John;
Thanks for putting the link up, I spaced it out on the original post.
Bob;
Yes, they do squirt one heck of a lot of gas out the back. Mr Newton says it's not just a good idea, it's the law.
This is a down and dirty about the original gun that I found.
ADDITIONAL TECHNICAL DATA  75 MM RECOILLESS RIFLE, M20
                       Length..................6 ft. 10 in.
                       Weight..................114.5 lbs.  
                       Rifling.................Uniform, right hand, 1 turn in 25
                       Breech..................Interrupted screw
                       Range...................7000 yards (HEAT round)
                       Muzzle Velocity.........1000 ft./sec. (HEAT round)
                       Projectile Weight.......3.19 lbs. (HEAT, shaped charge)
                       Armor penetration.......4 inches
Perhaps I'll put in a call to S.G. and ask about any not so obvious demil cuts in the tubes, like a full length longitudinal cut.
Seems like it would take a fair amount of pressure to achieve the ballistics listed above.
Bill