Riddle me this! Shortened barrel, increased velocities & pressure.

358Rooster

358Rooster

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Strange thing happened with my .260; the load I had sussed out last fall with Hornady 140 BTHP and H4831SC that shot well enough for steel to 1050 yards (not BR accurate but cheap and easy) is seriously hot now. Looking back in my notes, the temperatures I shot in last fall were roughly the same as they were the other day, which was about +6C. The temp the other day may have gotten to +18C.

Velocities last fall averaged 2740 in the 27" Krieger with about a 60 fps spread:redface:. This winter, I had the barrel shortened to 21-3/4" and velocities went to 2983 and 2990 before blowing a primer out on the third one. This, with the very same load - same bullet from the same lot, same powder from the same lot and same charge, same seating depth, same primer, same Lapua cases. In fact, I loaded about 200 rounds immediately after finding that 'good enough' load last fall.

Anyone have any relatively intelligent explanation for this one? At any rate, they're all pulled now and I've started load testing in earnest, now that work isn't taking up so much precious shooting time.

Rooster
 
nope not really, there is 3 of us shooting 6 Dashers, mine is 21.5" ex benchrest gun, one shooting 28" and the third shooting 30" pipes, I with the shortest barrel am having to load 1.2-1.5 grns more powder to achieve the same velocities as they are using same twist, brass, bullets, powder and primers, staying in the under 20 deviations
 
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I can't think of anything obvious - the temperatures were not that far off and don't explain a gain of 200 fps. You would normally lose about 25 fps per inch of barrel cut, but barrels have been known to lose a lot less than that or even "speed up" after having been cut, but not that much. MV = pressure. Could the crown have been affected, or the bore in some way, to partially obstruct the bullet's exit? How was the barrel supported when cut? How was it cut and crowned?
 
I think this may have happened to me, this is the only theory I could think of.
This is only a tinfoil hat theory.

Some powders have a very short burn period and pressures may come down rather quickly.
In the long barrel, the pressure may have peaked before the bullet left the barrel.
While the bullet is travelling in the barrel, with less pressure behind it, it actually slows down.
This may not show signs of overpressure as the pressure rises and drops at a timely rate.

In the case of the shorter barrel, the pressure spiked at the same rate, but the bullet did not
provide the needed resistance to drop it and absorb some of the "shock".
You now have a case where the pressure rises quickly and drops even quicker.
The steel hammer VS rubber hammer theory.

In my case, a change of powder solved the problem.
 
That's an interesting theory, but it has problems. All powders, regardless of "Burn Rate", amount and barrel length, reach their peak pressure within a very short time (~0.5 milliseconds) which translates into just a few inches of bullet travel (typically 3-6"), after which the pressure curve drops very consistently in relation to the position of the bullet in the bore, then drops off dramatically when the bullet exits the bore. Even though the pressure is dropping, it's usually still high enough that the bullet is still accelerating - it takes a very long barrel for MV's to start to drop.

The chart below is "borrowed", but powder "A" could be Bullseye, "B" Varget, and "C" Retumbo. Why slower powders can produce higher MV's is that you can use more of them at the same peak pressure - the pressure curve is "flatter" (has more area under it) which means more energy. As you see, even the slowest powder reaches Peak Pressure well before the bullet exits the bore - you'd need a very short barrel (pistol length) for it to be otherwise.

612px-Grain-determined_pressure_vs_time.png
 
dand883 said:
This was my first thought too.
Click to expand...

It's a big world and anything's possible, but seriously? Who would remove a barrel, bugger up the chamber end in the process, and then have to reinstall it, all just to shorten the muzzle end?
 
Thanks Andy..........you blew my theory all to hell. Or maybe not.........
I cut a 30" extra heavy .308 barrel down to 26" and while using the EXACT same load/brass/primer/bullet
had an increase in velocity of about 100-150 FPS.
No signs of overpressure, but bolt was sticky (hard to open at times).
Changed powder from Varget to H4831 and everything fell back in line.

Now, correct me if I'm wrong, but I allways thought slower powders like H4831 should
work better in longer barrels and faster powders like Varget may work better in shorter barrels.
The theory behind longer barrels is to take advantage of the slower powders.
I realize there is more to powder burning rates than just time, other factors come into play,
like initial pressure (COL), case fill, enviromental, twist rate (resistance) etc.

Any ideas/theories/guesses Andy? Because I'm running out.
 
