Interesting I found this on Quora maybe Terrain can explain it better
How do telescoping and caseless ammunitions work? Why do the gasses not just go around the bullet and out the barrel?
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, Competive shooter, former military
Understand that the chamber diameter and the barrel diameter are not the same.
In either configuration the bullet will move forward into the barrel when fired, forming a tight seal similar to any other modern bullet as it engages the rifling. There will be some gas escaping around the side, but only as the bullet moves the few centimeters it needs to pass the forcing cone into the barrel. Most of the gas will be trapped behind it.
The tech is not new, its essentially the same as whats used for a shotgun shell.
Telescoped ammunition
From Wikipedia, the free encyclopedia
Telescoped ammunition is an
design in which the projectile is partially or completely enveloped by the propellant. Examples include ammunition for both hand weapons and artillery. Some telescoped ammunition is also
.
Telescoped ammunition has advantages in comparison with traditional ammunition
. Telescoped ammunition cartridges can have reduced overall length with similar ballistics to a comparable mass traditional round. Also, telescopic cartridges may avoid the risk of damage to the projectile during the loading process.
Cased telescoped ammunition for the
has reached
.
In August 2013,
was awarded a $US2.05 million contract to continue developing parts of the US LSAT program. Part of the contract is to further refine 5.56mm cased telescoped ammunition, and develop 7.62mm cased telescoped cartridges.