Barrel Length vs FPS and Pressure


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sltm1
May 7, 2009, 06:04 PM
You guy's have been great on answering my questions on 45 Colt reloading so here's one more. At 8g Unique w/ 255g bullet & 7"bbl, my fps is approximately 850, don't know what the pressure is. I just got a Rossi 92 w/ a 20 bbl, with the pressure or speed increase because of the extra containment length going down the bbl?

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mickeydim468
May 7, 2009, 06:15 PM
Theoretically, Yes to both the pressure and the velocity. I would look at the burn rate of the powder you are using for your 7" and try to choose a slower one since you are nearly tripling the barrel length. Start low and work up!

Good luck!

helg
May 7, 2009, 06:46 PM
Alliant manual states that 5.4 grains of Unique under heavy 260gr bullet give 760 fps and 19,400 psi.

Quickload program shows that 8 grains of Unique under 255gr bullet of .64" length (approximated from the weight), which is seated to standard 1.275" OAL, give muzzle speed 1147fps and max pressure 32,300psi. This is WAY MORE than max SAAMI of 21,100. See the attached screenshot.

The manual and the program show that the load is way overcharged and should never be used.

You can see on the graph at the bottom of the screenshot that longer barrel does increase muzzle speed, but not too much, around 5% from 5" to 7" barrel. Unique burns fast, and max pressure occurs in less than an inch of bullet travel through the barrel. Therefore, with the charge max pressure does not increase in longer barrels as long as the barrel (less cartridge) is longer than one inch.

rcmodel
May 7, 2009, 07:03 PM
You have a lot of wrong information there on Quick-Load.
And are wrong about a rifle not noticably increasing the speed.

Normal .45 Colt 255 grain OAL would be in the range of 1.575" - 1.600".

8.5 Unique / 255 will give about 850 FPS in a revolver.
About 1,100 FPS in a 16" carbine.

And the load is within SAAMI pressure guidelines.

rc

Oyeboten
May 7, 2009, 07:10 PM
This was an interesting casual test of fps and Barrel lengths ( even if not the same loadings as are being discussed ) -


http://www.ballisticsbytheinch.com/45colt.html

helg
May 7, 2009, 07:34 PM
You are right, the question was on 45Colt, and I put 45ACP, as the screenshot show.

Alliant manual states that 8 grains of Unique under 250gr bullet give 850 fps and 11,800 psi fo7 7.3"bbl.

QuickLoad shows 15,800psi/982fps for 260gr/.692" bullet (overcharge) and 13,300psi/993fps for 230gr/.636" bullet (attached). Both seated to 1.580" OAL

The program is not aware of revolvers, and treats every barrel like it is continuous one. I doubt that this is why the calculated numbers are higher than test data from the reloading manual.

sltm1
May 7, 2009, 07:37 PM
helg, I looked at the thumbnail chart.
You typed in 45 ACP, I'm using 45 Colt (or Long Colt).
rcmodel, thanks. That's pretty much along the lines I was expecting. With that speed, I think I'll keep all rounds at 8-8.5g....1,100fps in a carbine is plenty of knock down power within 125yds. Can't see well enough to feel reliable any farther than that with open sights anyhow!! That way I don't have to seperate my hand gun ammo from my long rifle ammo, which was the purpose of having matching calibers in the first place.

Squeaky Duck
May 7, 2009, 08:30 PM
Green Dot powder will give you higher velocity with no more pressure since it burns a lot faster. And if you shoot at an indoor range it burns very clean with a lot less muzzle flash and boom than Unique. I tried it in my 45 auto and I got it to go from 800fps to about 900fps and still kept the working pressure to about 20,000psi. Look in the Lee Precision reloading manual. There's a lot of good PROVEN loads using Green Dot for the 45 round. That's where I found loads for mine.

Also I found using Green Dot I use only 5.4 grains which shoots the same as 6 grains of Unique when using 230gr bullets in the 45 auto.

