Powder speed vs barrel length--RCModel?

[jr_roosa]

Here's a question, and I think I remember a long time ago seeing an interesting comment from RC regarding speed of powder versus barrel length, but I can't find it. It was something like, "slower powders will allow greater velocities than faster powders regardless of barrel length" with the idea that slower powders just do even better with longer barrels. It seems a little counter-intuitive and I feel like I'm mis-remembering it.

I'm re-working loads for my model 19 with a 4" barrel. I have a pound of VihtaVuori N110 that I've been using and I get some odd results with Hornady 158gr HP XTPs.

Hornady manual lists a max load of 13.1gr with a velocity of 1200fps. I get the following:

11.5 934.8
12 987.7
12.5 1102
13 1061

I suppose that there is a small chance that I swapped the 12.5s for the 13s, but could I actually be getting a fall-off? I guess statistics could have buttted in and with a 5 shot sample I could be at the high end of one bell curve and the low end of the other. SDs were 18 and 26 for the 2 highest loads. The Vihta Vuori manual lists a 15.9 gr/1569fps max load in a 7" test barrel, and I'm nowhere near anything like that!

First question, there isn't an internal ballistic reason for the velocity to drop off in this range of loads?

Second question, how much velocity should I expect to lose with a 4" barrel versus a 6" barrel (the Hornady test barrel), ballpark? Is 100fps about right?

Third question, and this goes to the quote I'm trying to remember from RC, would I be likely to get better performance with a little faster powder (since I have a shorter barrel), or with a little slower powder?

Thanks!

-J.

 

[rcmodel]

Probably what I said was, whatever powder gives the highest velocity in a long barrel will also give the highest velocity in a short barrel..

Your 12.5 - 13.0 load results looks to me like you did in fact mix them up.

Lyman #49 shows 14.0 giving 1,139 @ 38,200 CUP in a 4" universal receiver barrel.

You would expect less in an actaul revolver with a barrel / cylinder gap.

Lyman shows MAX loads of:
Enforcer getting 1,323 @ 41,700.
2400 getting 1,279 @ 41,800.
AA #9 getting 1,357 @ 42,900.
H-110 getting 1,309 @ 38,400.

Enforcer, 2400, and AA #9 fall just slightly faster then N-110 on the burn rate chart.

H-110 is just slightly slower.

rc

 

[ranger335v]

Probably what I said was, whatever powder gives the highest velocity in a long barrel will also give the highest velocity in a short barrel..

Which is true, no matter the intuitive. Peak pressure occurs within 2"-4" of bullet travel, longer barrels simply allow the pressure to be applied for a longer time.

 

[jr_roosa]

That was it! It is more intuitive than what I was remembering.

Sounds like my 1100 fps loads are what I should stick with for the 357. Doesn't sound like I will get any better performance without a longer barrel or a bigger frame (where I would be less hesitant to push the envelope).

Thanks!

J.

 

[1SOW]

http://www.ballisticsbytheinch.com/

I found this interesting and somewhat surprising. Extensive tests with a very wide range of bbl length, by caliber and cartridge used.

 

[wanderinwalker]

jr_roosa,

I'd say if you're satisfied with the accuracy and performance, stay right where you are. I find it interesting that 1100-1150 is about what I get out of my 4" .357 revolvers with 158gr bullets, though my 19 runs a bit faster than my 586 (50-fps or so). Though I can report the 125gr UMC JSPs are advertised at 1450-fps run just about that in my 586.

Another common one I've found is getting unburned powder from shorter barrel lengths. As in "XX of 2400 gives me unburned powder flakes from my 4" gun, I think I need more barrel length." This is also unrelated to barrel length and purely suggests the load is slightly under the powder's sweet spot.

 

[helotaxi]

You will usually get a little more powder burned in the longer barrel however that isn't the source of the velocity increase from the longer barrel. The greater period of time where the gas is expanding and accelerating the bullet is the source of the extra speed.

The one exception that I've seen WRT to differences in powder between longer and shorter barrels is with the .357. With the excess capacit nature of that round, Lil Gun does better in the longer barrel while H110 is better in a shorter barrel. Just the nature of the pressure curves of the two powders.