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5.7x28 case + 80gr projectiles = ???

I got to looking at the 80gr Sierras and the 5.7x28 case and I thought, "Where have I seen this before?" Then I realized that the combination looked like a scaled down 240gr Sierra and 300 Whisper case. And that got me to wondering if it would be possible to use the 5.7 case as a basis for a subsonic 22 cal rifle. What do you think?

Until someone comes out with an appropriately scaled down bolt action I was thinking about having a Contender barrel made up. Put my can on the end and it might be a lot of fun. The questions I am left pondering are: How do I go about determining the proper twist for this project? I wonder what the external ballistics would be like with an 80 gr bullet starting out at 1040fps? And most importantly how come no one else (that I can find at least) has tried this?

According to Wikipedia at; http://en.wikipedia.org/wiki/5.7x28mm:

"SB193 Sub Sonic

The SB193 cartridge features a 3.6 g (55 grain) Sierra Game King FMJBT (Full Metal Jacket Boat Tail) projectile. The SB193's low muzzle velocity eliminates the distinctive "crack" created by supersonic rounds when used in conjunction with the P90 Gemtech sound suppressor. The SB193 suffers from greatly reduced penetration and range, but benefits from slightly lower recoil. The white colored bullet tip can identify SB193 subsonic ammunition. Its sale to civilians is restricted by FN."

Contender barrels are available here:http://www.tcarms.com/customShop/

Its been done, check this article at;
http://en.wikipedia.org/wiki/5.7x28mm:

"The SB193 cartridge features a 3.6 g (55 grain) Sierra Game King FMJBT (Full Metal Jacket Boat Tail) projectile. The SB193's low muzzle velocity eliminates the distinctive "crack" created by supersonic rounds when used in conjunction with the P90 Gemtech sound suppressor. The SB193 suffers from greatly reduced penetration and range, but benefits from slightly lower recoil. The white colored bullet tip can identify SB193 subsonic ammunition. Its sale to civilians is restricted by FN."

The T/C custom shop has barrels.

You can contact sierra for info on what twist rate would be required or some of the barrel manufacturers can help with that too. There are ways to calculate stability and spin rate but I don't know them. You can find software to calculate the external ballistics. I think it ships with a number of chronographs like the ones from PACT. You should be able to find it online somewhere too. Check reloaders nest.

One reason for the lack of interest might be the low energy available from an 80gr bullet at 1K fps. More than a .22 LR but with a lot more expense. It seems the subsonic .223 interest has died too though there was a lot of work done some time back. I wonder if the 5.7 case has the powder capacity to spit out the 100gr sierra slugs at 1000 or so fps....that might be interesting but I think the bullets are special order only and expensive. I know they have 90gr available too. All in all its a pretty steep expense to get reamers, dies, brass and building the whole setup when for the same money you can do a 300. Thats my guess for the lack of interest.....neatness ratio is not high enough to offset the expense or useable alternatives.
Frank

80 gn bullets need an 8 twist to stabilize.

Subsonic with the 5.7 and 55 bullets is easy.

I dont know if the 5.7 loaded with 80s would fit in a mag.

Twist rate can be calculated via greenhills formula - but it wasn't designed for subsonic ranges really and you have to take into account bullet construction/shape velocity/air density.

http://www.frfrogspad.com/miscellb.htm

Quote:

Q. How do you determine the rifling twist needed to stabilize a given bullet?

A. The needed rate of twist is effected by the diameter of the bullet, the bullets weight, and the bullet's overall length. Longer bullets need a faster twist to stabilize. As an example, a 1:12 twist in .30 caliber will adequately stabilized most commercial bullets of up to about 200 grains. To use a heavier (longer) bullet or to obtain optimum stability and accuracy with long pointed or boat tailed bullets of that weight requires a 1:10 twist. For best accuracy the slowest twist that will stabilize the bullet should be used.

Modern bullet stability calculations are based upon the work of the late Robert L. McCoy who was a ballistician with the Ballistic Research Laboratory at Aberdeen Proving Ground. His work, now used in advanced ballistics programs, accurately takes into account all of the factors involved in bullet stability and accurately describe the bullet's behavior.

There is an old formula called the Greenhill Formula that, while it was designed for estimating twists for boat tailed lead core bullets of moderate velocity, does a pretty good job of estimating twist required for flat based bullets under "normal" conditions.

T = Twist in inches
K = Greenhill's constant = 150 (This has to do with the specific gravity of a jacketed lead bullet)
D = Bullet diameter in inches
L = Bullet length in inches

T = (K * D2) / L

Using a 1.35 inch long .308 bullet (200 gr) and crunching the numbers we get about 10.5 (One turn in 10.5", which is pretty close to the 1:10 twist normally used in .30-06 rifles. The twist for the .308 is nominally 1:12 because it was based on the shorter bullet of the150 gr military ammunition from which the .308 commercial round was developed. Most match rifles in .308 have a 1:10 twist to stabilize the 180 and 190 boat tailed match bullets better. The results from the Greenhill formula are on the conservative side--indicating a faster twist than probably needed. That doesn't cause any problems because a little too much stabilization is better than too little.

However, the one catch with the Greenhill formula is that it does not account for the effects of temperature or muzzle velocity. As temperature or velocity decreases a faster twist is needed to maintain the same level of stability. Colder and thus denser air has a more destabilizing affect than warmer air. A lower muzzle velocity results in a slower rotational speed of the bullet and thus less stability.

As an example of the effect of temperature, the original M16 rifle for the M193 55 gr ball came out with a 1:14 twist which was barely stable at 68 degrees and which was totally unstable below about 40 degrees. They changed to a 1:12 twist to get stability (barely) at colder temperatures. The new M855 62 gr round is unstable below about 65 degrees with the 1:12 twist and requires a 1:9 twist to be stable. They went to a 1:7 twist because the M856 tracer round has a very long bullet, but the ball round does just fine in 1:9.

For velocity issues long bullets are more susceptible than short ones. For example a particular 210 gr .30 cal low drag bullet from a 1:11 twist barrel is stable from a muzzle velocity of about 2800 f/s and higher. Below that muzzle velocity it becomes unstable. With a 1:10 twist barrel the bullet will be stable at muzzle velocities of 1400 f/s and higher.

Another catch is that Greenhill assumes that the bullet's specific gravity is 10.9 (a lead cored jacketed bullet). For other bullet construction such as a steel core you need to apply a fudge factor by determining the bullets specific gravity. The formula would be:

Twist = [Square Root (10.9 / specific gravity of the new bullet)] * twist derived for a lead core bullet

You can determine the specific gravity of a bullet thusly:

1) Suspend the bullet at its balance point from the pan of a scale.
2) Weight the bullet.
3) Place a container of water under the scale so the bullet hangs fully in the water and weigh the bullet.
4) Subtract the weight obtained in step 3 from the weight obtained in step 2
5) Divide the weight obtained in step 2 by the difference obtained in step 4.

As an interesting exercise you can also determine the spin needed in mediums other than air under "standard conditions" by multiplying the spin for air by the square root of the number obtained by dividing the density of the medium in question by the density of air. As an example water is about 900 times as dense as air: 900 / 1 = 900 and the square root of 900 is 30. Thus you need a twist 30 times as fast to stabilize a bullet in water.

http://www.z-hat.com/twistrate.htm

calculator on the net
http://kwk.us/twist.html

downloadable
http://www.nfa.ca/content/view/134/197/

purchase
http://www.corbins.com/dc-twist.htm