One guy posted similar post in another forum and he got an answer
"Did you get around to it, and, if so, what were the gun transporting arrangements, as I and a couple of mates, fancy the idea too!
I travel through Europe lots with my RIFs.
I always use DFDS Dunkirk / Dover so I am a bit of an authority on their procedures.
1. Make your booking either on line or by phone. It is cheaper online.
2. Phone up the customer services number with your booking reference and ask for a "code 100" against your booking.
3. Answer questions:
Q.What type of weapons are you carrying?
A. Airsoft replica firearms - they are working models, but look real so need to be booked on as though they are real.
Q. Do you have all the required licences for these weapons
A. They do not require a licence. They are only age restricted for purchase
Q. Will the bolts be removed for transport?
A. Yes (for my GBBRs) or They are electric powered, the batteries will be removed.
Q. How much ammunition will you be carrying?
A. 6000+ rounds **wait for gasp on other end** but they are only plastic balls.
Q. How will the weapons be transported?
A. In a gun case, in the boot of the car, out of public view.
Sorted! It is a 2 minute phone call and it is free to book them on.
4. On arrival at Dover surrender your vehicle for search at the customs bay (with the little owl symbol). Somebody may try to usher you onwards - DO NOT GO ONWARDS - explain you have a code 100 and they will sort you out.
5. Inside the inspection simply lay the gun cases on the table and let the old boys have a look over them. You will need your passport and booking reference.
6. They will then call ahead to the ferry operator and confirm that you have booked the weapons on and send you on your way. DO NOT LEAVE THE INSPECTION BAY UNTIL YOU HAVE A RED STICKER ON YOUR WINDSCREEN. Failure to get the sticker will result in you being turned away from check in and back to the inspection bay.
7. Proceed to check in. You will normally be assigned a lane away from the others.
8. When boarding the ferry you have to put your hazard lights on so the loadies know you are the one with weapons. The used to take your car keys off you for the duration of the crossing. The no longer take your keys so you may no longer have to so the hazard light thing.
I have never been searched coming out of Dunkirk as it is a smaller dock without many of the facilities. I have never done the trip through Calais.
I imagine all ferry operators and indeed the tunnel will have a similar process.