My guess is the chamber is very rough. If the barrel is Nitride/Melonite/QPQ the process can leave salts and other residue from the hardening process. Cleaning and polishing the chamber could help but there is also a possibility the chamber is just rough. Its not unheard of for a manufacturer to use worn out reamer. The manufacturer may have failed to run a finish reamer in the barrel. Poor quality control is relatively common with some 6.5 Grendel barrel manufacturers.
One common problem with reloading the 6.5 Grendel is bumping the shoulder back so the brass fits the chamber. You can size the case and see if the sized case fits the chamber and will fall out when the muzzle is raised. If the case doesnt fall out lower the sizing die a little more. The shoulder normally needs to be bumped back .003-.004. Once you have the case sized take a loaded cartridge and color the bullet with a black marker. The cartridge should fall out of the chamber via gravity when the muzzle is raised. If the bullet is into the lands seat the bullet deeper. If the bullet is into the lands some of the black on the bullet will be rubbed off. It is not uncommon to get a short chamber with a worn out reamer. These particular chamber problems are fairly common on cheap 6.5 Grendel barrels. Rack grade cheap barrels are luck of the draw on accuracy and QC.
One common problem with reloading the 6.5 Grendel is bumping the shoulder back so the brass fits the chamber. You can size the case and see if the sized case fits the chamber and will fall out when the muzzle is raised. If the case doesnt fall out lower the sizing die a little more. The shoulder normally needs to be bumped back .003-.004. Once you have the case sized take a loaded cartridge and color the bullet with a black marker. The cartridge should fall out of the chamber via gravity when the muzzle is raised. If the bullet is into the lands seat the bullet deeper. If the bullet is into the lands some of the black on the bullet will be rubbed off. It is not uncommon to get a short chamber with a worn out reamer. These particular chamber problems are fairly common on cheap 6.5 Grendel barrels. Rack grade cheap barrels are luck of the draw on accuracy and QC.