50BMG yielding low velocity's

M

Mausermolt

New member
Any 50 BMG reloaders here?

I have a Bushmaster BA-50 that I have been reloading for that is giving me really low velocity's. Curious if anyone has any ideas why.

-750 grain Barnes Solid over 220 grains of H50BMG, yields about 2430FPS out a 30" barrel

And 647grain API surplus over 220grains of US869 yields 2340FPS, out of the same barrel.

That's waaaaaaay slow according to the books....
Any ideas?
 
How are you measuring the velocity? With an optical chronograph, the high blast sphere volume and powder particulate count can trigger the first screen ahead of the bullet, but not the second, leading to low readings. We had one member who had to set his chronograph out to 18 feet before it got reliable readings from his 338 Lapua magnum. With your 50, you might have to go to 25 feet, depending on the unit's sensitivity.
 
Barnes tested with a 32" barrel with a 1 in 15 twist. You'll only get close to their numbers with the exact same rifle, powder lot and ambient conditions.
And they don't mention where their chronograph was.
220 grains of US869 is below minimum for a Barnes 647 grain bullet too.
 
It's not that low, with a 36" barrel and a charge of about 235grs, one of the guys I shoot with was getting 3,000 FPS. That was with H50BMG, didn't get any signs of over pressure but that's at the upper end of the scale.

Most guys are getting about 2,600 FPS with the load you're using. So you're not that far off and other factors could've given those results. Try bumping it up to 230grs by going up a couple grains at a time and chronographing them to check your results.
 
Mausermolt,

It dawned on me to check to see if perhaps your problem was imaginary. I put your load of 220 grains of US869 in QuickLOAD with a 30" barrel and set the bullet weight to 647 grains and it predicted a velocity of 2332 fps, pretty close to what you are actually getting. If I adjust the burn rate up half a percent (well within normal lot-to-lot variation) I get exactly 2340 fps, and predicted pressure peak is just 32,907 psi, which is 40% below the cartridge's maximum. Hodgdon's data for an M33 bullet is 265 grains of US869, so you are about 20% below that maximum, which, indeed, takes about 40% off the peak pressure number of 55,000 psi.

A normal starting load is 10% below maximum, so I would go to 238.5 grains and work up to 265 grains in 2.5 grain steps. QuickLOAD thinks you will be at about 2800 fps right at 260 grains. It also thinks that is the maximum load, so the powder model is just a little bit at the high end off by the time you get to 265 maximum, or else the Hodgdon number is off, which I doubt. At 265 grains your barrel length should be at about 2850-60 fps, depending what assumptions I fiddle with. The Hodgdon test barrel is 45" long, so you won't be reaching their velocity numbers.
 
I don’t reload but shoot a lot of BMG.
And your speeds are way low if correct.
I’d suggest you go over to 50 cal shooters association @ FCSA.org and ask the question there. Lots of guys there that can help.
 
One thing to be careful of if you want to drop a powder charge of 265grs. is a compressed load. The 50 BMG case is large but I'm not sure if it can hold that large of a charge and not be compressed. If it is compressed it will increase the pressures to levels that might not be safe, even in a .50!

Plus the length of those AMAX being at about 2 1/2" seat pretty deep in to the case, another thing to think about. I wouldn't use a charge over 245grs of any powder/bullet combo due to safety reasons unless you really research that high of a charge.

Be safe and ask plenty of questions!
 
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