Attaching Constructor's method here to keep everything compiled in one location. Due to the scarceness of the SMK's I figured providing other methods to determine freebore might help people.
It seems there is a bunch of bad reamers going around. If you can load a 90gr SMK to 2.29 without jamming the bullet into the lands the reamer used to cut the chamber was out of spec. The SAAMI spec for freebore is 1.676 from the base of the cartridge.
The easy and rough way to find the dimension -Turn a flat based .224 bullet around and seat it upside down in a 224 valkyrie case. Seat it so it barely touches the lands in your chamber. Measure from the base to the forward most part of the .224 diameter of the bullet. It should be very close to 1.676"
The correct reamer drawing that matches the SAAMI spec drawing is attached below.
It's pretty easy to check your chamber. Don't wait for someone to prove it to you check it yourself. The 1.6767 dimension is the distance from the base to the start of the freebore.
The most accurate way to find the freebore dimension- Take a .224" diameter inspection pin. Cut it in half, 1" long. stick it in a Hornady OAL gauge and adjust it in your chamber. Measure from the front of the inspection pin to the base and there's your number.
ETA-7/3
The jump to the lands effects accuracy. If there is too much jump the accuracy may never be what it could be. The ARs mag length restricts us to how long we may load the cartridge. Most 6.8 mags allow loading to 2.295". If you load a 90gr SMK to 2.26" like the factory rounds and the barrel was chambered with a reamer that was not to spec the jump to the lands may be .060". Most bullets shoot best when jumped .005-.020" or some VLDs shoot best when jammed .005".
You may get lucky and find a sweet spot where the bullet jumps at .040 or .060 and it produce 3/4" groups. Mine did that with factory ammo but when I loaded the bullets out .010" off the lands the exact same barrel shot some 1/4" groups. If the barrels were chambered with a SAAMI spec reamer the jump with factory loaded 90gr SMKs is right there at .010".
If your barrel was chambered with a out of spec reamer and you are happy with the accuracy don't worry about it.
This thread will be used to identify what barrel you have and identify if the chamber is to SAMI spec or not. It would also be nice to provide freebore measurements of barrel makers.
