Idea on increasing muzzle velocity

[BlackRifleShooter]

Ok, I have had an idea rolling around in my head for a couple weeks and I thought I would bounce it of off you guys.

This deals with 5.56x45mm ammo and the M16/AR15 platform. The primary load for these platforms is XM-193. For the 55 grn fmj bullet of the xm-193 load to be effective as a fighting round it needs to achieve fragmentation. We know that it readily achieves fragmentation at velocities above 2700 fps. Fragmentation Range for an M16a2, M16a4 or its civilian counter parts(hereafter reffered to as full size for simplicity) is around 200 meters as this is the furthest distance that the 20 in barrel of the full sized rifle can provide sufficient velocity to achieve fragmentation. The M4,Car-15 etc (carbine) with its 14.5 to 16 inch length barrel can only provide fragmentation velocity levels out to +/- 100 meters. With the M4 becoming the mil/le weapon of choice and it being very popular with civilians due to its handy size, it would be nice to extend the fragmenation range of the carbine.

Other than developing some type of new propellant that will burn faster at lower pressures than what we currently have, we are stuck with what the NATO specs of XM-193 can give as far as velocity. I have been thinking of something else that could enough FPS to extend the carbines fragmentation range a bit further out. possibly another 50+ meters.

I have read on this forum and other places that Weatherby rifles use a longer throat to achieve higher velocities. I have also been told this is only done to lower pressures on the extremely hot Wby rounds. I can find no documentation either way. So I put this info in the maybe box.

Another thing that we have leanered is smooth bores can achieve faster velocites than can rifles bores due to less friction.

My idea plays in large part on the fact that very short barrels can be just as accurate as longer barrels. I know as a kid the BS talk around rifles was always that the longer the barrel the more accurate the rifle was. While this can be true when accounting outside forces, wind, distances, etc it has nothing to do with the inherrent level of accuracy from the rifle. As a point proven a custom AR-15 builder that builds alot of SBR uppers for Mil and civilian contractors (as well as though in free states that pay the tax) has repeatedly turned out 10.5 in barreles Carbines that can shoot Sub 1/2 MOA groups at 100 yards.

Another fact that I have been recently learning about is Free Bore Boost. FBB occurs in rifles and pistols that are outfitted with certain types of supressors (silencers for those of you speak hollywood). If pistol X fires load y at 843 fps with out the supressor Z installed it will achieve 890- 918 fps with the supressor installed. The reason for this is you have added 5 inches of powder burning barrel length with no rifling and the bore is slightly larger than the bullet being pushed down it, so there is no friction slowing it down.

My idea is to take a 16in barrel and bore out the rifling from the muzzle crown back towards the chamber for 10 inches. I can not see why this would not work. It should produce MVs 50 - 75+ fps faster than a standard 16 in barrel and be just as or close to as accurate as a standard barrel. I do not know, that it would be possible to cut a crown, basically in the middle of the barrel. If not the barrel could be cut at the desried length for a rifled barrel and the hollowed out smooth bore section permantly reattched. For Mil/Le I do not see why a quick detach system like a supressor could not be made to work. This would add several precious meters to the carbines fragmentation range and greatly improve its fragmentation with it its currently established distances.

Obviously if this idea has any validity it could improve many other firearms platforms as well.

Please share your thoughts and thanks for reading.

BlackRifle Shooter

[John Taylor]

First off the idea of free boring or back boring has been around for a long time and never proven to work. Think of it this way, the bullet has to spin fast enough to keep it stable. As the bullet picks up speed in the barrel it is also increasing it's spin or RPS ( revs per second ). If you stop the rate of increase on the spin by doing away with the rifling you might as well have a slower twist barrel which will not stabilize the bullet at long range. Part of the reason a smooth bore will have higher velocity is the fact that some of the pressure from the powder is used up to spin the bullet. A slower twist barrel will give a higher velocity but may not stabilize the bullet. Going to a shorter bullet will work in a slow twist barrel but will not maintain velocity at long range. The only 2 ways to get higher velocity is more pressure or holding the pressure over a longer time ( longer barrel).
Another thought, when I was working on 50 BMG single shots with long barrels (45") we were having trouble with metal fouling the last 8" of barrel. The answer was to Teflon coat the barrel and bullet. The side effect was that we gained 100 FPS.

[BlackRifleShooter]

John the rate of twist would already be established in the first few inches of the bullet contacting the rifling would it not? The fact that a supressor tube provides an increase in velocity via free bore boost, to me, proves the idea will work. The question is only how well.

How long will teflon coating last in a bore? I would also be curious to hear more about the 50 bmg out of a 45 in barrel sounds interesting!

