Future Firearms Ammunition Technology 007: Squeezebore Ammunition – Celeritas Et Accuratio

The long-necked Colt 7.62mm round on the left combines the principles of triplex and squeezebore rounds (together called "salvo-squeezebore"). When fired from a modified M60 with a tapered muzzle, it would spit out three 55gr .224" caliber projectiles per shot. In the center is the duplex (not squeezebore) M198 7.62mm round, and on the right is the M80A1 EPR round, for comparison.

The long-necked Colt 7.62mm round on the left combines the principles of triplex and squeezebore rounds (together called "salvo-squeezebore"). When fired from a modified M60 with a tapered muzzle, it would spit out three 55gr .224" caliber projectiles per shot. In the center is the duplex (not squeezebore) M198 7.62mm round, and on the right is the M80A1 EPR round, for comparison.

Previously, we discussed the benefits of and challenges facing saboted projectile ammunition, including the advantages of decoupling the diameters of the bore and the projectile, and the problems of accuracy during sabot discarding. One concept that could possibly provide many of the benefits of saboted projectile ammunition without the drawbacks is the idea of having a malleable projectile that is forced through a conical section of bore, squeezing it down to a smaller shape. This increases, to a degree, the swept volume of the barrel, while not requiring any discarding sabot and not producing “wasted” energy that goes into propelling the mass of the sabot out of the barrel.

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Three bore contours. Top is a conventional bore profile with no taper. Bottom is a fully tapered bore that gradually decreases over its whole length. Middle is a bore with taper only at the end, as with an attachment.

 

Squeezebore barrels require a tapered section, but this can be anywhere in the barrel so long as it is stiff enough and gradual enough to squeeze the projectile down without quickly wearing out. The best location for both barrel life and swept volume is therefore at the end of the barrel, which can be accomplished via a barrel attachment. This affords interesting opportunities for squeezebore barrels, as a rifle could come equipped with a squeezebore attachment for firing supersonic projectiles, which could then be swapped out for a full-diameter suppressor attachment for firing full-caliber subsonic projectiles, if necessary.

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A WWII-era British Mk. VII Tetrarch tank with a “Littlejohn” squeezebore adaptor for its 2 pounder (40mm) gun. This attachment had to be used with special squeezebore ammunition, but gave the guns equipped with it considerably higher muzzle velocity and penetrating power when used so. Image source: wikimedia.org

 

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Cross sections of two different designs of squeezebore bullets. On the left is a hollow conical design, which could potentially be stacked in a SALVO arrangement, while on the right is a flanged design, similar to that of armor piercing squeezebore projectiles used in World War II.

 

The additional swept volume coupled with lightweight projectiles allows muzzle velocities close to that possible with sabot-firing designs, but the squeezebore concept (also called the “Gerlich principle”) does have some limitations. At least historically, squeezebore barrels do not last very long relative to conventional barrels (whereas the sabots typically reduce barrel wear), something that is partially alleviated by making the squeezing portion of the barrel a removable attachment. Also, squeezebore projectiles may be difficult to make, and a squeezebore weapon may limit the kinds of projectiles which can be fired through such a barrel, and there may be less dimensional control over the fired projectile’s shape once it has been squeezed down. Finally, there is a limit to how much a projectile can be squeezed down by a barrel; the few large-caliber squeezebore artillery guns that have been made have typically stayed at a ratio of about 0.70-0.75 for the final diameter versus the initial diameter.

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A close-up of a Littlejohn adapter. Note the venting ports. Image source: flickr.com

 

Still, the squeezebore concept offers less technically challenging way to gain much the same advantage as a sabot-firing weapon, despite its disadvantages.



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


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  • bmartin79

    Vary interesting. Seems this would be good for tanks and artillery . Not small arms . Not to sure a small arm that has a barrel attachment that needs to be replaces would be popular.

