Does anyone know if there's an equation relating barrel length to muzzle velocity for the AR platform?  Specifically I'm interested in M193, M855, the 77gr Nosler NATO round.

I know I can find data for 24, 20, 16, and sometimes 14.5 inch, but I'm interested in performance out of 11.5 - 14.5 in 0.5 inch incriments and their respective fragmentation range.

So basically I'm interested in two or three bits of data:  the fragmentation velocity for each round (~2700fps for M193 and M855, but I don't know for the 77gr), and either the muzzle velocity for each out of the above barrels (or an equation to compute it) along with the ballistic coefficient, or the fragmentation range for each round out of the above barrels.

Unfortunately, this is purely to satisfy curiosity.  I don't have the money to go about actually playing with such firearms

If there is an equation it will be round and powder specific.

I have done some testing on bbl's 10.3,11.5,14.5,16, 20.

IIRC Tat and Brou have this in the ammo oracle - or ask them in the ammo forum.

Failing that I will dig out the data we got - but it wont be till end of the month when we get back from Minnesota

The ammo oracle has frag data for 11.5, 14.5, 16, and 20 inch barrels ... but I'm more interested in the 12.0, 12.5, 13.0, 13.5 inch intermediate lengths.  However, I figured it was a tossup to post either here or in the ammo forum.  

I figured an equation would be specific to each type of ammo, which is why I was curious mainly in those three (M193, 855, and MK262) since they might be tested enough to have an equation worked out, or at least velocities for more incrimental barrel lengths.

I figure if nobody happens to know (or at least knows where to look) by late Monday or early Tuesday, I'll cross post in the ammo forum.

[b]but it wont be till end of the month when we get back from Minnesota[/b]

Oh great. The Cannucks are invading us again.

Listen. This time when you burn Washington DC down, would you please do a thorough enough job that the damn place doesn't grow back?

Thanks in advance.

SD

[img]http://www.bushmaster.com/images/faqchart.gif[/img]

Mike

voilsb:

The equation you are looking for is called the Powley Calculator.  It is a simplified calcualtion, assuming a fixed chamber pressure and some other shortcuts like instantaneous power burn.
But it's very close and very quick: compared to Bushmaster's numbers, the Powley Calculator is about 50 fps slower.

BM is above Bushmaster's figures, PC is above the results from the Powley Calcuator:


Looking at the results, the FPS gain per inch from 11.5 to 14.5 falls from 60-40 fps/in for both 55 ang 62 gr rounds.

If anyone has data on the real muzzle velocity of the 77gr rounds, we can see how well Powley holds for the heavier rounds.

[:(] it skips the lengths I'm after.

Sweet data! It looks like the drop is fairly even from 14.5 to 11.5.  You've got 30fps for 14.5 - 14.0, 100 fps for 14.0 - 12.0, and 30fps for 12.0 - 11.5.  That 14.0 - 12.0 = 14.0 - (0.5*4), or (estimated) 25fps per 0.5 inch ... I wonder if that's true ...

By the way, one of the XM-8 configs is with a 12.5 inch barrel, which is one reason why I'm interested in external ballistics out of those barrel lengths.

Aight, I downloaded an excel worksheet for the Powley Calculator and plugged in data and measurements to get what appears to be M193 out of it.  I had an unknown variable (Water Capcity of Case), so I fiddled with it and calibrated it to both the BM and [b]blickbok[/b] data.  Here's what I got:



Granted, these numbers are estimated based on a rough calibration to [b]blikbok[/b]'s data, and measuring ammo specs from a diagram, so they're not perfect, but they're also probably not too terrible, either.

[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=17971[/img]
[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=17972 [/img]
[img]http://photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=17973[/img]

Yay for the 20" Rifle crowd! [:)]

(sorry for hijacking, but had to do it, because I needed a way to prove that 4" isn't just hanging off at the end of barrel doing nothing)

Well when you graph everything out it basically is 4" of excess lenght and weight...

Speaking of graphing it all out, here's a graph of calculated muzzle velocity to barrel length in 0.5 inch incriments from 1.5 to 24 inch barrels with M193 ammunition:

[img]http://voils.net/albums/Misc-Gun/m193.jpg[/img]Barrel length to velocity from 16 to 24 inches actually looks prettty darn linear.  Anything shorter than 14.5" unreliably fragments past 75 yards (yes, we knew that before this little experiment).

I would love to see similar data with the Mk262 mod 1 ammunition, though.  I'd also love to see it corroborated with field testing.

Now, if only I have rifles in those configurations and about 150 rounds each to test it ...

Hmm those velo's for the 4" and 6" dont jibe with what I have previously recorded.

Unfortunately my schedule till next Feb is pretty busy - as I would love to try this in depth.

Well, that's one reason I'd love to see this data corroborated.  And obviously, the 0" barrel is a bad data point, but I had to add it to get the axes to line up at 0,0.  But those are the figures generated by the equation.

Bummer.  I'd love to see the results (since there's no way I'll be able to try it myself for a rather long time, at the absolute soonest).

Brian,

Actually I forgot about my summer leave block in July...

I can do 4, 6, 10.3, 11.5, 14.5, 16, 18, 20

I think I know where to get G36C and K data too.  This time I must remember to bring my Kestrel to take environemntal data too.

