Primer compound mass - effect on velocity ?

When weighing primers, how do you know if the extra weight is from the anvil, cup, sealant, paper or primer compound?

The assumption, and it's just an assumption, is that there is greater weight uniformity between stamped metal bits than priming compound that is applied to the cups by hand.

The assumption, and it's just an assumption, is that there is greater weight uniformity between stamped metal bits than priming compound that is applied to the cups by hand.

I think the problem is that the metal makes up the major part of the weight so small variations in that part would dwarf big ones in compound. Given that the weight and internal dimensions of the metal parts aren't critical to function at least as I understand it they probably don't hold it to superfine parameters.

Agreed. I think averages from a good sample size can point in the right direction.

I've used 5 of each primer, unfired and fired, and it shows 6.5 times more priming compound in a CCI 35 than in a CCI 250.

This tells me when I use the same 236 grain charge over a CCI 35 and a F215M, there will be lower velocity for the LR primer.

I think it's the kind of question that can only be answered by testing.

The test will show how much.

Can the primer compound mass being so miniscule and the input it has so miniscule in relation to the charge have an measurable affect ?
I think not.
Different brands may have a small measurable affect.

My .270 Win shoots same 3 group with different primers, only the pattern of the group changes

The priming compound is usually not inserted by hand, but rather by a mechanically operated rod that is dipped in the wet priming compound, then dipped into the cup. Check out a few of the videos on the internet.

Primer manufacturers often have different names for their normal and match primers, for example CCI has CCI200 and BR2. These are made to the same specs, but the BR2 versions are advertised as being more consistent. Some people are willing to pay lots of extra R for the BR2 versions. So they actually believe the variations to be smaller. Some tests do indeed show smaller ES values, but not always. There are other variables at play here too.

Taking the mass of the priming compound as a percentage of the total primer, it is quite small. Something like 2gn of compound in a 10gn primer. Will any weighed difference be attributable to the compound or to the cup and anvil?
Do not forget the sealant and paper layers on top of the priming compound. They will also affect the weight, but not the energy value.

I think just find what works best for you and try to stick to that.