[John Taylor]

The rate of spin would be what ever it is at the end of the rifling, the velocity would continue to increase in the smooth bore section but would not increase the rate of spin, if anything it might slow the rotation down. Say you have a 20" barrel and the rifling ends at 10". If the velocity at 10" is half that of the 20" barrel then the rate of spin will be half what is needed to stabilize the bullet. Say you get 2400 FPS at 20" and only 1200FPS at 10", The rate of spine with a twist of 1 in 12" would be 1200 RPS at 10" and not change at 20" but if the rifling went to 20" then the rate of spin would be 2400 RPS.
When I was working with the 50 BMG bolt actions 30 years back, we found that military ammo would not shoot strait because the core was off center. It was made that way for what the military called "a cone of fire" more like a shotgun effect. We started making solid brass bullets on an automatic screw machine after trying a bunch of other ideas that didn't work. The bullets weighed 615 grain and we were getting 3400 FPS. The Teflon coating was Tryflon (spelling?). Heated the bullets up and sprayed them then left them in an oven to dry. Sprayed the bore and swabbed it out. This kept the metal fouling to a minimum and lasted all day. A solid brass bullet at that speed will put a 1/2" hole going in an elk and a hole out the other side about 1 foot across at 450 yards. The bullet does not expand or blow up, it's all from the shock of the high velocity.

[Treeman]

BRS, WHile the "rate of twist" is the same in your backbored barrel, JT is correct that the rpms will be set at whatever was achieved at the velocity where the rifling ceases ......but most of the velocity is achieved in the first few inches of bore with a steadily decreasing rate of velocity gain as bore length increases. Your idea might work. I suspect that slightly increased velocity with a decreased RPM rate may prove self cancelling interms of fragmentation.....though if stabilization winds up on the margin where accuracy is acceptable but tumbling is inevitible........... It is an interesting concept.

[John Taylor]

Treeman, I disagree about the amount of the velocity being achieved in the first few inches. When testing magnum calibers a number of years back it was discovered that every inch of barrel length equaled about 50 FPS. On a 22 LR the best velocity is from a 16" barrel, any shorter and the velocity start to go down. That's the reason we were using a 45" barrel on the 50 BMG, to get higher velocity As long as the pressure is up the bullet will continue to gain speed, the longer the barrel the higher the velocity till the pressure drops below a point that it is no longer effective.

[BlackRifleShooter]

There have been test done to show that a 55 grn 5.56 can be stabilized with as short a barrel as 1.3 inches(or at least I have read that on the intranets?) Once the bullet starts its rate of spin it isnt going to speed up its revolutions? 1-12 twist can be established in a 10 in barrel.... shorter than the length of twist. If you barrel is 10 inches and you have a 1-9 rate of twist that bullet will make one revolution for every 9 inches of travel. You arent going to make a 1-9 twist bore cause the bullet to revolve 2 times in 9 inches by extending the length of the barrel..... I dont understand the RPM comments unless my understanding of how rifling works is way off base.

[yondering]

I see where you're going with the freebore idea; the reduced friction should increase velocity for a given barrel length. A very smooth bore with a tight slip fit to the bullet would be good for boosting max velocity.
However, I'd start the freebore at the chamber end, just like a really long version of what Weatherby does. With the rifling at the end of the barrel, the bullet could still have the right amount of twist, as long as the jacket was tough enough to resist stripping by the rifling.

Regarding the RPM comments, I'll try to explain: When a bullet is fired out of a rifle barrel, it has a certain velocity and certain rate of twist at the instant it leaves the muzzle. Velocity drops off rapidly due to air resistance, but twist does not drop off as fast, so the twist rate of the bullet in the air is actually increasing.
Now, consider your proposed barrel, with a few inches of rifling and the rest smoothbore, which is the opposite of what I described above: When the bullet leaves the rifling, the twist rate is set, but the velocity increases, which causes the twist rate to drop. You could certainly compensate for this by giving the first few inches of the barrel a very fast twist; determined by the velocity you expect to achieve at the end of the barrel.

For example (I'm just guessing at these numbers), let's say your 55gr bullet needs a 1-12 twist to stabilize at 3000 fps from a "normal" barrel. We'll guess that your "hybrid" barrel can push the same bullet to 3200 fps, and the rifling stops at some point "X" inside the barrel. Let's say velocity at point X is 2000 fps.
So, bullet rotation for a 1-12 twist at 3200fps needs to be 192,000 RPM at the muzzle. Assuming twist rate will stay the same between point X and the muzzle, this gives us 192,000 RPM at point X, at 2000 fps, which is a 1-7.5" twist.

