View Full Version : Ballistics questions...

July 19, 2007, 04:32
I'm probably gonna get a general "ROFL! No, it wouldn't work, or it would be the norm by now..." response, but I'm gonna risk looking like an idiot and asking anyway...

So my questions are thus...

1. Medium-long or long-range FAL possibilities... Would an FAL benefit from a longer barrel? I've seen 21" barrels all over, what about a 23" or 25"? Possibly 27"? There's a certain length at which the bullet is affected negatively, so that is to be taken into question as well, but that's getting into the meat & potatoes of developing a longer barrel... My question is simply, "do you guys think it would be beneficial to have a longer barrel, maxed length until it starts affecting the bullet travel negatively? Is the 21" barrel already that long?"

2. Rifling... Would a smidgeon tighter in the rifling give a quicker spin and thus a closer group or a better distance? Say it has 9 turns per foot or however it is measured... Would 10 turns per foot/inch/meter/hectare/etc. improve ballistics? Longer distance or better grouping or even a bit less bullet arch?

3. I'm relatively new to the science behind how they go "bang..." What's the advantage of a fluted barrel?

- CK

July 19, 2007, 06:12
Barrel length is not the major contributor to accuracy, some very accurate barrels are quite short. What (within reason) it will do is give you a slight increase in velocity, so a flatter trajectory. I was dealing with one of the custom match grade barrel makers a while back & in his opinion 24" was the absolute maximum for a .308 round. IIRC thee velocity difference is about 100 FPS per inch. If you had a custom 24" BBl made for the FAL you'd pick up maybe 200 FPS and that's all.

Rifling pitch (tighter turns) will match to bullet weight. If it's too tight then accuracy will suffer as the bullet will be "over stabilized" and this will loose accuracy. 1 in 12 is about optimum for 150 & 170 grain bullets, anything heavier will need a looser (1 in 10) twist. That's why a lot of 30-06's were rifled in this pitch as the 30-06 was capable of using 180~200 gr bullets effectively.

Barrel harmonics are what's important. the bulging, stretching, whipping movement of a barrel is what contributes to accuracy. The keyword here is repeatability. Ideally every time you fire the whip will be the same direction, speed, and pitch. Bear with me a bit here as the answer has to be a bit involved.

Stiffness of the barrel is the key. To do this barrels were made heavier (thicker), but this was discovered not to be the whole answer. Thicker barrels absorbed heat from firing better and so shot more consistently as they heated up. The down side to this was weight. an 24" Bull (heavy weight) barrel for a .308 weighs almost 6 pounds alone, and, in addition to this weight the balance changes also. The answer to this was fluting the barrel. It gives three advantages. First it reduces weight without loosing stiffness. Second it gives a bigger surface area to allow for more cooling. Thirdly it shifts balance back towards the shooter a bit.

Something I haven't mentioned is this. All these things are only really applicable in match type rifles. Top see any real world advantages from a heavy, fluted, barrel you'd need a custom free floated, glass bedded rigid stock type rifle. In a FAL the mounting of the gas block & the movment of components of the gas system willl completely hide any differences these things would make to accuracy.

July 19, 2007, 11:59
Just to add to BK's post about - when he talks about 1 in 12, it means the rifling does one full 360 twist every 12 inches.

Example: 1 in 8 would be an example of a faster twist (bullet rotates once in 8 inches), and 1 in 20 would be slower (bullet rotates once in 20 inches) - see? ;)

It's actually the length of the bearing-surface that has to match the rate-of-twist, not so much the specific weight - but generally the heavier the bullet, the longer the bearing-surface anyway ;)

July 19, 2007, 12:02
You lose about 50 fps for each inch of barrel you remove.

Palma rifle competitors run .308 barrels to as long as 32 inches.
Trust me, you WILL get laughed-at if you show up with that barrel on a FAL.

You can play with the numbers at http://www.biggameinfo.com/BalCalc.aspx

For the distances you are likely to be shooting, and in the context of the general precision accuracy (or lack thereof) of a FAL, I think you will conclude that the ergonomic issue of barrel length greatly outweighs the trajectory issue for FAL operators.

