Here's how I made mine:
Put 2 three inch mag shells in the tube, then unscrewed bbl. retainer/mag tube cap. Put in a long dowel or other material about 3/8-1/2" thick and mark it at the top of where the cap screws on, remove and cut rod about 3/4" below mark. Drop into tube, replace cap and you're good to go. To remove, just unscrew cap hold gun muzzle down and it'll fall out.
I used an old fibreglass arrow I had laying around, but anything sturdy and straight, not too thick in diameter works.
In the 870's, everyone used pencils years ago, but they're too short for an M4.