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They allready do Scribe (I dont cos I cant shoot far enough to warrant it)

Surely that is what I just said as simply as possible ?

Its the chemical reaction thing. Powders can react(burn) faster(or slower) depending on temp. Usually hotter means faster but not always due to some retardant coatings etc.
Its like a jar full of...

My shooting crony will vary +or- 80fps shooting a known load over it(over an ohler) and this with being pedantic about it being fully open(changes spacings) and inline with bullet path in both...

Possibly a head wind or downdraft? just throwin it out there.