The formula reduces to this:

Hybrid twist rate = (Normal twist rate)*(Point X velocity)/(Predicted muzzle velocity)

7.5" = (12")*(2000)/(3200)

Hopefully that helps. Keep in mind I made up those numbers, so they are probably wrong, but the calculations are correct.

I'd like to say that I think you have a good idea, and it's worth trying. You might really be onto something; you never know until you try. Don't let other people tell you it won't work until you see proof. A lot of our inventions and technological advancements have come from ideas like yours.
Please keep us updated on your results.

[JeffinNZ]

Strikes me the whole M4 scenario is the tale wagging the dog.

If a shorter weapon is required go bullpup and keep the barrel length the same OR longer.

This kinda reminds me of the problem of writing in space where NASA spent mega bucks on developing a pen that would write in nil gravity. The Russians took pencils.

[Southron Sanders]

If I had a government grant and all the lab equipment, to increase velocity without increasing pressures dramatically, I would experiment with the "Lancaster Oval Bore" rifling systems.

Back in the 1850's when the Brits first adopted the .577 P-53 Enflield rifle-musket, they did a lot of experimentation with various rifling systems. What they found to BE THE MOST ACCURATE was Lancaster Oval Bore rifling.

With the Lancaster system, the bore appears to be smooth, but in actual fact, the bore is slightly oval and has a twist in it. Since the rifle was a muzzleloading rifle firing a variation of the Minie Ball, the conical projectile made of lead had no problem expanding opon firing to fill the bore.

The Royal Engineers of the British Army actually adopted a variation of the Enfield Rifle that had Lancaster Oval Bore rifling. The problem with the Lancaster Oval Bore was that initially these rifles shot much more accurate than the standard Enifield with the three land nd groove rifling. But with the Lancaster system, after several hundred or thousand rounds, accuracy deteroriated dramatically.

Keep in mind that the barrels of those rifles were made of IRON! Roughly the equivalent of modern "wrought iron." So the deteroriation in accuracy of oval bored rifles can most likely be traced to the poor metallurgy of the barrels.

It would be interesting if someone were to make up some 5.56 mm barrels with oval bored rifling and experiemnt with them. Theoretically, the 5.56 mm bullet with the lead core would be malleable enough to be "squeezed" into the oval shape when it entered the "rifling."

Theoretically an oval bore with that was highly polished should produce somewhat higher velocities.

With barrels made of modern steels rather than soft iron (as was the case with the oval bore rifles of the 1850's, the barrel should remain accurate for a much longer time. Interestingly enough one of the problems with the muzzleloading Lancaster rifles might have been the RAMROD as the iron ramrod head contacts the sides of the barrel during the loading pocess-"ramrod wear" in muzzleloaders can actually account for more barrel wear than shooting lead bullets! That is anoher reason that a 5.56 mm oval bore rifling should be re-investigated. There is no such thing as "ramrod wear" in modern breechloaders.

As for the 5.56 mm cartridge-

[Larry Gibson]

Rate of spin is defined as the distance a bullet travels to attain 1 revolution. Thus the rate of spin is established with one bearing length of bullet travel. This is why 2" S&W .38s with a 1-18 3/8" twist will stabilise WCs out of the old 1 7/8" M36 barrels. RPM is established by using the rate of twist as part of the formula with muzzle velocity being the other important part. RPM and "rate of spin" are not the same.

Now as to increasing the pressure of M193 ammuntion. I recently pressure tested 3 different lots of U.S. M193 and some Winchester white box M193 (headstamped WCC 07) purchased at a local sporting good store. The APP (Average Peak Pressure[M43 Oehler]) was as follows;

LC82; 60,500 psi

LC91; 56,200 psi

LC00; 56,900 psi

WCC 07; 65,700 psi (that is not a misprint!)

SAAMI MAP (Maximum Average Pressure) for the .223 is 55,000 psi. CIP MAP, which includes the 5.56 NATO, is 62,000 psi. M193 ammuntion is most often used for training. The M855 ammunition with the 62 gr FMJBT with steel penetrator is most often the standard issue ammunition in theater (Iraq & Afghanistan). The APP for a lot (LC 94) was 62,300 psi.

As we see the pressures of the issue ammo, both M193 and M855 are right up there. I doubt freeboring would improve much. Perhaps Hornady's "Lite Magnum" technology would. The samples of that ammuntion I've tested in other cartridges indicates a higher velocity at lower or acceptable pressures might be possible in the 5.56 NATO cartridge. That technology is not available to handloaders but may be of use to the military. Interesting thought you've brought out.

Larry Gibson