Ditto on what
colt_saa has said. Sounds like powder residue induced.
If your 300gr bullet is plated, like HSM's 300gr P-HP, using very slow powders, like W296/H110, are problematic because there's no cannelure to roll crimp into to ensure powder ignition starts properly.
The same thing applies to the jacketed bullets in
colt_saa picture. I believe what he, as well as myself & others use on jacketed bullets w/o a cannelure is Lee's collet crimp die vs. a roll crimp die. Same for Hornady's 300gr XTP for the 50AE, if used with reduced loads below 1400fps.
Moderate speed powders are better for reduced loads, which also allows for deep seating & crimping of the case mouth over to the bullets ogive (~.498") to eliminate jump crimp & ensuring good ignition.
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HSM 300gr - Acme 330gr for the 500 S&W Mag
(-02b)
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