What you have made will probably work OK, but for U.S. GI-spec blanks only. If the gun short cycles with the above blanks, you made need to weld up some of the gas escape holes in the BFA. The best packing is asbestos, and next-best is teflon string used for pipe packing (1/8 inch dia.). Dont use 0-rings, as they will break and cause you no end of hair-pulling. With the above packing, installed correctly, back pressure is not a problem. Packing is a craft learned from experience, but the object is to have it tight, but not so tight you can't manually pull the bolt back and have it go back forward. If you are a re-enactor, and you encounter HOT .30-06 motion picture blanks made BAR's, DO NOT USE THEM or you will destroy your gun. I've been building and renting MG's for movies for 20 years, and if you have any questions, please e-mail me at [email protected]. Good Luck!