English Mike
July 19, 2007, 16:57
The following is from memory:

If you take a heavy barrel & flute it, then it will not be as rigid as it was. The barrel will be only slightly more rigid than an unfluted barrel of the same mass & contour. The fluting DOES help to remove certain harmonics from the barrel.

July 20, 2007, 04:20
Originally posted by W.E.G.
You lose about 50 fps for each inch of barrel you remove.

The shorter the barrel, the slower the bullet!?

I thought the friction and combustion would have played a lot larger role in this...

Friction, bullet dragging down the barrel, only so much the air between bullet and primer can be expanded, etc...

Maybe I should take a physics class... O.o

I think I might just invest in a fluted barrel... I thought I saw them for sale on DSA's website...

Thanks for the info, guys! :)
- CK

July 20, 2007, 07:06
Originally posted by CeeKay
The shorter the barrel, the slower the bullet!?

I thought the friction and combustion would have played a lot larger role in this...
Rifle powders are slow-burning (compared to handgun powder) because the pressure has to be maintained all the way down the long barrel to overcome the friction of the rifling - bear in mind that the volume of the available space behind the bullet is increasing as the bullet itself travels further down the tube.

Now, because the powder is slow-burning, the bullet hasn't reached its peak velocity yet if you shorten the barrel and it exits the muzzle - all the remaining pressure of that still-burning powder is then wasted (but visible as an increase in muzzle-flash).

The longer the barrel, the slower-burning the powder and vice-versa (with the exception of smoothbore shotgun barrels, which use fast-burning powders as there is minimal friction due to lack of rifling).

July 20, 2007, 07:46
There was an experiment done some years ago where a crazy long barrel was test fired & then cut by an inch. This was repeated untill the barrel went from something like 50" to 16" at the end of the test. The velocity & accuracy were recorded at each stage & compiled at the end with some surprising results.

Basically there was a drastic velocity REDUCTION with the over length tubes. This velocity started climbing rapidly as the most efficient length was reached & the dropped off slowly as the "sweet spot" was passed. at 16" there was no chage to velocity so they stopped testing at that point.

What the testers figured out was that after a certain point there was no more "push" to the compressed gases from the propellant and friction took over, rapidly slowing the bullet in the barrel. As the tube shortened to the point where the gas just ran out of power as the bullet exited was where they got the maximnum velocity. any shorter than that & muzzle blast and flash increased while velocity slowly dropped off. The blast & flash was just the combustion of whatever propellant hadn't finished burning at the point where the bullet cleared the muzzle.

Of course this is only relevant to velocity, not accuracy. the super long barrel was so whippy that most of the bullets at long tube lengths were making something like 2 foot groups at 100 yds.

July 20, 2007, 07:52
Rifling twist rates are supposed to balance the weight, length, bearing surface, and a bunch of other things to make the bullet stable in flight. 10" to 12" doesn't sound like much, but if you calculate back to bullet rpm's in flight its a tremendous amount of change. We're talking 30~40 THOUSAND rpm's here.

A good example of mis matched spin stabilizing is watching a football (american, not soccer) player throw a ball. If the ball wobbles it is mis-matched to the rotating/speed/shape/mass equation. It's easier to see with the bigger projectile & slower speed, but the same thing happens to a bullet in flight.

July 20, 2007, 12:23
I believe I recall that barrel test. It was about circa 1990 or so; at least I don't believe Clinton was stinking up the Oval Office yet. Now, keep in mind the "sweet spot" will change with calibre, but this was either .308 or 30.06 and the optimum length was about 24 inches IIRC.

As the just-fired bullet begins its travel down the barrel, the area behind the bullet is getting larger every millisecond. In automotive terms, this is "lowering the compression", which means less energy is extracted from the burning, expanding gunpowder gasses. As mentioned, this is compensated for by a slow-burning gunpowder continuing to burn as the bullet travels down the barrel. The "push" on the bullet is continuous as it travels down the barrel. It's as if you're a child on a swing and your Dad doesn't just give you a quick push, he keeps his hand in the small of your back and starts running... better hold on tight!

