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Doesn't more powder mean more velocity??
lksmith03
Member Posts: 1,742 ✭✭✭✭✭
I am trying to work up a load for my 50BMG
using WC872, and 750gr Barnes Tac-lr bullets which are a solid brass lathe turned projo.
The thing that has me perplexed is that my loads and velocity averages are below:
200grs 2541fps
205grs 2397fps
210grs 2296fps
Anybody have any idea what the heck is going on?
Same Bullet type, same gun, same can of powder, same conditions, same headstamp brass. only thing that is different is the amount of powder.
using WC872, and 750gr Barnes Tac-lr bullets which are a solid brass lathe turned projo.
The thing that has me perplexed is that my loads and velocity averages are below:
200grs 2541fps
205grs 2397fps
210grs 2296fps
Anybody have any idea what the heck is going on?
Same Bullet type, same gun, same can of powder, same conditions, same headstamp brass. only thing that is different is the amount of powder.
Comments
The other thing you find is with a retardant on the powder, which I have no idea about WC872, may cause reduced velocities if you get to an overpressure situation with the round. That's intended so rounds continuously coming in don't get hotter and hotter. Until they cause a MAJOR malfunction.
The round might supposed to be always getting to 2700 fps, but no one's going to chrono it and calculate trajectory. They just follow the tracers until it's on target.
So, my advice is to check and see what another powder does. Or, two or three. That way you can see if something isn't right with your rifle or the powder you're using to reload.
What barrel lenght do you have?
Go to www.riflebarrels.com and look at the article on 50bmg barrel lenghts.
All bullets obturate in the bore. That means: under intense acceleration, they are compressed from the rear. As a result, they become slightly shorter and fatter. The softer the bullet, obviously, the more the effect. Lead bullets obturate a lot, jacket and core bullets somewhat less and monolithic bullets the least of all. The heavier the bullet, the more the effect, in all cases, because it is the bullet's mass that causes obturation.
So, my theory is that because the bullet gets fatter, it fits tighter and clearly therefore creates more resistance. At some point, if you "hit" it hard enough with enough pressure, the friction between bore and bullet becomes high enough that velocity actually drops compared to loads that "hit" the bullet easier.
Hence, the more powder, the lower the velocity.
Note also, that if velocity actually drops with more powder, pressures have soared - possibly to destructive levels if the trend continues.
Finally, Barnes bullets are very different. People buy and use them because they ARE different. But that difference means that NO OTHER load data should ever be used with Barnes bullets. Follow Barnes load instructions and data ONLY.
Max load for the barnes bullet and that powder is 218gr with a starting charge of ~195gr. I am well within the safe level and am about in the middle of the 2 levels. I would expect velocities to either go up or stay about the same. Pressure isn't an issue at all since there are no signs of pressure and even fired cases can be easily chambered and extracted both right after firing and several minutes after. plus the cases really arent very hot right after ejection
MOST of the time we are working in a very narrow window that does show that relationship, but outside of that window (above or below) all bets are off.
One of the "classic" pressure signs is the powder increase vs velocity increase curve. IE: look at the fps/gr numbers. When it drops off you have reached the maximum charge for that combo (note that the max this way may well be in excess of the SAAMI pressure max).
My Barnes Manual #3 shows that bullet, but not that powder for the 50BMG. Did you get your load data from Barnes or is it data for some other brand of bullet?
It was listed in my Die set.
The Container that the powder comes in says to use load data for AA 8700
and is on the slow side of it.
My question is: is that data for the Barnes bullet specifically or just for "a" 750-gr bullet? Because that may be the root of the problem. As I -and Barnes- have said, you can NOT use data for other bullets when shooting Barnes.
It says "750gr Jacketed bullet" I emailed Barnes and they said that AA8700 is too fast burning for a 50cal, but the powder I use and the powder listed in the US military small arms data sheets both say to use AA8700 load data and the powder I am using (WC872) is on the slow side of the WC860 that is stated for 50 cal.
He said that that powder has exceeded the efficiency somethin or other and that it should fill the case more but even then more powder should equal more velocity since the case fill is higher.
In short, do what the Barnes people told you - and use their data ONLY with their bullets.
More powder and less velocity = CAUTION may well be DANGEROUS. That seems universal for every cartridge. Thank you.
I've held off writing anything until I had time to gather some of my notes to pass along.
Here is some data that has been published recently:
I am sending the supplier's address by e-mail so you can contact them for the complete data sheet. This e-mail also contains other information for you to use in this project.
I haven't finished writing the e-mail yet so wait a bit if it doesn't show up right away.
Best.