Science: Flash Hiders and Accuracy

 

An F89 LMG in a test rig at DSTO.

An F89 LMG in a test rig at DSTO, as part of these tests.

Here’s an Australian study that looked into the effects of flash suppressors on machine gun accuracy (burst dispersion). Australia uses a locally-made variation of the FN Minimi, with some local modifications including, significantly, a parallel-sided rather than tapered barrel, as the F89 in the squad automatic/LMG role. The F89 closely resembles an early (pre-Product Improvement Package) M249 SAW.

Muzzle whip was measured by these optical proximity switches, to try to determine why some suppressors were more accurate than others.

Muzzle whip was measured by these optical proximity switches, to try to determine why some suppressors were more accurate than others.

A test by the Aeronautical and Marine Research Laboratory of the Australian DOD’s Defence Science And Technology Organization looked at flash suppressors’ influence on accuracy. The test mostly looked into dispersion of rapid fire, not cold-bore accuracy, but is extremely interesting. One of the things they discovered: that an adapted FN-MAG flash-hider is superior to the standard Minimi arrangement, at least with respect to accuracy.

It has been found that the addition of a flash suppressor from a MAG 58 machine gun Will reduce the size of mean radius dispersion by as much as 41% over a Minimi barrel fitted with a standard flash suppressor, and by 35% over a bare F89 barrel.

Some part of that is due to the weight of the flash suppressor:

The evidence suggests that the parallel sided F89 barrel is stiffer than the taper ended minimi barrel and that a muzzle mass would reduce flexing in the Minimi barrel but have minimal effect in the F89 barrel.

A very interesting finding was that each flash suppressor changed the point of impact of individual rounds in the same direction and amount, regardless of the indexing of the flash suppressor.

As everyone’s come to expect from a scientific document, the scientists conclude by suggesting that what is needed is more research.

7 thoughts on “Science: Flash Hiders and Accuracy

  1. Aesop

    All interesting, but there’s one other noteworthy point: where bullet hoses are concerned, a little spread randomness is a good thing for the beaten zone. You don’t want to put a full battle pack into the same ragged hole, you want to make colanders out of what was enemy infantry squads on the move.
    LMGs and MMGs are the one place where trigger-jerking spray-and-pray is a feature, not a bug.

  2. Hank

    I made an interesting discovery with my Colt AR-15 shooting from a Caldwell 7 rest. After zeroing the rifle at 100 yds. I removed the A-2 flash hider and fired another string which grouped well low on the target. Replacing the flash hider restored the gun to zero. Repeating the test several times gave the same results.

    I have no explanation for it.

    1. Hognose Post author

      There are a couple of possibilities, and they involve both the weight and the change to gas flows caused by the FH. The DSTO testers were bemused by a similar result. From P. 17 of the document:

      It was also observed that the minimi FS does not reduce dispersion any more than if no FS were used and that the addition of any form of FS shifts the MPI down and to the right of the original target impact point. The latter effect appears to be the result of mounting asymmetry. To validate this observation disk-shaped screw-on weights were attached to the muzzle end of the barrel…. The shift in MPI down and to the right still occurs.

      US testers at BRL/ARL could not replicate the Aussies’ other results using an M249 barrel, but a previous ARDEC test, using a Minimi barrel, did, according to the BRL report Dan links:

      It is not clear why the TID results did not concur with DSTO’s findings. One consid- eration is that the gun barrel used for the flash suppressors was different (DSTO used a FN Minimi barrel, ARL used the M249 SAW). The U.S. Army Armament Research, Development and Engineering Center (ARDEC) reproduced DSTO’s results by firing the flash suppressors with an FN Minimi barrel (Zisa 1992).

      Reading this, we thought that we, and Dan (which is a rare thing), had missed a relevant document, but that is not the case. The reference is to a personal communication from T. Zisa of ARDEC to Doug Savick in 1992, not to a published document.

      As noted, the Minimi barrel has a lighter, more tapered profile than the F89 or M249 barrels. It would be rational to expect weights or flash suppressors to have a greater effect on this barrel’s dispersion. Likewise, you would expect the flash suppressor to have a greater effect on a pencil barrel for an AR, then on ethic bull barrel. But this is all speculation without actually doing the testing.

      For most of the BRL/ARL tests it seems that they did not fire the guns from a fixture or rest, instead from bipod-supported prone position; and they do not appear to have measured mean point of impact, only total impact dispersion (not location of hit, only size of group, in science-free English). They did use a fixture for the part of the testing that was conducted on an indoor range.

      Sounds like there’s still room for research!

  3. ensitue

    so in other words the study was a huge waste of money as the results of increasing barrel mass have been well known for decades, if not 100s of years

    1. Hognose Post author

      Not really. There were greater effects for the flash hiders than for barrel weights of the same mass, suggesting that (1) there is something happening with the gas flows or (2) the OZ contingent have an instrumentation problem of some kind.

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