Nice rifle! Check your once-fired military brass carefully inside and out. Occasionally you'll find a case that has stretching in the head area from being fired in a bad headspaced military weapon. Use the bent sharpened wire feeler method to feel for the stretch ring near the head of the case. Discard any that you can feel the depression on the inside of the case wall. Trim all cases for length. 3.910 maximum and recommended trim length of 3.900". I HIGHLY recommend a primer pocket uniformer such as K&M to true the bottom of primer pockets of the 50 brass. It will insure primers seat flush or below and especially important on a semi.
I have'nt loaded for a 50 semi but only a bolt M33. Load data for the 50 especially with surplus powder and components doesn't exist. Recommended start charges with either WC860 or IMR5010 with 647-650 grain bullets is 210 grains. 690-700 grain start at 205 and 750 grain start at 200 grains. You can work up in larger 3-5 grain increments. Surplus powders can vary in burn rate and require working up with each different lot number of powder. I've used a few different lots and found one that was 200-300 fps slower than the others.
I think you'll find with 860 or 5010 that around 217-222 grains will cycle your rifle and get the velocity and accuracy wanted. In my 33" barrel 220 grains of 860 gets approx. 2775-2800 fps. 210 grains should get you 2500's near 2600 fps. I load the API and 647 gr. ball the same loads. I seat all 50 cal. surplus bullets to military length, 5.450" and I wouldn't seat deeper than 5.400".
My RCBS press and dies takes hard firm contact and cam-over between the die and shellholder to get the shoulder pushed back to zero. Don't want cases with too long shoulders that may not fully chamber in your semi. Which CCI primers? The CCI 35 primers in the blue boxes have flat heads while the new CCI 35 Arsenal primers have domed heads. CCI recommends to use the flat primer seater punch rather than a cupped punch. When seating the domed primers the flat punch will slightly flatten the dome of the seated primer. Just insure flush or below.
Here's a military load chart. DO NOT USE FOR LOAD DATA!! FOR REFERENCE ONLY! These loads are military and may be too hot in your rifle.
Here's one persons personal data using mostly 5010 and I find his data to be realistic and safe.
You might ask your question over on the Barrett discussion board under the "50 ammo" topic.
Be careful and have fun!
Also might ask at the Biggerhammer site:
Another good forum that supports 50 shooters: