Calculating muzzle velocity?

[dunkel]

Ok, pardon me if this is a dumb question...my reloading setup is quite basic and I've been loading very safe loads, but am curious about moving up a bit. So...

Right now I'm mostly loading .38 SPL...158gr cast lead on top of 6.7gr of 2400. Still working on getting a chronograph and all that, but so far, these rounds have been very adequate for what I'm doing...just plinking at cans and stuff. Recoil is almost non-existent out of either my Blackhawk or Rossi 92. The wife likes to shoot it, too. Accurate enough to hit cans and a steel plate at around 75 yards, but I haven't actually grouped it yet.

Anyway, the question is...is there a rough formula for calculating muzzle velocity based on published load data? For instance, I found this:

http://www.handloads.com/loaddata/d...&Weight=158&type=Handgun&Order=Powder&Source=

158gr LSCW on top of 7.5gr of 2400 produces 990fps. Great, but what about my 6.7? Is it a simple calculation where you cross multiply and come up with about 844fps? Obviously, the best thing is to get a chronograph, but as I said, I have not picked one up with.

The biggest concern that I have is that I'd like to load some .357 and I'm concerned with pushing the lead bullets too fast and getting leading. My manual says starting load of 13.9gr of 2400 pushes it to 1487fps...I think that's probably too fast. Would like to try 9.4gr, which using simple math comes to about 1006fps...much better, I think.

Anyway, just trying to work all this out. I appreciate any tips anyone might have.

 

[kevinosborne]

I ran circles with this a few years back and realized that it's much less aggravation to buy (or even better borrow) a chronagraph If you are trying to achieve a specific velocity (say for SASS). I've looked a bunch of places and talked to a lot of people and the ones that I trust all said the same thing. There is no hard and fast rule where by you can extrapolate velocities. My theory is that it's because the pressure curve changes so radically with slight changes of really fast powders.

One place to look may be to get in touch with someone at Lee Precision. In the Second Edition of Modern Reloading Richard Lee published some interesting stuff about reduced loads for cast bullets in rifles and had numbers to use to calculate velocities and pressures for reduced loads for a given powder, but only for rifle powders. Most were on the order of .95xx to .98xx per grain reduced.
Good luck, and if you find some info on this, please post it...I'm interested.

 

[loadnplenty]

Kevinosbourne, prety much hit it on the nose. It is hard to tell ,some other things to consider is some guns just shoot a higher velocity than others, for what ever reason. Age of powder, and the tempature for which it has been stored in also contribute.

 

[dunkel]

That's what I was afraid of.

I do have the Lee reloading manual and it's been extremely informative. I've only scratched the surface of all the information in there. Going to just keep studying it and see what I can find out.

Yes, I'm just going to need to break down and get a chronograph. If I was reloading commercial jacketed bullets of some sort, I wouldn't be as concerned about it, but with the cast, I really don't want to overdo it. In the meantime, I reckon I'll just keep loading plinking ammo with .38 brass and save the .357 for when I get a more sophisticated setup.

Thanks, guys.

 

[RET E-7]

Get you a inexpensive Shooting Chrony. It pays for itself!

Tom

 

[Ga_Nemo]

http://www.handloads.com/loaddata/d...&Weight=158&type=Handgun&Order=Powder&Source=

158gr LSCW on top of 7.5gr of 2400 produces 990fps.

Would like to try 9.4gr, which using simple math comes to about 1006fps...much better, I think.

Anyway, just trying to work all this out. I appreciate any tips anyone might have.

I would not be concerned about the difference. You will get 16fps difference in rounds loaded as identically as possible. Factory variations are easily that far apart.

 

[OldGunbunny]

That's what I was afraid of.

I do have the Lee reloading manual and it's been extremely informative. .

Oh, you haven't started. Projectile shape, weight, propellent type and amount, temperatures of weapon, propellent and projectile, as well as how tight the projo fits in the bore (a worn tube will give lower velocities).

The reloading manual can be used as a baseline, but as RET E-7 points out, you need a chronograph to measure your weapons. Then you can compare your readings against the book to derive your VE (velocity error).

Google FM 6-40 and read chapters 3 and 4. While this manual applies to field artillery, these two chapters will be the same for all projectile firing weapons.

 

[kevinosborne]

Dunkel...Glad to hear you're going to buy a chrony....Can I borrow it sometime?......lol

 

[kevinosborne]

Hey, on a serious note, shoot your .38 spl load with lead bullets till the barrel falls off and push them as fast as ya want, WATCHING OUT FOR ONLY escessive leading in the groves...which leads to excessive preassures quicker than any hot load you could reasonably dream up because of that old ally and nemisis, friction. If I had a nickel for every .38 that blew up in my face.....well...I could afford reconstructive surgery.....Then my avatar wouldn't have to wear a hat....This is where I say "But don't take my advice....."...but seriously, if you pm me I'll share some pet .38 loads from 125 to 180 with ya.

 

[Dr. Strangelove]

To the OP, no, loads don't work in a linearly progressive manner, as in .10gr of X powder equals 50 more fps. You really need to get a chronograph to see where your particular gun is, velocity wise.

Until you get a chronograph, you can call the powder manufacturer or the bullet manufacturer, tell them what you want and what you are firing from, and they will be be able to give you an approximate load. When working up loads for rifles when the powder I want to use isn't indicated in the manual, I've gotten some good advice from the powder companies, they all have 800 numbers, call them and ask for the technical department.