ok
so what i've found so far is:
you can't take paintball equiptment with you on the Eurostar as 'Carridge luggage' as it is deemed to be a safety risk.
What you can do is send it out with another company;
this is from the Eurostar website:
The luggage allowance on Eurostar is two cases or bags and one item of hand luggage.
For any of the above requests*, please contact Esprit Europe on +44 (0)870 5 850 850 who will arrange for your baggage to be transported separately for a small cost. Please note that baggage needs to be taken to Esprit Europe to be registered at least 24 hours prior to your Eurostar departure. This will enable you to reclaim it on your arrival.
basically it can't travel with you which sucks.
they also say this:
What security measures do you have in place?
We x-ray all luggage and security screen all passengers and their hand luggage. We also have measures in place that are not visible - we do not discuss these for obvious reasons. Eurostar works with the authorities in France, UK and Belgium to ensure its security measures meet their requirements. We also employ extra measures over and above what the authorities ask for.