So the other day I ordered a 9" .300blk conversion kit for the MCX.

As expected I got a new barrel.
However it also included a new bolt.
This confuses me.
Why do I need a new bolt?
Both the .223rem and the .300blk use the same bolt in every other gun I've heard of.

Anyone have any guesses why they sent me a new bolt too?

Probably because of the recall that was associated with 300BLK which required sending in our BCGs for swap. So just to cover their bases, they’re probably shipping conversion kits with new bolts in case you never responded to the recall.

but it was only a bolt - not the whole BCG

Now that is weird.

The new Virtus barrels require a bolt with rounded lugs. The original MCX 300BLK barrels could use the same bolt as the 5.56NATO. The change was made to improve reliability with short barrels and cans.

Bill