Lead cores are “pre-formed” at the cutting from wire stage, so that the “core seating” &“swaging” process is simplified. So a lead core for a Ball 7,62M43 Jacket will be “flat pointed” for weight considerations, and even if air does escape during core insertion, there will remain a “Pocket” at the Point because there IS NO LEAD there. No “compressed air” pocket or “un-escaped” air about it.
The Core and the Jacket are engineered to make sure the jacket is the standard length after seating and base forming ( Boat tailed and rollover of jacket)…the resultant air pocket may assist in expansion, tumbling or whatever for the bullet, but it is just an artefact of Bullet production.
The US, in WW I, made .303 “Mark VII” Bullets without the aluminium tip…in order to get the same Over-all weight ( 174 grains) the Bullet was actually a bit shorter than the British Spec. Mark VII ( with Fibre/Aluminium tips)… A proper mark VII jacket filled with lead is about 190 grains ( the Portuguese made some like this in the 1920s ( from samples in my collection)
but by adjusting Jacket length, brought the weight back down to 174-175 grains by the 1930s ( in 1928, they reloaded Old W15 and other US made projectiles from dismantled cartridges into new AE made .303 cases ( also from my collection of Portuguese .303)…The US made projectiles were shorter of course…see above.).
Forty years ago, I did an analysis ( 1922 to 1937) of AE made .303, from Powder weight and type, primer shape, Flashhole/Berdan style ( Roth and RWS) Bullet length and weight, and crimping style. I will have to dig it out and re-print it…I also still have the entire 1922-1937 collection of cartridges ( courtesy of a mixed Loose crate of Portuguese .303 from Mozambique…included as well all the US Contract dates, as well as British “z” loadings of 1917 and 1918…all ammo donated by Britain to Portugal in 1919, or otherwise dumped in the Channel and North sea as being “Defective”.
Sorry about wandering away from the original thread, but the “Bullet weight” question brought this other signal event to mind…