Location: The Wild Turkey Capitol of the World......Missouri
Posts: 1,027
Barrel length and velocity
What is the loss in feet per second per each inch less of barrel length? If a loading manual claims 3500 fps with a 26" barrel, then what would be the approximate velocity say with a 22" barrel with the same load? I used to know this but seem to have forgotten what the formula is, so I figured it would be quicker to ask on here than to search around for the answer. BTW, I plan on getting a good chronograph sometime in the near future, I have my eye on this one by PACT...
I have a PACT digital scale so I think I may get this chronograph one of these days and that will answer all my questions on muzzle velocity.
mv^2 = 2as
Where mv is muzzle velocity, a is acceleration, and s is barrel length. Solve for acceleration using the loading manual data and then plug it back in using your barrel length.
Remember to convert your barrel length to feet. a=3500^2/(2*2.167) a=2826488ft/s^2
mv^2=2*2826488*2 mv=3362
But sloth, he does not know the acceleration, cause he does not know the initial velocity. I am not understanding how your equation gives this? Your building pressure in a barrel, and accelerating, and keep accelerating until your bullet leaves the barrel, then start decellerating.
His problem is not linear and has many varibles other than length. Like powder capacity, powder type, and I believe the bullet used, rifling (resistence or lack of to build pressure), etc.
From what i have seen, 25 to 30fps per an inch from 18" to 26", after or before that, it starts dropping off do to powder and pressure capacity of the caliber. Kinda like having a 50gal compressor vs. a 10 gal.
There was a good article in rifle mag that showed 44 mag tests cutting off the barrel and measuring velocity. What it showed is velocity loss was significant at when above 4", but below that it leveled off. I will try to find it.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.
The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.
These are approximations, of course, but can serve as a reasonable guideline. I'm sure that much of the loss (or gain) depends not only on the muzzle velocity range but also a great deal on the type of cartridge you're talking about. A large case volume magnum rifle shooting a very slow powder probably suffers much more velocity loss for each inch of barrel removed than a small capacity cartridge shooting med to fast burning powders with similar muzzle velocity (e.g. a .300 Win Mag shooting a 150gr bullet vs. a .223Rem shooting a 55gr bullet).
I'd advise that you invest in the chrony you've had your eye one. I own one and I wouldn't do load testing without it.
Location: The Wild Turkey Capitol of the World......Missouri
Posts: 1,027
RE: Barrel length and velocity
Driftrider, your charts were very helpful, they pretty well answered my question. In particular, I'm looking at loading a 90 gr. Speer TNT in a .270 Win. using Varget powder. A Hodgdon manual I have says 55 gr. Varget is max. for this load and the Speer manual says this also. The Hodgdon manual gives 3596 fps with the max. load out of a 26" barrrel. I will be shooting it out of a 22" barrel. The TNT bullet is not to be loaded at muzzle velocities exceeding 3500 fps. The way I figure it using your tables above, with the max. load of 55 grs. of Varget, I will be getting approximately 3476 fps., give or take in my 22" barrel. I will probably start out a couple grains below max. so I should be OK. I've got to get that PACT chrony, I feel handicapped anymore without one! Thanks for the help!
Initial velocity would be zero since the bullet is at rest. That's why I left the inital velocity out of the equation.
If you follow the chart that driftrider posted and the formula you'll see you get pretty close result. With what I entered there was a 138fps drop from the 26" barrel to the 22" barrel. Using the chart 3000-3500fps the velocity should drop 30fps per inch. 4*30=120fps.
Since they are averaging fps loss over a 500fps range I would say that the formula is more accurate.
Yours may be a touch more accurate for certain rounds, but using the chart has the benefit of simplicity. Besides, in both your formula and the charts that I posted give an educated guess at best. Every rifle is different in many ways, and every cartridge, powder and bullet combination is different as well.
In reality, as long as the estimate is +-100fps for a rifle shooting 2500fps or faster, the difference downrange is fairly small. A coyote or prairie dog will be just as dead after being hit in the boilerroom with a .270, whether the muzzle velocity is 3500fps or 3300fps. The downrange trajectory is also very similar until the range gets very long as well.
It is a nice little formula, I will admit. I just think that it's a bit more work that needs to be done to make what amounts to little more than an educated guess. The chart will get one into the ballpark, and are easy to remember and apply without sitting down with a pencil and paper and calculating average bullet acceleration, etc....
I agree, I just found it handy for myself as I do most of my hunting with a 243 that only has a 18" barrel. Pretty significant velocity change going from a 24" or 26" barrel that most reloading guides use.
I was just looking in the lyman reloading manual...they have most rifle loads listed for handguns too...with 16 inch barrels..that might give an idea of the difference in MV
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There are a couple of other things to consider also. such as bullet seat. Yep that makes a difference. if you have your bullet seated so it has to jump to the riflings you loose a little pressure build up. Think of it this way. If you squeese a tube of tooth past till the cap pops off you must have had a lot of pressure build up to do that and the cap lauches pretty far. If you don't screw the cap on and you squeese the tube it pops off with very little pressure and it doesn't go very far...........I know, I know that analogy sucks, but you get the idea. I've said all that cause I have different barrel length .308s and some times the longer barrels don't shoot faster. cause the chambers are different and the bullet seat needs to bet set out farther on some guns.
I personally like the 26" barrels. I have one in 30-06 and .300win mag. The slow burning powders have room to work .........plus they look cool.
Also did you know the recoil is less on the longer barrel?......I made that last part up.
Barrel length and velocity
