5.7x28mm out of a 24 inch barrel

[F1V3-53V3N]

Has anybody come across chronograph data with 5.7x28mm coming out of a 24inch barrel? I have some data from a guy that had a custom 5.7 rifle built with a 20inch barrel, but haven't found anything on a 24 inch.

If nobody has info on this, is there a mathematical formula that allows for ballpark estimates on velocity increases gained per inch of barrel length? I would figure there isn't as longer barrels will obviously offer diminishing returns at some point.

[HK SD9 Tactical]

There are too many variables for an accurate answer. Figure about 50 fps per extra inch of barrel if the powder has not expended all of its energy by the time the bullet reaches the muzzle. Alot depends on the load, powder used, and weight of the bullet.

The factory ammo would probably loose fps due to the power curve of the powder (ie the powder has expended all of its energy prior to the bullet leaving the barrel - this is just a guess) which would remove most of the gained velocity due to friction of the bullet against the barrel while traveling down the barrel. There is also the volume of the barrel that needs to be taken into consideration as that directly affects the velocity curve.

However, I have never chrono'd the velocity of 5.7 out of a barrel that length so this is just a best guess, but between my SBR and PS90 stadard, the average is 50fps extra per inch on nominal loads between the two. Hotter loads will garner better velocities.

[mxgobig]

SS197SR will get you between 2130-2160.
http://www.*******************/viewto...p?f=16&t=11213

[jathtech]

even if all the powder has burned, the gasses are still not finished expanding, so a longer barrel will never result in a lower velocity.

[F1V3-53V3N]

SS197SR will get you between 2130-2160.
http://www.*******************/viewto...p?f=16&t=11213

Ahhhh, so that's your gun? Nice! I was about to ask, "how do you know it was a 24inch barrel because the thread doesn't specify?", but then I noticed your profile name.

It does look like you lose a bit of velocity going from a 20inch barrel to a 24 inch. There was another guy that built a custom 5.7 bolt gun in a 20 inch barrel and he got SS197 to go 2176fps. What type of chrono were you using? Did you perhaps get any numbers from other rounds?

[F1V3-53V3N]

even if all the powder has burned, the gasses are still not finished expanding, so a longer barrel will never result in a lower velocity.

At some point you would have to see diminishing returns. Barrels don't create energy, they just conserve/manage/contain... whatever. I'm no engineer.

[mxgobig]

Yep, I built that one in early July of this year.... still haven't had time to work on the magazines yet.

I was using an Oehler chrono.... not sure what model it was. I belongs to a gunsmith friend of mine.

I haven't put any other ammo through it yet. I'm sure the real velocity will come with reloads and a little slower burning powder.

[F1V3-53V3N]

Yep, I built that one in early July of this year.... still haven't had time to work on the magazines yet.

I was using an Oehler chrono.... not sure what model it was. I belongs to a gunsmith friend of mine.

I haven't put any other ammo through it yet. I'm sure the real velocity will come with reloads and a little slower burning powder.

:P

You are the only person I know who has a 5.7 rifle with a 24 inch barrel. That makes you a VERY valuable information source. Any way I can convince you to chronograph some more types of ammo through your rifle? I can send you just about any round made for the 5.7mm. EA's hotter stuff seems to really take advantage of longer barrels. I bet S4M would create a time warp as it left the barrel.

I would absolutely LOVE to get another 5-shot average of 197 (just to be sure), along with 195, S4M, and ProtecTOR 1, 2, and 3, and perhaps the VarminaTOR.

Whattdya say?


*EDIT* Post 300!!!! Weeeeeeee What do I win?!! What do I win?!!

[jathtech]

haha, you win a new t shirt, a gold one... Cept you can't have it, or see it, or touch it...

[jathtech]

even if all the powder has burned, the gasses are still not finished expanding, so a longer barrel will never result in a lower velocity.

At some point you would have to see diminishing returns. Barrels don't create energy, they just conserve/manage/contain... whatever. I'm no engineer.

diminishing returns, yes. And the bullet will eventually stop gaining velocity at some length as well, but that length is so long, you would only find it on a Boeing weapon platform.