    • George

      It could last thousands of rounds, and short sections of Ta-W10 easier than whole barrels…. (Or Stellite, or Inconel, or…).

      I am more worried about accuracy. That may be overcome. But hasn’t been sufficiently yet.

      • PK

        A short section of tantalum/tungsten alloy would only quintuple the cost of the gun, after all…

        • George

          It’s at the end when pressure has dropped a lot. A couple of inches may do. If 2″ of 0.625 Ta-W10 rod costs $2,000 I would be slightly amazed, I was getting weight quotes a couple or more orders of magnitude less after you pointed it out to me a couple of weeks ago.

          • PK

            The application of a gradually reduced boresize liner of Ta10W or similar wouldn’t have the alloy itself as the primary cost, not in a short segment. It wouldn’t be anything approaching cheap to apply it, though! Some day… some day. We just need better explosives, more brisance and predictability all rolled into one.

            I get why my phrasing was less than clear, I did only say the alloy itself. My mistake on that!

          • George

            I would definitely try the cylindrical bore and short conical section at the end approach first, as the cone’s going to take the wear and as two materials joined like that is easy fabrication.

            No idea how it would work out…

      • Amplified Heat

        Consider that submachinegun barrels routinely shoot out squibs leaving only a faint swelling of the bore, and the idea of lower power squeezebores seem more realistic. I believe a lot of the wear issues are not that you are swaging (after all, it’s not like the throat rifling of most chamberings wears out quickly despite carving grooves into the bullet with every shot) but that you are swaging an already high velocity projectile at the highest speed point of its travel at the muzzle. I also don’t think the idea has been tried with modern coatings, which would reduce the wear/fouling further, even in extreme high velocity cases.

    • The Bob

      Has been done with small arms – Have a search for the Myra Extruder. Was a rifle that that swaged a .22LR round down to .20 or .17

      Ahead of it’s time, but made obsolete by the introduction of rounds like the .17HM2 (and by extension, .17HMR)

    • Amplified Heat

      On the contrary, I happen to believe it is most useful for *pistols* up against body armor
      -You get the benefit of a sabot as far as much larger driving area early on when pressure is high
      -The rate at which the bore volume increases as the bullet moves slows down, helping to flatten out the pressure curve & more efficiently extract energy
      -You get the velocity benefit of a longer barrel, and the BC/SD benefit of a long & skinny bullet that would ordinarily be difficult to fit in a pistol grip magwell
      -Pistols are almost all recoil operated, so no worries about swaging effects around gas ports or mistiming the bullet’s exit
      -Bore volume is much higher than for a necked cartridge striving for the same performance, so even ignoring the powder efficiency gains, the result will be far less overbore so not as loud/flashy as a 5.7×28 for instance

      So the summary is that you get a lot more efficient use of powder/pressure which are both in high demand but short supply in handguns, out of cartridges that can be shorter/fatter which fits magwells better. Bonus benefit is that the pressures & velocities involved will be much lower than for high powered rifles or artillery, so barrel wear and brass safety are less of a concern even for large diameter reductions (and no full-auto to worry about). A 45 to 30 or even 22 caliber shooting ~60gr copper bullets with polymer-coated collapsible driving flanges that is designed to break in half on impact flying at 2500fps or so would be hellacious whether it is clad in kevlar, steel, or denim.

      Not quite as cool as the ultimate conclusion of the squeeze bore concept, which is to do away with the barrel altogether and use high explosive to form a copper disc into a super high velocity conical penetrator that vaporizes upon impact, but I think packing EFPs into a handgun is probably a bridge too far.

  • iksnilol

    I like the idea of an attachment. Just a short tube that you screw on the end of a rifle or pistol for better trajectory and range.

    Reminds me of the AP attachment for the pistol in Deus Ex.

  • Aono

    I’ve read that Colt Canada makes barrels for the C7/C8 that have a “taper bore” that goes beyond the usual CHF process. Also that Centurion uses the tapered end of the mandrel to narrow the last few inches of bore to the muzzle on their CHF barrels. Could these technically qualify as squeeze bores?

