There might be an easy explination,I do apologise if the answer is too apparent!
When I seat bullets,I notice some slide into the neck,you can actually pull them out by hand.This is after trying everything I could think of to get the neck size down,including:Using an RCBS sizing Die,I unscrewed the "button" that inside sizes the neck after full length sizing.This to me says that the neck is squeezed as tight as the die would allow,and the "button" doesn`t open up the neck again.But I still get bullets falling through into the case.Anyone come across this before?

Measure the Diameter of the Bullets you are trying to use and let us know what that is. And what Cartridge you are working with.

Thank you for the swift response.The bullets i`m using are Swift Scirocco 2s,180gr,308 cal,Cartridge is 300 Remington Ultra Mag,brass is Remington,got the components from 2 different friends in your fine country.I see where you are going with the measurement of the bullet,but I take 1 bullet and fit it to different cases,some won`t fit,other slide in.Don`t think it`s the bullets,it`s either the cases,or I`m doing something wrong in the reloading process.

Anneal your brass and see if that helps.

.300RUM,

If you've got your RCBS Full-Length Sizing Dies set up correctly and are actually Full-Length Re-Sizing then the issue could be the Bullets, Die or the Cases.

The Bullets: Could be they are undersize. Are you positive they are .308" caliber? It wouldn't be the first time some 7mm 175 grain bullets got put in the wrong package with the wrong label, either. A quick measurement should tell if all's Hoyle.

The Die; as you've already tried without the Expander Ball; you're correct in that if you are using an RCSB Full-Length Re-Sizing Die that this particular Die won't re-size (contract) the case neck any smaller than than you already have if you've removed the Expander Ball. What are the markings on the Die? Should be .300RUM F/L?

The Cases: The Case Necks could be too thin; thereby not enough grip/tension on the Bullets. Somehow can't imagine that .300RUM cases would be so work-hardened from a coupla firings (or maybe one) that they're too hard to size; although annealing them may make a differece. Measuring the outside diameter of the Case Necks after sizing ought to let you know if that is the issue. According to the drawing of .300 RUM on Quick Design the outside neck wall diameter should measure .344".

Until you take a micrometer to measure all of the Bits & Pieces above you won't know which is causing the issue.