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Andy said:
It's a big world and anything's possible, but seriously? Who would remove a barrel, bugger up the chamber end in the process, and then have to reinstall it, all just to shorten the muzzle end?
Click to expand...

I was thinking maybe the whole gun doesn't fit in his lathe, so took barrel off, cut end, crowned it on lathe and reinstalled, and something was different after vs before putting the barrel back on.
 
Andy said:
It's a big world and anything's possible, but seriously? Who would remove a barrel, bugger up the chamber end in the process, and then have to reinstall it, all just to shorten the muzzle end?
Click to expand...

it isn't so much with buggering up the barrel but one of wear ..... you will see many target guys do this , they start with a long barrel and once the groups start to open up they knock a few inches off the muzzle end , then cut the chamber off , and rethread and rechamber .
from what I've been told , if this is done properly the barrel will return to being just as accurate.
 
burnt_servo said:
it isn't so much with buggering up the barrel but one of wear ..... you will see many target guys do this , they start with a long barrel and once the groups start to open up they knock a few inches off the muzzle end , then cut the chamber off , and rethread and rechamber .
from what I've been told , if this is done properly the barrel will return to being just as accurate.
Click to expand...

Anything's possible, but the OP would have mentionned that I would hope. He really needs to answer a couple of questions before we speculate any more.
 
cet said:
Thanks Andy..........you blew my theory all to hell. Or maybe not.........
I cut a 30" extra heavy .308 barrel down to 26" and while using the EXACT same load/brass/primer/bullet
had an increase in velocity of about 100-150 FPS.
No signs of overpressure, but bolt was sticky (hard to open at times).
Changed powder from Varget to H4831 and everything fell back in line.

Now, correct me if I'm wrong, but I allways thought slower powders like H4831 should
work better in longer barrels and faster powders like Varget may work better in shorter barrels.
The theory behind longer barrels is to take advantage of the slower powders.
I realize there is more to powder burning rates than just time, other factors come into play,
like pressure, case fill, enviromental, twist rate (resistance) etc.

Any ideas/theories/guesses Andy? Because I'm running out.
Click to expand...

MANY will disagree, but in barrels longer than 6", the powder that produces the highest MV in a long barrel, will produce the highest MV in a short barrel. Tests and theory confirm this.
 
cet said:
Thanks Andy..........you blew my theory all to hell. Or maybe not.........
I cut a 30" extra heavy .308 barrel down to 26" and while using the EXACT same load/brass/primer/bullet
had an increase in velocity of about 100-150 FPS.
No signs of overpressure, but bolt was sticky (hard to open at times).
Changed powder from Varget to H4831 and everything fell back in line.

Now, correct me if I'm wrong, but I allways thought slower powders like H4831 should
work better in longer barrels and faster powders like Varget may work better in shorter barrels.
The theory behind longer barrels is to take advantage of the slower powders.
I realize there is more to powder burning rates than just time, other factors come into play,
like initial pressure (COL), case fill, enviromental, twist rate (resistance) etc.

Any ideas/theories/guesses Andy? Because I'm running out.
Click to expand...

I've got nothing to explain the velocity increase, but powders that produce the highest velocity in a longer barrel will also produce the highest velocity in a shorter barrel - just less velocity.

This is a pretty odd circumstance. Is there any way the velocity measurements themselves could be the real issue? Different chronographs? Different lighting? Different distance from barrel?
 
Just a wild guess, could the powder in your cartridge have settled differently, resulting in a different rate of ignition?
 
I'd be more inclined to think that the velocity increase was instrumental error. I have had wide variances in velocity readings from a chronograph depending on light conditions. Bright sunlight from the front/back is the worst. Your load was probably "over max" before, the symptoms were just not as obvious.
 
If the ammo that shot well is the same batch of ammo (i.e. it was all loaded at the same time) that is now too hot, then the barrel has changed. A new chamber/throat cut with a different reamer would do it.

If the ammo was loaded in a different loading session, later, the powder has been contaminated with some hotter powder.
 
WOW.............have I got some learning to do, never too old to learn I guess.

Thanks for all the input folks. I'll be googling for a while.

TGIF................have a good weekend!!
 
I'd guess different lot of powder is a little faster, lots of copper fouling or powder fouling in the throat, a tight spot in the bbl from a bbl vice if the bbl was removed, or welding/gluing of the bullet to the brass over time. That's my guess. Also check your case length may be to long and jamming into the throat
 
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