The "flame" you see shooting out of the barrel is the gunpowder gasses STILL burning, and this may represent lost "work" from a too-short barrel, depending on the pressure -vs- friction factor. Note that the same barrel may be too long, or too short, depending on the powder used in the rounds... i.e. any given barrel may be "too long" if you use a fast-burning powder that stops "pushing" before the bullet leaves the barrel, or "too short" if you use a slow-burning powder that throws a humongous fireball when the bullet exits the barrel. People in the accuracy business go to great lengths to match all these factors to achieve an optimum balance.

There is probably more overspeeding of bullet twist than underspeeding. Not only will this contribute to inaccuracy due to the bullet wobbling or even keyholing, but overspeed will also contribute to poor terminal performance as centrifugal and hydrostatic forces explode the bullet into fragments that won't penetrate as far.

This is all merely an interesting theoretical discussion, as the FAL is the wrong tool for this particular job. It is a battle rifle, not a tack-driving match rifle. Go get a custom bolt gun for that function.


July 20, 2007, 12:36
I see...


Thanks guys!

- CK

July 20, 2007, 20:56
That's probably the same one. It ended up witha long thin picture of all the cut offf barrel sections laid out end to end.

If you want to up accuracy a bit then I think a free float handguard would be a better way to do this on a FAL. Like Radio says this isnt a target rifle, so pretty soon you'll run out of "improvments" that contribute much.
Powder doesn't "explode", it burns fast. How fast it burns is dictated by the pressure that it is burning under. "progressive combustion" is the term IIRC.
There is a quick build up of burning rate as the bullet starts out and then a fairly steady push as the gasses fill the ever expanding volume for them to fit into. eventually you just run out of fuel & the pressure starts dropping. Whenb this happens the burn speed drops, that reduces the gas more, and so on.

If you reload you can actually tweak the powder type to the barrel length by using faster or slower powders.

Going back to your original post I'd think that 21" is fine for anything out to 600 yds or so. Now for a FAL that's about practical maximum range.

You can do an easy test to see if a free float will benifit you.
Sandbag resr the rifle at the rear of the handguard, right up against the mag well. Now fire a 10 round group at 200yds. (100 really won't show things up anywhere as well). Without changing anything else move the sandbags to the middle of the handguard & repeat. If there is a big difference in group size then a free float will be a good bolt-on accuracy enbhancer.

Of course this all assumes that your barrel is in good condition as all this starts with a good bore.

July 20, 2007, 21:37
Originally posted by CeeKay

Is the 21" barrel already that long?"

Yes it is.

Sorry, couldn't resist. :D

Stay basic, at least till you get a few k rounds down range in this platform - there's a reason they built them the way they did. Only JMB and a few others could actually reinvent the wheel, plus the shooters ability is generally the most important factor in the equation anyway.

Spend your excess dollars on good ammo and :beer:

July 21, 2007, 10:43
Originally posted by shootist87122

Yes it is.

Sorry, couldn't resist. :D

Haha ass. :tongue:

(I do appreciate a good pun as much as the next guy, though... :p)

There'd be no way in hell I'd be able to build a barrel... And if I actually did... I wouldn't fire it!

I've only turned a wood lathe about 10 times in my life... That's not enough machining experience to run a metal lathe and produce a barrel for a firearm... O.O

If I ever wanted to get into long barrels, I'd do as brownknees said, and get a 50-some-inch barrel... I'd be able to shoot 1/4" groups all day at 25 feet... ;) I don't care if you'd all laugh at me... I'd have the most accurate short-range FAL EVER! ;) I wouldn't even need sights! I'd just nose the end of the barrel up to the target and click away! HAHA!!!

Or, I could just talk to DSA or someone and see if they wouldn't turn me a 24-inch barrel...

Or, I could buy a receiver and have an FAL that is capable of firing... ;)

- CK

(btw: Sale is still on... By Monday, I'll have decided on which of the thousands (okay, like 5 or 6) of beautiful kits to build. :tongue: