Tapered barrel on HK rifles

richalexander97 said:

or literally because the bullet is slimmer when it leaves the muzzle (and thus has better ballistics)

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It’s not this.

richalexander97 said:

The MR556 version of the hk416 has a tapered barrel.
Given that 556 is 5. 7mm in diameter, how small would the caliber reduction be... would the bullet be crushed to about 5. 5mm?

It's claimed to increase velocity but I don't known if that's due to a higher force behind the bullet, or literally because the bullet is slimmer when it leaves the muzzle (and thus has better ballistics)

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A .224 inch diameter bullet is 5.7mm (or more exactly - 5.689mm), this is also the land diameter. The bore diameter is .219 inch or 5.56mm. As far as I am aware, the MR556 uses standard 5.56mm x 45 ammunition.

There are no "sqeeze-bore" weapons made any more, they were some experimental ones made for very high velocity, but the same result can be achieved with a sabot, for less manufacturing difficulty. They get high velocity the same way a saboted projectile gets its high velocity. A large piston area and a light projectile, but require special projectiles (see below).



More exactly, to increase velocity, you have to increase the force pushing the projectile, while decreasing the weight, or mass, of the projectile. The mass of the projectile has a lower limit due to required terminal ballistic requirements, and the maximum pressure is limited due to material constraints, so what do you do?

1) Full bore low density projectiles - the HVAP idea, but these have poor exterior ballistics, they loose velocity rapidly.

2) Sub-caliber projectiles with discarding sabots - the APDS idea, however in the early days there were problems getting clean sabot separation, especially when a muzzle brake is present, so these had lower accuracy.

3) The sqeeze-bore, these have low density and full-bore projectile, but as the projectile exits the muzzle the flanges are sqeezed in to make the projectile more aerodynamic (better length to diameter ratio). The barrel on the US designs were actually made more along the lines of a shotgun choke, with a full bore down to the last one or two calibers of the barrel then rapidly choking down the the final diameter. This makes rifling the bore easier and the choke is an external fitting on the barrel.

All of the "pencil" AR barrels are externally tapered; most "bull" heavy barrels are, too. This is common in rifle barrels as it helps reduce overall weight.
This can be cut into the external contour of the barrel as part of the contouring process.

Tapering the bore is rather a different proposition. This because you'd need a tapered reamer and/or boring bar. Both of which require a second machining axis while performing the first. It also adds to the complexity of rifling the bore, for needing that additional axis.

And, as noted above, it is not particularly successful as a strategy for firing projectiles. The Germans, in WWII used the concept as a way to get a lighter-weight anti-armor gun into the field. The pressures involved required the use of solid tungsten projectiles, which ran into wartime material shortages.

@lysanderxiii & @CapnMac - The OP is talking about MR556’s, which do indeed use a tapered BORE.

Allegedly, these will typically be ~.220” bore for a few inches in front of the leade, with the bore then tapering to .218” by ~2” from the muzzle. I can’t speak to the relative groove diameter. Chrome lining does come into play, and if memory serves, that tighter-than-SAAMI .218” bore is reflective of the chromies, not the unlined barrels.

As far as anyone I have ever spoken with on the topic is concerned, it’s really just a silly feature they created for themselves, and without hammer forging, really wouldn’t be viable for any other production method.

It’s certainly not anything I would ever get worked up over, and I find it really silly for anyone to claim it makes their barrels faster than others - reducing pressure early in the combustion and increasing pressure during the expansion phase isn’t a great way to increase your integral net pressure when using a progressive smokeless powder. Want a fast barrel? Use polygonal rifling. Otherwise, this taper bore thing is just a feature, largely irrelevant. I might buy a claim it increases throat life - but ever so marginally to the point I’d bet it would take several barrels tested side by side to actually prove it out, so again, irrelevant.

Centurion arms tapers their bore too. So does Hodge Defense.

I have a polygonal gain twist squeeze bore 5.56 9.5" barrel on a Noveske Lower in replace of their Diplomat 7.5" pistol one I bought it with. I am not at liberty to say exactly how I got it or from whom or who made it. It was an experimental barrel and is fairly heavy profile . It shoots 77 grain Sierra Match kings stabile and 1 moa to 300 yards . It does much better with 69 and less grain bullets so I think it must end up at 1 in 7" at the end, I can't measure it. What I DO know , in my chrono graphing of it and 10.5 " Colt barrels it is a little faster than those 1" longer ones. M855 Green tip ammo is 2650 fps from the 9.5" barrel which is almost 100 fps faster than the two Colt made 10.5" barrel I measured .

richalexander97 said:

So your saying that the bullet will exit at 5. 53mm instead of the usual 556.
Seems like the taper is almost nonexistent then.
I wondered if you could make a 556 barrel with a. More pronounced squeeze, creating a 5.3mm bullet with better trajectory

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Bore and groove diameters are different things.

Plus, you’re drastically overestimating how much a ballistic coefficient would be influenced by squeezing the bearing shank a couple thousandths. The opportunity to malform the bullet into a secant profile is far more likely than the opportunity to actually improve ballistic coefficient in any productive way.

The opportunity to compromise the jacket is also greater if you undersized the bore - again, the groove diameter notwithstanding the bore diameter.

Gordon said:

I have a polygonal gain twist squeeze bore 5.56 9.5" barrel on a Noveske Lower in replace of their Diplomat 7.5" pistol one I bought it with. I am not at liberty to say exactly how I got it or from whom or who made it. It was an experimental barrel and is fairly heavy profile . It shoots 77 grain Sierra Match kings stabile and 1 moa to 300 yards . It does much better with 69 and less grain bullets so I think it must end up at 1 in 7" at the end, I can't measure it. What I DO know , in my chrono graphing of it and 10.5 " Colt barrels it is a little faster than those 1" longer ones. M855 Green tip ammo is 2550 fps from the 9.5" barrel which is almost 100 fps faster than the two Colt made 10.5" barrel I measured .

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Sounds like something Triarc would mess with.