Use a rubber O ring, not a washer. It seams like about 1/2 inch inside ,easurment. I have reallly never found that it helped mush but its worth a try. Most Handi rifle shoot the lighter bullets per caliber best. I have had good luck with the NEF rifles EXCEPT for the 243. Both of the ones I had were always getting the casings stuck and i had to carry a rod to knock them out with. They work best with rimmed casings like the 30-30, 44 Mag, 357 Mag 22 Hornet and 45-70.
Go to the NEF forum There you will find out everything you will ever need to know .
www.go2gbo.com/forums/index.php?board=126. Great bunch of guys too.