I'd like to hear anyone's theory as to why some barrels are faster than others, given that they are the same length. I'm especially interested in any responses from an internal ballistics point of view.

Given 2 barrels, both 24" long, and shooting the same load. If one is consistently 100fps faster, does that necessarily mean that it must be operating at higher pressures?

I would have to guess that it's a difference in a couple tenths of a thousandths from one barrel to the next, either in the bore, chamber, length of throat, or even finish smoothness of the bore. I believe it could be some any combination of these to create pressure variations, hence, different velocities with identical ammunition.

Had a savvy old-timer explain to me that long ago Winchester rifled 10 barrels using the same rifling cutter (excuse me for my lack of proper technical terms). With each use the cutter would get slightly smaller due to wear, hence cut a slightly tighter bore diameter. (Again, we are only talking about tenths of a thousandths.) By barrel #10, the bore would indeed be a couple "tenths" tighter. If that was the case,then it stands to reason that there would be a slight difference between barrel #1 and #10 ballistics.

The barrel builder guys on this forum can probably better answer this. The modern computer controlled equipment takes tool wear into account for much of this, but there probably are still enough variations between one barrel to the next to make a slight difference.

The same would apply to accuracy as well: one barrel being more accurate than the next when all other variables are the same.

Thats an excellent question which I have studyied for months. I picked up an Interarms in 375 H&H, when I chronoed some handloads through it I was getting up to 150 fps more than with my Win mod 70. I tried the 300 gr Hornady Heavy mags and I got 2805 fps and thats no typo. I don't think it is operating at any higher pressure than the mod 70. I have miced the web of cases fired from both and they are both within safe specs. I hope someone with better info than I have responds because I am dying to know why this is so.
Sooner

[This message has been edited by Sooner (edited 12-12-2001).]

I'm glad to see some others wondered about this as well. I hope that some of the barrel experts can shed some light. Perhaps I'll post this as well over on the gunsmithing page.

I've ask several barrel makers and custom 'smiths and the basic answer is "We don't know" and "If we knew we would make them all that way".

Personally, I strongly suspect that for whatever reason the faster barrel is because of increased peak pressure and higher velocity is simply the artifact of that.

It is entirely possible, however, that the barrel -- and we must include the chamber in any barrel discussions -- is somehow affecting the burn rate in some subtle way. This might increase the area under the curve without actually causing increased peak pressure.

All of this, of course, is pure speculation on my part because I haven't done any side-by-side comparisons with two rifles that are nominally identical. I have done quite a bit of side-by-side comparisons with rifles for the same cartridge, but they were different brands.

All internal dimensions should not have an equal impact on barrel speed. I would think that throat length would be one of the most critical, and I think it's one of the ones that seems to vary most (even over the life of the barrel).

Otherwise the fastest barrel would be the one with a bore diameter just small enough to seal the bullet to gas pressure and with just enough land height to grip the bullet well.

Also a very smooth barrel should be faster than a rough one.

Some of these factors could well result in higher speed with lower pressures. I don't think that the two are necessarily directly correlated.

I was talking to D'Arcy Echols about some cases the other day, and he mentioned this subject in reference to cut rifled barrels vs button rifled barrels on the Lott cartridge. He said there was a reliable 50 fps difference. The button rifled barrels would be faster.

He still tends to use more cut rifled barrels than button.

FWIW,

Don

Very intersting topic ;just a hint,I will go for smoothless of the bore,how they can"polish" the bore...

I think that the elapsed time a bullet has maximum or near maximum pressure acting on it, the area under the curve, is a major factor as to why some barrels are faster than others. That said, there are probably several factors that contribute to this. When the combination is right or optimum, the barrel is fast.
A bore that has slight taper from throat to muzzle, with the muzzle just under bullet diameter could be a major contributor to this. The overall dimensional uniformity of the lands and grooves and twist rate would figure in. As could a bore surface that is not to rough, but not to smooth. Just the right amount of friction to stabilize the bullet but let the pressure perform work as well. The way the chamber and throat are cut(max or min)...
The factors influenced by the metallurgy of an individual barrel could be important. The harmonics, the expansion ratio...
This could go on for an entire career and probably never have an absolute, conclusive answer that would make sense.

Lots of barrels will differ as much as 100 FPS and thats not uncommon....

It is basically very simple..the barrel that offers the most resistance for whatever reason will build up pressure and that increases velocity..

Cut rifling I suspect will hang on to the bullet. a tight chamber with the same load will have more pressure and velocity...A tight barrel will give you more velocity as will deep rifling or steeper twists....and on and on.

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Ray Atkinson

ray@atkinsonhunting.com
atkinsonhunting.com