Originally Posted by
MTWeatherman
All else being equal, it’s a rare firearm indeed that shoots a PB bullet more accurately than a gas checked one. With proper pressures, good barrels etc...equally well yes...but rarely better.
Gas checked bullets allow increased velocity performance in nearly all rifles and some magnum class handguns... exceptions largely being those designed for black powder where pressures must be held low... well within the strength of a non-checked bullet.
Largely repeating what others have already said...but since the question was asked, I believe gas checks yield those positive results primarily in 4 ways. Gas checks:
1. Provide a scraping action to remove lead. This allows some leading to occur as each bullet is fired...the gas check restores the barrel to its pre-fired unleaded condition and prepares the barrel for the next shot.
2. Eliminate or greatly reduce gas cutting...important in higher pressure loads and in firearms with somewhat undersized cast bullets, large throats, wide spots in the barrel etc. This is accomplished by the greater yield strength and melting temperature of the gas check. Note some gas cutting or leading could still occur as long as it is not enough to overwhelm the scraping action of point 1 or the riflings abillity to grip the bullet.
3. Provides increased bullet yield or shear strength for the rifling by replacing what would otherwise be lead with a copper alloy. In many cases, this also results in more bearing surface providing additional strength.
4. Provides a uniform base to the bullet...important as the bullet exits the barrel. Non-uniform bases adversely affect accuracy...always a possibility with PB bullets unless care is used in casting and seating.
(A contributor to both effects 1 and 2 above may be the “spring back” quality of the check and likely expansion of the check itself that takes place when pressure forces out any air remaining between the check and the shank and obturates the shank itself.) This would enhance the scraping and sealing qualities of the check.
Gas checks do not accomplish their mission by preventing the melting of the base...far too little time exposure to accomplish this by temperature alone. Exposed lead bases in jacketed bullets fired under high pressure have long ago refuted this theory. It is high velocity gas cutting along the sides of the bullet which erode and destroy the bearing surface. This lead is then condensed in the barrel ahead of the bullet as the pressure and temperature drops.
What effect(s) comes into play is dependent on the bullet, load (read pressure), firearm, lube, etc.. Low enough pressure in a smooth barrel with a proper fitted boolit with good lube and nothing is to be gained by the check. None of the 4 points above are necessary. However, as pressure goes up, barrel quality goes down, bullet quality and lube deteriorates (to include loss of bullet bearing surface and size), etc. and some or all of the above become important.
In short, a gas check allows many things to be less than perfect while maintaining acceptable accuracy and higher performance and velocities than we could otherwise achieve without it. In many cases it makes life much easier for the casting novice as it reduces the need for the more extensive testing with bullets, lubes, and loads that would otherwise be required...to say nothing of bore lapping.