Barrel length and velocity.

[paultap]

Hi guys, most of the common reloading manuals I have are of American origin. Our American friends tend to use longer barrelled (unmodified) hunting rifles compared to what we tend to use in the UK.

If you take 243 and 308 for example, the trend in the UK seems to lean towards 20 inch barrelled rifles. Most of the reloading manuals I have read have loads developed for the above calibres in barrels that are at least 2 inches longer and in many cases 4 to 6 inches longer!

Logically, if we assume we are getting the same velocity that our American based reloading manuals are telling us for a certain powder charge weight, we are probably significantly off the mark, With the actual velocity being significantly lower. Obviously the only real way of testing this out would to use a chronograph to measure the exact velocity of each load produced. In reality this is not a piece of equipment that the average reloader is likely to possess.

If we take the case of the 243 for example, it is my understanding that with a 20 inch barrel, some factory produced Ammunition struggles to make the UK legal minimum muzzle energy requirement of 1,700 foot pounds, that is the legal requirement for use on our larger species of deer.

I would be very interested to find out the average reduction in velocity in comparison to reduced barrel length for a given load, compared to the length of barrel that the load was developed in. For example, for a load developed in or fired from a 24inch barrel, what velocity reduction is likely from a 20 inch barrel?... 150 fps... 200 fps maybe?

I would be interested in your views on the above and it would be very interesting to hear from anyone who has actually carried out such a test.

Many thanks.

 

[stephentri]

Hi I have a Tikka t3 6.5 x55 with a 20 inch barrel my friend has a Sauer 202 same calibre with a 24 inch barrel same load 140 grain sst I get 2650 fps he gets 2825 fps

 

[paultap]

Hi I have a Tikka t3 6.5 x55 with a 20 inch barrel my friend has a Sauer 202 same calibre with a 24 inch barrel same load 140 grain sst I get 2650 fps he gets 2825 fps

That's interesting , muzzle energies by my calculations work out as follows

20 inch barrel gives 2650 fps = 2183 ftlb

24 inch barrel gives 2825 fps = 2481 ftlb

So thats a drop of 175 fps and 298 ftlb for 4 inches of barrel length, quite a significant drop.


Incidently, here's the formula used to work out the muzzle energy -


velocity (in fps) x velocity (in fps) x bullet weight (in grains)
------------------------------------------------------------------- = Muzzle E in ftlb
450240



example for 2650 fps using 140 grain bullet is as follows -

2650 x 2650 x 140
---------------------- = 2183 ftlb
450240

 

[Greener Jim]

To be honest that's a pretty good retention of velocity. It's only 43.75 fps/inch loss.

More overbore cartridges such as 22-250 would do much much worse.

If you compare two cartridges from the same 'family' such as 243, 308 and 358 the velocity drop per inch lost is less in the larger calibres as the cartridge is less overbore.

Here is an interesting article where they chop a 308 barrel down from 26" to 13.5" and they get a 25 fps/inch loss
The Truth About Barrel Length, Muzzle Velocity and Accuracy - The Truth About Guns

 

[robertw]

You should chronograph your ammo anyway I have two 6mmBR rifles both barrels are from the same manufacturer same length same action with the same load one is 50fps faster than the other
so if your load is marginal it would pay to know exactly what the velocity is running foul of the law is not a good move.

 

[Polly]

Just did a lot of " web research" into this myself and concluded that for most stalking calibres around 50 fps per barrel inch is a reasonable rule of thumb

 

[Polly]

Oh and meant to say that most factory data is based on a 24inch barrel

 

[NigelM]

Loss of velocity per inch of barrel length is very dependant in calibre. An overbore calibre will loose much more than an underbore one and a heavy for calibre bullet will loose less than a light for calibre bullet. A 20" rem mag with a 140 grain bullet will loose heaps, not worth doing, but a 20" .308 using a 180 grain bullet will not loose too much.

There is always a tipping point you cannot dip below. I wanted to build a 16" .308 tracking rifle but the loss of velocity/energy for those additional 4" was huge.

 

[Greener Jim]

You can still be deer legal at 15" with a 308

Have a gander at pistol data
Set your sights on pistol reloading data | Hodgdon Reloading

 

[Muir]

Just did a lot of " web research" into this myself and concluded that for most stalking calibres around 50 fps per barrel inch is a reasonable rule of thumb

Thirty-five fps per inch has always been my 'rough estimate' number when used in a ballistically balanced round. Not always right but, hey! That's why everyone should own a chronograph. It cracks me up that a reloader can spend $$$$ on a bit of shiny, micrometer read out reloading gear but won't spend a bit of cash on an inexpensive chronograph.~Muir

 

[8x57]

Bruce Potts did two very good real world write ups in Shooting Times on progressively shortening both a .243w and a .270w in the last year or so. See if you can find copies of the articles as they both made intersting reading.
His conclusion on the .243w was that if you want to stay deer legal don't cut a barrel any shorter than 20". The .270 was less affected because of the greater mass of the bullet but I believe that once again he recommended not to go any shorter than a minimum of 20".

