There is one aspect to the size of the shot you are using that
hasn't been mentioned - and that is that the smaller the size
the more likely you are to be able to "fill" the space with a
maximum amount of individual BBs. So smaller - when we
are talking about something like the space between the
frame members on a freight car ... wait for it ... WILL get
more weight. Exaggerated case in point - if the BBs are not
able to fit beside each other they won't 'fit' the space as
well. You end up 'wasting' more space if the BBs are larger.
Smaller BBs also fill in around where brake and other under
car details are attached.
Someone gave a link to MidwayUSA - they have both 5 and
10 pound quantities ... and I even saw 10 pounds of #12 shot
(very tiny) for about $4/pound. I do not know if that price
including shipping or not.
I like using lead shot for car weight - I hold it in place using
Krystal Klear (essentially white glue) - and if I need to (rarely)
I can put some water on it and let it set and it will let go. But
in over a decade I have never had any of the glue let go
without applying water (humidity isn't enough to even soften
it). KK is also nice because it 'flows' around the lead - I put
in a layer of lead and then add KK on top and let it dry ...
repeat as necessary for multiple layers. And KK can be
painted without affecting it ... if you really want to 'hide'
the fact that you added some weight.
I have also used shot and KK to add weight to brass
steam - one of my tricks for that combo is to glue some
to the underside of the cab roof ... in order to balance
the steamer over the center of the drivers (actually just
ahead of center and -never- behind). Many brass steamers
that have derailing problems are easily fixed by getting them
My personal standard for car weight is "NMRA RP -plus- 1 oz"
per car. That extra ounce translates to added tracking and the
extra doesn't seem to seriously affect train length. YMMV.