Holy sweetness, that's cool!  Any chance you can get a 9.5 and 12.5" barrel, to simulate the XM8 and PDW systems?

Edit:  D'oh, as suspected, the G36C and K are the 9.5 and 12.5 inch guns, I'm guessing.  I found data saying the C model has either a 9 or a 12.5 inch barrel.  The K seems to have a 12.5.  I knew the XM8 was based off the G36, but am not really too familiar w/ that platform.

voilsb:

Case capacity of the ".223 Remington" is 31 grs of water for a 1.760" case.  I'd take off a grain for "5.56mm NATO" cases.

KevinB:

The Powley equation was originally designed in the 60's for pen-and-paper calculation, so it takes some severe shortcuts.  The biggest corner cut is the assumption of 45k psi peak chamber pressure, and an instantaneous burn rate of the powder.  Right after that, it's the assumption of common "hunting rifle" barrel lengths-- 16-24".  

Still, it is within 50-100 fps without correction to real-world values.  Cheaper than buying a BM 26" varminter barrel, and chrono after we chop and recrown every inch. :)

I would love to have more hard data.  The more we have, the more we can correct the model.  

Here's the data I used to get numbers that agree pretty well with what's posted in the Ammo FAQ and on the MD-AR15 website (for 11.5, 14.5, 16, and 20 inch barrels and M193):
And I also used 0.86 for the estimated charge weight constant.  Most of the data I wasn't 100% sure on had a negligible (1fps or less for all barrels) effect.  The only "real" variables were the Water Capacity and whether I used .80 or .86 for the charge weight constant.

I think I've asked this before but I'm not sure.  Why doesn't an ammunition manufacturer develop a round made specifically for the shorter barrel CAR/M4.  I know next to nothing regarding different powder types but can't a slightly faster burning powder be used to make sure it is all burned by maybe 14 inches.  This should increase muzzle velocity, accuracy and reduce muzzle flash, right?  The military market alone should make the cartridge economically viable.

"I think I've asked this before but I'm not sure. Why doesn't an ammunition manufacturer develop a round made specifically for the shorter barrel CAR/M4. I know next to nothing regarding different powder types but can't a slightly faster burning powder be used to make sure it is all burned by maybe 14 inches. This should increase muzzle velocity, accuracy and reduce muzzle flash, right? The military market alone should make the cartridge economically viable."


BINGO.  who knows? I doubt there is much they can do with the 14.5 but I'm sure they could make a round for the 10.3/10.5 with signifigantly higher velocity.


"Still, it is within 50-100 fps without correction to real-world values. Cheaper than buying a BM 26" varminter barrel, and chrono after we chop and recrown every inch. :)"


Buy a 24 inch model one sales barrel and hack a half inch off at a time with a saw.  The crown doesn't matter for muzzle velocity.  (only accuracy.)  I don't think very many people here are concered with values over 24inches.  After you get past the gas port simply hand-cycle like a bolt action.  (this assumes someone cares enough to waist $150-$200 on it.

The big problem with these tests is that different ammo lots can have significantly lower velocity.  (Black hills made some impotent .223 and some loaded higher than 5.56 pressure.)  

Those numbers with 77grain ammo look low, I have seen written data for 2400-2450FPS with a 10.5/77grain combo and that is approx where KevinB has the 14.5 pegged.  I would guess that this might have something to do with ammo lots.

The 77gr Data I posted was the Hunter Shack 77gr SMK - it is much slower than the BH.  This was the first 'Navy' Match ammo that then mutated into the highly sucessful Mk262

So I don't have to start another thread (in the ammo forum, since this is more pertinant to that) ...

Does anyone have the necessary data (I'm guessing it's reloading data; I'm not a reloader, so I wouldn't know) for the Mk262 Mod 1 round?

Case Length, Bullet Length Cartridge Length, and Case Bullet Diameter should all be the same and Bullet Mass should be 77.0 grains, so the figures I'm interested in are these: Full Water Capacity of Case, Height (axial) of boattail, Tail Diameter of boattail, charge weight constant (.80 or .86?), Ballistic coefficient, and reliable frag velocity.  I'd like to run the numbers for that ammo, too.

And if this is too out of place here, I'll post it in the ammo forum instead.

Volsib:

5.56mm NATO:
1.760" case, 2.26" cartridge OAL.
I can't find data on the length of the bullet on the web, but
http://nosler.com/chartcustomcomp.html
has a picture.  It has a boattail.  If the picture wasn't so dark, we could count pixels to get dimensions. :)

Great thread guys.

I'm going to be building a longer range flattop scoped rifle than I use my M4's for.  I was undecided about a Bushmaster National Match 20 or 24 inch barrel but the data provided here indicates that the extra 4" (24") is not worth the gained velocity vs. the extra length of the rifle.

20" it is for me.

This thread deserves a bookmark.  Thanks for the great info.

CMOS

I had 5 Nosler 77s laying so took some measurements. Sorry, no SMKs laying around. Chinese measuring devices were used so take it with a grain of salt.

bullet: Nosler 77gr HPBT
Claimed BC: .340
mass: 77.1gr
bullet length: .980in +/- .008
height of boattail: ~ 0.150
tail dia. of boattail: ~ 0.195

This is great information.
Thanks,

Frank
POF-USA