody for.
The original, primary-source document. Technical Memorandum 2-79, Aiming Point Displacement from Firing a Rifle from the Open-Bolt Position by Dominick J. Giordano, is available: