Relationship between Barrel Length, Burn rate, and velocity

[Master Blaster]

Hi ALL,

I was wondering if or how any of you have worked the relationship between the barrel length, burn rate and velocity into your reloading. It seems to me that in order to get a higher velocity one would normally use a slower burning powder as a rule of thumb. But it seems that this relationship is also dependant on the barrel length of the firearm and in some situations a faster burning powder will give you a higher velocity in a shorter barreled firearm than a slow burning powder. This seems to me to be most evident when I am reloading for revolvers and I compare my 6 or 7.5 inch barreled revolvers to my 2" barreled revolvers. The goal is to get a complete burn inside the gun and produce the maximum velocity possible for that short barreled gun. I think a large fireball outside the barrel is indicitive of a less efficient load.

Right now I am trying to get better velocity form those short barrreled revolvers. While a load of H110 or 2400 will produce the highest velocity in my 6" .357 mag, Unique might produce higher velocity in my 2" barreled .357, because more if it burns inside the gun as evidenced by the huge fireball H110 produces versus the much smaller fireball produced by Unique.

Has anyone done any experimentation with this idea????

 

[Zak Smith]

But it seems that this relationship is also dependant on the barrel length of the firearm and in some situations a faster burning powder will give you a higher velocity in a shorter barreled firearm than a slow burning powder.

The best way to get a feeling for this is to buy a copy of QuickLoad from NECO and play with it while varying the parameters. It does internal ballistic modelling and can produce pressure/time/distance curves.

A big load of H110 in a 2" barrel won't do much good because over half the powder isn't being burned inside the barrel (ie, in a very long barrel, over half if the powder would burn between 2" and the muzzle).

-z

 

[P95Carry]

Permutations are near infinite but as a general rule of thumb, short tubes will do better with a (safe) load of fast powder. The longer the barrel then the slower burn rate powders come into their own, there being significantly more time for pressures to rise and near full burn to take place.

The slow powders will generally permit the much higher velocities safely because the pressure spike is lower for a given max average pressure - the fast powders like Bullseye burn so fast that dangerous pressures can be easily reached if pushing for too much velocity.

Some of this can be a ''juggling act''!

 

[Zak Smith]

Also note this type of effect is most noticable in handgun barrels between 0 and 10". Beyond that, or in rifle barrel lengths, it does not apply since most of the area of the time/pressure curve is already developed. In other words-- don't generalize to rifles.

 

[fecmech]

Guys--This ground has been plowed many times before. Whatever powder gives you the highest velocity out of a 6" barrel will give you the highest velocity out of a 2" bbl. In magnum pistols that is with slow powders. If you don't believe that, load some ammo, get a chrono and go at it. Yes you will get more blast, yes you will get more flash, but you will get more speed! Nick

 

[Zak Smith]

fecmech,

That is true for almost all rifle loads (ie, long barrels), but not necessarily true for very short barrels (e.g. 0 - 6")--

According to QuickLoad:

357MAG, 125GD, near max with H110, 37800 psi max, 6", 1698fps, 76.8% burned
same but 2" bbl: same max psi, 877fps, 43% burned

357MAG, 125GD, near max with BULLSEYE, 38400psi max, 6", 1563fps, 100% burned
same but 2" bbl: same max psi, 911fps, 94% burned

 

[P95Carry]

Nick - I think I'd qualify things by adding ''efficiency'', and I'm not trying to be pedantic! Sure a hefty load of slow thru a short barrel can still get velocity but - somewhat wastefully.

 

[The Bushmaster]

Master Blaster...Load up some rounds using W-231. I don't care what bullet you use. 125 to 158. If you are using jacketed bullets in 125 grain. Try 8 grains of W-231 (Please work up to this load slowly). It will give you around 1130 fps from a 2 1/2 inch barreled .357 mag. And around 1100 fps using 7.7 grains of W-231 under a 140 grain JHP. If you are shooting a 2 inch barreled .38 Special try 5.3 grains of W-231 for a rousing 750 fps with a 125 grain JHP (This is also a top of the scale or max load and should be approached carefully). Or a 140 grain JHP over 4.5 grains of W-231 for 670 fps. If you are shooting lead bullets I would still look into W-231. I don't load lead bullets, but W-231 just can't be beat for high velocity and clean burn and minamal muzzle flash. And yes it has a fast burn rate. Give it a try. You will also find that it works well in many of your revolvers and autos. Check out Winchester's Components Catalog for load data.