    • Arathar

      They have some advantages and are often mentioned in discussions about squeezbores. But they dont directly aim for the main goal of Squeezbores -> maximum piston effect and better pressure retaining (instead of one huge peak), and then having a small, thin, aerodynamic bullet that leaves the muzzle / or serval.

      The taper bore does none of the 3, but still have less friction at the beginning which maybe makes them a tiny bit faster than with a normal barrel. Than they get there accuracy at the end part of the barrel. Everyone have to decide themself if they see it as actual Squeezbore or not.

  • Sunshine_Shooter

    I feel like the squeezebore is an attempt to replicate the advantages of a sabot-projectile without wasting the energy required to accelerate the sabots, but in so doing created a whole new host of problems that far outweigh the benefits gained and retained.

    • Amplified Heat

      It hasn’t really be tried outside a few rather crude prototypes and a rather successful anti-tank role. I happen to believe a lot of the ‘barrel life’ issues ultimately stem from the same limitations felt by any projectile trying to go that fast down a ferrous barrel, and would not be nearly so significant if the tech were used to get to say 3000fps more efficiently than to go faster than 4000fps (which is rough on even a conventional 25-06 barrel). The benefits are not only enhanced velocity and ballistic coefficient, but gains in efficiency through not accelerating a sabot as you mentioned, and also the very important ability to shoot a heavier payload than a sabot arrangement. Accuracy is also been found to be superior to sabot concepts, especially in small arms where they tend to really screw with accuracy (and not only when misused in barrels of the wrong twist, a-la Remington Accelerators)

  • roguetechie

    I have a short PDF of a very strange taper bore technology that used powder somewhat inefficiently in an effort to let you use what would ordinarily be extremely overpressure rounds.

    It’s in the SAR archives somewhere and is interesting, but it confused me quite a bit to where I never really understood the point.

  • Amplified Heat

    Or just swappable barrels. Not like there’d be much sense in your large-bore integrally suppressed barrel being more than 12″ or so long, and I’d be doubtful good long range rifle accuracy can be maintained over a removable barrel section –it’s bad enough accounting for silencer effects and the bullet isn’t even touching those.

    • roguetechie

      Yeah that and a bunch of other stuff would be issues.

      Personally that’s why when I can do expensive and in-depth development of my own preferred solution I’ll be going with a saboted 5.56 round based off the AR-2 FABRL bullet in a case as small as .30 carbine loaded to 5.56×45 lengths. I call the idea 5.56 minimum because it will weigh at most 50% of what a loaded M193 round does.

      My bet is that better sabots are the future.

      • Arathar

        Can you send me a link to a AR-2 FABRL bullet? What should make it better than EPR construction?

        • Arathar

          *Edit i just remembered it was this Aluminium cased, conical nose bullet.

          At what velocity would you want shoot it? With how much overall energy?

        • I think he is referring to the shape of the AR-2 FABRL bullet, not the construction.

    • CommonSense23

      Accounting for silencer effects?

  • demophilus

    I knew a former Austrian soldier who claimed they had a slight squeeze bore barrel for the AUG that swaged their 5.56mm down a bit. He said their JAG nixed it for general issue on account it made bullets shatter. They issued it to CT/sniper units instead.

    IMHO, now that we sinter bullets and do powder metallurgy, we should take another look at squeeze bore. Might not work for some applications, but it might for others, especially if it’s a modular system like a barrel extension.

  • DanGoodShot

    As a reloader, an attachment for a .308 down to… hum… (honestly, I dont know enough to say how much you could squeeze a round down too effectively) would be a lot of fun to play around with to see what kind of velocity, accuracy, distances and ballistic properties you’d get from it. I do think having the sqeezebore section a separate attatchment is the way to go. Imho, I don’t see a better way to deal with the excessive wear on a barrel.