Steve C
May 7, 2009, 08:52 PM
I just got a Rossi 92 w/ a 20 bbl, with the pressure or speed increase because of the extra containment length going down the bbl?

You will get higher velocity from the longer carbine barrel but the pressure generated by the cartridge will not change.

Revolver loads produce less velocity than a fixed breach weapon of equal barrel length as gas leaks out of the system at the cylinder and barrel gap.

As long as the volume of the gass produced is sufficient to keep pushing (pressure) against the bullet being pushed down the barrel for the time its in the bore, the longer barrel produces higher velocity.

An example I've chrono'd is a .38 spl 158gr LSCW driven by 4.7gr of W231 leaves the muzzle of a 4" revolver at 879 fps and 1,101 fps from a 18" IMI Timberwolf carbine.

Oyeboten
May 7, 2009, 09:58 PM
Squeaky Duck,



Would you happen to know, is 'Blue Dot' good for say .38 S & W Special, or for .357 Magnum, when Bullets are 148-160 ish grn weight?


My re-loading Book is not arrived yet...and just wondering..!

Ol` Joe
May 7, 2009, 11:05 PM
Green Dot powder will give you higher velocity with no more pressure since it burns a lot faster. And if you shoot at an indoor range it burns very clean with a lot less muzzle flash and boom than Unique. I tried it in my 45 auto and I got it to go from 800fps to about 900fps and still kept the working pressure to about 20,000psi. Look in the Lee Precision reloading manual. There's a lot of good PROVEN loads using Green Dot for the 45 round. That's where I found loads for mine.

Also I found using Green Dot I use only 5.4 grains which shoots the same as 6 grains of Unique when using 230gr bullets in the 45 auto

To get equal velocity with Green dot over Unique the pressures will have to be higher with the faster burning powder (the green dot).
The velocity is the result of average pressure over time, not peak pressure. The faster powder will possibly give higher velocity with equal charge wgt, but the peak pressure will also be higher then a slower powder of the same charge wgt.

To put it in perspective, you may very well get 900 fps from a 5.4gr load of Green Dot where the same 5.4 gr load of Unique only goes 800 fps but, your pressure although still under 20K psi for the Green Dot load may only be 15K psi for the Unique load. Add a few gr to the unique load and you very possibly could now be getting 1000 fps and still be at a lower pressure then 20K psi of the 900 fps Green Dot load.

The reason for the louder blast and added flash from the unique load is the higher muzzle pressure. It is higher then the the Green Dot load at that point, but likely still is lower earlier in the barrel at its PEAK. Higher PEAK pressure too aides in the powder burn as higher pressure results in higher flame tempratures which equals less ash and smoke. The Green dot wins here due to the peak it reaches as you noticed.

sltm1
May 8, 2009, 12:17 AM
I gotta tell you all, I'm pretty knowledgeable in cap and ball, but it's a pure pleasure to have your combined wisdom shared with a neophite in a new field. THANKS A BUNCH!!

Walkalong
May 8, 2009, 12:21 AM
Would you happen to know, is 'Blue Dot' good for say .38 S & W Special, or for .357 Magnum, No good for .38 Spl. pressures are much too low. Decent for .357, but there are better options IMHO.

Jim Watson
May 8, 2009, 12:25 AM
I loaded low end .357s to make IPSC Major (a 158 at 1130 fps) with Blue Dot and it did ok but there are better powders for full charge loads.

arizona98tj
May 8, 2009, 01:39 AM
And if you shoot at an indoor range it burns very clean with a lot less muzzle flash and boom than Unique. I tried it in my 45 auto and I got it to go from 800fps to about 900fps and still kept the working pressure to about 20,000psi.
Just wondering what you were using to measure your working pressure?

Oyeboten
May 8, 2009, 02:02 AM
Thanks Walkalong and Jim Watson on the 'Blue Dot' .38/.357 question...

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