Yes Fecmech...We have covered this subject many times before and we will cover it many more times before they put my ashes over the side at sea.

 

[fecmech]

Bushmaster--Masterblasters original post was somewhat contradictory.

"The goal is to get a complete burn inside the gun and produce the maximum velocity possible for that short barreled gun. I think a large fireball outside the barrel is indicitive of a less efficient load."

I am not arguing efficiancy, muzzel flash or blast, simply velocity. If you pick any published max safe load with the slower powders in magnum handguns you will have a higher velocity out of a 2"bbl than you will have with a max safe load of bullseye or 231 or any other of the faster numbers. It will not be "efficient", it will have a big fireball, and it will be damn loud, BUT it will be faster!

Zak--I know very little about Quickload, But I would be willing to bet a lot of money that any load that does 1700fps out of a 6" bbl is not going to drop to 900fps out of a 2'bbl. Real world data that I've shot in the past is 125 grainers run in the 1500fps range with 296 out of a 6" and do mid 1200fps range out of 2.5" combat Mags. I'm also suspicious of the Quickload putting the 2 powders so close out of the 6". Most all of the published data I've ever seen shows a 250-350fps velocity advantage for the slow burners out of 6" tubes.
But to repeat if you don't believe, chronographs are only about $80. go see for yourself. Nick

 

[Zak Smith]

fecmech,

I've compared my rifle loads vs. QuickLoad and they're generally real close.. ie, within 10-40fps for a 2800fps load. I have not done so for pistol loads 'cause I mainly just do "IPSC" type loads for handguns.

best
Zak

 

[fecmech]

Zak--Almost all my chrono data is packed away as I'm in the process of trying to sell my home. However I just found some margin notes in my cast bullet handbook that may interest you. I have 2 loads, one with 2400 and another with 296 that were chronod out of a 6" K-38 and a 1 7/8" Model 36 at the same time. Both loads are with a 168 gr. Cast swc Lyman "Keith" bullet, I am not recomending these loads( they are serious +P .38 loads) but is is real world data that I personally shot.
#1 10.5/2400--1013 fps in 6" and 804 fps out of the 1 7/8"bbl.( 11fps sd.)
#2 11.3/296---1064 fps in the 6" and 868fps out of the 1 7/8" bbl.(15 fps sd.)

Chrono data was for 12 rds out of the 6" and 10 rds out of the 1 7/8"
This should give you some idea what of the speed relationship between 6 and 2" bbls with slow powders. Nick

 

[The Bushmaster]

Fecmech...I have played with my 2 1/2 inch combat mag using several powders and of the 4 or 5 powders W-231 ended up working the best in my Mod 19. In the process I discovered that it worked very well in my three .38 specials. One being a 2 inch and the others 4 inch barrels. If you are having better luck with slow burning powders...Outstanding, but I didn't. Besides I'm (if you hadn't noticed) partial to W-231. It has served me well in 9mmX19, .38 special, Short barreled .357 mags and Colt .45ACP.

EeeHaaaww. This is a good site. I will stay.

 

[rbernie]

But it seems that this relationship is also dependant on the barrel length of the firearm and in some situations a faster burning powder will give you a higher velocity in a shorter barreled firearm than a slow burning powder.

Where this gets really fun is when you factor in the overbore of the chambering, the length of the leade, and the 'fit' of the bullet to the barrel. It's been my experience that the more overbore the cartridge, the more it wants slower powders notwithstanding the actual barrel length. On the other hand, the less overbore the chambering the more it wants a fast powder and the less it'll respond to slower powders regardless of how long you make the barrel. Also, the more freebore in the throat or the more wear to the barrel itself, the more it seems to want faster powders than would be expected.