Velocity variation vs. barrel length

gunnut505

Velocity variation with barrel length.
This information was taken from Speer's Number 7 reloading Manual circa 1967, and involved S&W 357Mag revolvers using various popular loads.
DO NOT USE THIS DATA FOR RELOADING!

3.5" 5" 6.5" 8 3/8"
3.5gr Bullseye, 148WC, 779 826 845 868
4.0gr 5066 148WC 770 796 834 854
16.0gr 2400 146 swchp 1231 1411 1436 1478
8.0gr Unique 146 swchp 1170 1253 1276 1327
14.0gr 2400 160swc 1084 1198 1248 1301
7.0gr Unique 160swc 995 1093 1138 1192

As an aside; did anyone else notice how much more energetic 2400 is nowadays? 16gr behind a 146gr wadcutter would damage the gun as it lined the bore with lead.

sandwarrior

gunnut505,

On the flip side of this it seems to me that IMR rifle powders have definitely toned down from the past.

As for velocity changes in rifles it still seems to be between 25-50 fps per inch of barrel gained or removed.

Tailgunner1954

SW
Just some more data points:
The actual (rifle) chamber has as much or more to do with velocity variations between rifles than the barrel length does.
I went from a 26" barrel (chamber A) to a 25" barrel (chamber .
Chamber A couldn't get to 2750fps with any of 3 powders, even with a 2gr overcharge.
Chamber B hit 3000fps at the same powder charge the book did.
Same receiver, same chambering (30-06), same Shilen barrel. The only difference was the actual chamber/throat itself (and it cost me 1" off the breach end to fix). IIRC The cast of chamber A showed it to be .004 over bullet diameter, while the cast of chamber B is .0015 over bullet diameter

I recently got involved with a strain gauged barrel project (friends Capstone project), mine (B) above and a 20" Handy rifle. The HR had a throat that was .003 over bullet diameter and it was 250fps slower than my 25" barrel with the same factory ammo (150gr Federal, 3000fps from mine, 2750 from the HR).
Check Dan Lilja's site for his velocity/barrel length study, also

sandwarrior

Tailgunner1954,

I'll agree with you there that a loose chamber will not launch a bullet as fast as a tight chamber will. However, along that same axiom, if you set that same barrel back a couple times (inches each time) with the same reamer used originally to chamber it, your going to find a little bit of velocity lost.

Tailgunner1954

Oh, I agree that BOTH have an effect, but IME the amount of throat slop plays a bigger role than the barrel length, and is the biggest "bug-a-boo" when saying that a X length barrel will be Yfps faster/slower than a barrel of X +/- 3". Also the actual chamber dimentions/amount of slop will also account for the mythical "fast barrel", which in reality simply is one that has the minimum throating dimentions.

A interesting experament might be to take a Handy rifle in 308Win and re-cut the chamber with a match spec chamber and NO throat IE with a reamer that requires a seperate reamer (don't worry about the shoulder bulge point, as we'll be scrapping the barrel when we're done). Fire a series of test loads (we'll need to watch the bullet seating depth). Next run in a standard 30cal throating reamer and repeat the firing test. Finaly run in a 7.7 / 303 Brit throater and repeat the firing tests. IF at this point you want to start cutting the barrel back (staying above legal length of course[:D] ) and moniter the change from that also

Rifleman34

Hey Guy's
Just join your forum and your interest in the topic i like and it sounds like you know what your doing but my Question is what about a casing that is fire formed to the chamber.Tested ten years ago on calibers,243,270,30-06,308 and 7mm Magnum factory verses reloads.This test included heavy and standard barrels lengths 20" to 26",muzzle velocity and down range performance out 600 yards and everything else that complies.
To make a very long story short the best load for your barrel is the load you make from a fired casing with only the neck re-sized = consistent pressure =steady velocity rate= better accuracy.
NO FACTORY BULLET CAN DO THIS ONLY THE RELOADER!!!!!

He Dog

For what it is worth, Jon Sundra has done some range tests on barrels using the same ammo and hacksawing off an inch of barrel at at time starting with a 24" barrel. In a couple of trials (I don't recall the calibers used) he found that each inch lost equated to about 50fps lost velocity.

Tailgunner1954

HeDog
The larger the case capacity relitive to bore size the greater the loss/inch. It all comes down to the "expansion ratio" the powder has to work with. A 22LR peaks out at about 18", while IIRC the Hornet peaks out at about 24".
In a 30cal for example, the 30-30 won't suffer as much loss/inch as a 300RUM when going from 24" to 18". However IF you went the other way, say from 24" up to 30", the 30-30 would show little to no improvement while the RUM would really come to life. IOW the barrel can be to short OR to long for the cartridge in question.
You can see the same thing in the other calibers (easiest way is to look at the Contender/Encore "handgun" loadings and compare those to the same chamberings in a rifle length barrel.

The Lilja article I mentioned in a earlier post started with IIRC a 42" barrel, and (again IIRC) a couple of the loads he tried gained speed when cut back to 40", and from there to about 34" were only losing about 5-6fps/inch. Once he got down into "carbine" length barrels, than the loss/inch went way up.

gunnut505

I just figured that the topic comes up about once a month now, and shared some of my vast library of esoteric gunarabiliae.
I'm trying to find Hatchers' study with a long-barreled '03, and the one with the 8' pistol barrel.
There are so many facets to this lifestyle, that some questions have been asked and answered thousands of times; some correctly. I just assumed everyone knew it was the internet.

Chuck S

Each bullet make and model travels down the bore with x amount of friction. That's why most reload manuals show the load for a select bullet, style and weight. For instance a Gameking 165 gn, etc. gives one set of data while a matchking the same weight will have slight changes in the loads. Each powder also has a different burn rate and reaches a peak at a different time from ignition. The burn peak largely determines peak pressure but as long as there is pressure enough to overcome bore friction, the bullet will keep accelerating. This occurs even when the powder stops burning in longer barrels. Hence the powder burns quickly but until the pressure is reduced by the rapidly enlarging space as the bullet travels down the barrel, the bullet will always continue to accelerate.[:)]

It would seem then that the longer barrel equates to more velocity but this isn't always the case. Why you ask?[?] At some point, with longer barrels, the bullet, powder, chamber and barrel combo comes into equilibrium during this cycle and the pressure is no longer high enough to overcome bullet friction in the bore. Following that point the extra length of bore to travel, will slow the bullet down until it is launched. A generalization that says a bullet's velocity is reduced by x amount for each inch of barrel lost ignores physics and reality.

A good example is the lowly 22 lr. The sweet spot for that cartridge is somewhere close to 18 inches. Add barrel length and you lose velocity for each inch added. Cut the barrel shorter and you lose velocity for each inch cut off.[8D]

bpost

Odd, I still use at least one of those loads..... It still works for me.[;)]

Rocky Raab

The one thing not touched on here so far is bore diameter. VERY small variations in bore diameter have a quite measurable effect.

According to John Barseness, a change in bore diameter as small as .0005" will change the pressure of a load by about 1000 psi, which in turn can change muzzle velocity by 100 fps. Smaller bores such as lab pressure barrels always develop higher pressure and velocity. That's good and bad, because even though we probably can't achieve the velocity the lab can with a given load, we'll also be operating at a lower and safer pressure.

How much do factory bores actually vary? As much as .002" larger than lab barrel standards. Yes, that's FOUR TIMES the number above. Meaning you might get 400 fps less than "book" numbers with a given load.

Given those parameters, it is probably pointless to ask "How much slower will my barrel be if it's two inches shorter?" Because length is by far not the largest factor.