.300 Win Mag barrel velocities? Help please.

[shooter79]

Evening all

 

I'm trying to help a friend with his possible new purchase of a .300 Win Mag. It has a 20 inch barrel and i think it will be too short to get the full benifit from that cartridge. What will be the golden rule per inch trimmed off the barrel regarding velocity loss? Overall will a 20 inch barrel suffice out to longer ranges?

 

If you have any experience in this field please share your thoughts?

 

Regards

 

Carl.

[Danpd]

A 20" barrel isn't optimum for a 300 WM, looking at quick load you'll be approx 200fps down on standard 180gr load. So realistically all the rifle will be is a very loud powder hungry 30-06. at close range you won't notice much difference, but if longer range is on the cards then I'd be looking at a 26" barrel.

[MOOSE]

If you have at look at the Hornady website there is a clip on there about their superformance range of ammo, in it they discuss barrel length for a 300 win mag

[Laurie]

I'd expect an even greater loss than Danpd's 200 fps compared to a 26-inch barrel, substantially greater in fact. There is no single figure per inch of barrel difference as the amount gained / lost by different barrel lengths is a function of calibre + powder charge energy (in terms of gas volume / pressure). A relatively large calibre allied to a relatively small propellant charge as in revolver cartridges used in leverguns won't see a great deal of difference between a 16-inch barrel carbine and a 24-inch barrel rifle when shooting a light lead-bullet target load of say 6-10gn in .44 Magnum. Take a severely over-bore-capacity rifle magnum cartridge like the 7mm Shooting Times Westerner with maybe 80gn powder behind a 7mm bore and you get a big MV change per inch of barrel.

 

Then on top of that factor, it depends on the pressure appertaining at the point immediately behind the muzzle - and that depends on barrel length as the pressure behind the bullet isn't constant as it travels down the barrel. It peaks just ahead of the chamber and drops rapidly for a short period then continues to fall more slowly as the bullet travels further along the barrel. Let's say a particular cartridge / calibre combination sees 60,000 psi peak just ahead of the chamber, 30,000 psi when the bullet had travelled 10-inches from that point, maybe 15,000 psi when it's now 20-inches beyond the chamber and say a bit below 10,000 psi immediately before exiting the muzzle 24 inches down the tube. If you chop the barrel at the 20 inch mark you've lost four inches of barrel where there is an average gas pressure behind the bullet of ~12,000 psi. The velocity gain over that section is a function of average pressure times distance minus the friction caused by the bullet to barrel fit. As the latter is pretty well constant throughout he barrel it's we can partlly ignore it, although as the pressure is dropping the remaining amount of useful energy reduces too. So its 4 inches X the energy imparted by 12,000 psi gas pressure on the bullet base. Cut the barrel back another four inches and 4-inches X a rather higher average pressure, say 20,000 psi, so it's a greater velocity loss per inch of barrel - and so on, each successive loss of an inch giving a greater MV reduction as you move closer to the chamber.

 

Here's what QuickLOAD says for a max load of N165 behind a 180gn Sierra SBT

 

20-inch - 2,853 fps

 

22-inch - 2,927 fps

 

26-inch - 3,050 fps

 

30-inch - 3148 fps

 

20-26 inches is ~200 fps difference as reckoned by Danpd, but I'm wary of QuickLOAD on this issue thinking it's too linear. Note too that muzzle pressure rises to 14,881 psi in the 20 inch barrel compared to a bit over 10,000 in a 26-incher according to QuickLOAD. Anything much over 10,000 psi gets noisy and can also reduce accuracy as the high speed and volume of the departing gasses can affect bullet flight as they overtake it just ahead of the muzzle.

 

Many US sporting writers and gunsmiths have wtitten very disparagaing remarks about the 10-yearly or so recurring trend for the American firearms industry to sell 22-inch barrel magnum lightweight hunting rifles to gullible shooters. As Danpd says, the point is well made that a 22-inch barrel .300 Win Mag has similar if not less performance than a .30-06 with the same bullet but four extra inches of barrel. It's also VERY unpleasant to shoot - muzzle blast and in a lightweight package, recoil. Chop it down to 20-inches and you actually double that effect, so it's become an inefficient low velocity .30-06, or a very noisy and unpleasant to shoot rifle with not that much more performance than a longer barrel .308 Win.