Factory ammo is ammo made by a commercial organization and retailed in shops (made in a factory) - not hand-loads. The ammo will be sized at the factory to SAAMI specs and to fit all likely chambers, so will be fairly sloppy in most chambers.
Your group-buy barrel will have been chambered for a 0.136" bolt so making it shorter by using a 0.125" bolt may not permit factory ammo to go into full battery and fire. I say may because it depends on where in the permitted range the gunsmith has reamed that chamber. Grendel, SAAMI permits a variance of 0.01" in chamber headspace; Min 1.2201" to Max 1.2301". SAAMI also specifies cartridge length as 1.220" Max, with up to .007" more headspace permitted. Theoretically then, the gunsmith could ream the chamber to the max 1.2301" and the ammunition manufacturer squash the cases to the min 1.213" and it's all still within spec. So, longest chamber with shortest ammunition is 0.017" of headspace. The difference in the two types of bolts is only 0.011". You might be lucky and get factory ammo to function reliably through your gun with a 0.125" bolt but the likelihood the gunsmith has gone generous with the chamber, and the manufacturer has gone tightest with the ammo is unlikely. More likely the bolt wont go into battery and you will have to use handloads only with that bolt. But, it is worth a try and perfectly safe.
If you have a box of factory ammo on hand let us know if it functions reliably through your group buy barrel with the 0.125" bolt.
SAAMI blueprint for Grendel
[ATTACH=CONFIG]17722[/ATTACH]
Your group-buy barrel will have been chambered for a 0.136" bolt so making it shorter by using a 0.125" bolt may not permit factory ammo to go into full battery and fire. I say may because it depends on where in the permitted range the gunsmith has reamed that chamber. Grendel, SAAMI permits a variance of 0.01" in chamber headspace; Min 1.2201" to Max 1.2301". SAAMI also specifies cartridge length as 1.220" Max, with up to .007" more headspace permitted. Theoretically then, the gunsmith could ream the chamber to the max 1.2301" and the ammunition manufacturer squash the cases to the min 1.213" and it's all still within spec. So, longest chamber with shortest ammunition is 0.017" of headspace. The difference in the two types of bolts is only 0.011". You might be lucky and get factory ammo to function reliably through your gun with a 0.125" bolt but the likelihood the gunsmith has gone generous with the chamber, and the manufacturer has gone tightest with the ammo is unlikely. More likely the bolt wont go into battery and you will have to use handloads only with that bolt. But, it is worth a try and perfectly safe.
If you have a box of factory ammo on hand let us know if it functions reliably through your group buy barrel with the 0.125" bolt.
SAAMI blueprint for Grendel
[ATTACH=CONFIG]17722[/ATTACH]