Unfortunately I loaned my copies of the articles to someone at the rifle club and I haven't had them back. They obviously found the articles to be just as informative and well researched as I did.

 

[Greener Jim]

What happens when you shorten a barrel? - Shooting UK

Found it this one is only for .270 mins. They must've combined two for the magazine and I can't find the other.

 

[JabaliHunter]

Some more here

.300 Win Mag cut in 1" increments from 24 1/4" to 16 1/4" 300 Winchester Magnum: How Does Barrel Length Change Velocity- A 16 300 Win Mag? | Rifleshooter.com

Another older one on the .300 Win Mag and .308 SWAT Article BARREL LENGTH

.308 from 26" to 13.5" with multiple bullet weights The Truth About Barrel Length, Muzzle Velocity and Accuracy - The Truth About Guns

.223 from 26" to 16.5" 223 Remington/5.56 NATO, velocity versus barrel length: A man, his chop box and his friends rifle | Rifleshooter.com

and on for the 7.62x54R http://honors.usf.edu/documents/thesis/u82488180.pdf

I'm sure there are more, if you have links, why not post them here?

 

[Heym SR20]

It used to be that 24" was pretty much the standard barrel length, but with 26" being used on high velocity / magnum type cartridges. With these lengths of barrels most of the powder is burnt, you don't get a lot of muzzle flash and quite a bit of te noise is taken care of. Take it down to 20" a lot of powder is not burnt, you get a lot of muzzle flash and a lot of noise and probably need a moderator - given the fad for mods, that's why many UK sold rifles have 20" barrels, and because many have 20" they are very noisy so need a mod - a self perpertuating circle which all equals more pennies for the dealers.

And with a mod (given other threads) you have the dded exitment or having a muzzle launched projectile going down range!

 

[Southern]

The fall off in velocity with the shortening of the barrels is not linear, and at some point it really starts to fall off. As already stated, and demonstrated in tests, the cases with a lot of powder in relation to bore diameter, like 7mm Rem Mag, .270s, .25-06, lose more FPS than the smaller cases, like 7x57, .308, and 7mm-08.

I have found some loads with really lose velocity in my 7x64 Mauser fullstock carbine, and others which lose little. Same for my .270 Mannlicher. I have found some loads - not hot-rods, just normal - which still get to over 3,000 fps in both of them. The slower powders which work well in the 24-inch barrels work best in my carbines. A friend with a 7mm RM carbine for hill hill rambling elk rifle has found the same thing with 140-grainers at 3,100 FPS.

I have some 7mm-08s with 20-inch barrels, some hunting .308s with 20-inch barrels, and a .270 Win Steyr Mannlicher carbine. The .270 is noticeably louder than the 7mm-08 and .308. Same for a Mannlicher MCA I had in .30-06: a flamethrower. My Mannlicher in 6.5x54 was not loud at all.

And longer barrels really make the rifle quieter and move the muzzle further from your ears, too. My 8x57s and 8x60 with 23.5 and 24 inch barrels just make a nice boom. So do the .303 Enflields, with their 25-inch barrels, and my 27-inch 7x57R combination gun. A 28-inch Browning 1885 in .30-06 is a pleasure to shoot, both in lower blast and in less recoil and muzzle flip.

 

[JabaliHunter]

.308 from 28" to 16.5" 308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28) | Rifleshooter.com

 

[Greener Jim]

My 32" barreled 45-120 is relatively quiet considering the amount of powder burnt. Still need ear duffs but it's no louder than an unmoderated 24" .308. Seems less spikey if that makes sense.
As Southern said, just a nice boom.

 

[hybridfiat]

Unless the barrel is an un-turned blank or a heavy varmint profile how much weight does that 2" add? Not much, certainly not enough to make up for the loss of velocity and the increase in muzzle blast and noise.
A far better option would be to go with a light sporting or even ultralight profile barrel and keep those inches.
My experience of 'drinking straw' barrels is that they shoot straight for long enough to get what you are shooting at. Just not so good on a range after 20 rounds in succession.
If weight is a problem, resist the temptation to put a massive over powered 'Tactical' scope on it.

 

[JTO]

No doubt, a hotter powder would give the same MV in a shorter barrel, but would the shorter barrel last as long before being 'burn out'?

 

[tackb]

I've run a 20'' 308 for years , never felt the need for those extra 50-100 fps and nor have the piles of deer I have shot !