224 Valkyrie Forum

224 Valkyrie AR Platform => 224 Valkyrie Troubleshooting => Topic started by: Alaska_guy on July 11, 2018, 07:35:58 PM

Title: Measuring freebore.
Post by: Alaska_guy on July 11, 2018, 07:35:58 PM
This post is to help identify current barrels on the market and their freebore.  Due to the amount of people experiencing accuracy issues I would highly recommend checking your freebore.  This is an inexpensive and easy way to determine your barrels freebore.

This is most likely the easiest method I know of to measure freebore within

Purchase cerrosafe to make a chamber casting.  Measure chamber casting to determine freebore.


Below is a quick video explaining how to make a cast.  Removal of your AR barrel will be required.


If you are a reloader, this method has worked very well for myself over the years.


Title: Re: Measuring freebore.
Post by: Constructor on July 11, 2018, 08:55:31 PM
Much easier way to do it posted elsewhere on this forum.
Title: Re: Measuring freebore.
Post by: Alaska_guy on July 11, 2018, 10:45:39 PM
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.
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.
Title: Re: Measuring freebore.
Post by: Alaska_guy on July 21, 2018, 11:26:07 PM
Update on PSA barrel.

Well I had a chance to measure the PSA barrel using a hornady comparator gauge.  I consistently measured 1.763" to the lands using 75gr eld projectiles with a COAL of 2.313.  I also tried some 70gr gmx bullets and I measured 1.750 to the lands and 2.153 COAL.

If I flipped the 75gr eld around so the base was facing the lands I measured 1.880.  Turning the 70gr GMX around so the base was facing the lands I measured 1.840.

Hopefully that helps.  @Constructor, any thoughts?
Title: Re: Measuring freebore.
Post by: Constructor on July 22, 2018, 08:27:33 AM
If the measurement was to the forward most .224" diameter part of the bullet then I would say the freebore is long.
Title: Re: Measuring freebore.
Post by: Alaska_guy on July 22, 2018, 01:29:28 PM
Thanks.  I figured if I split the difference 1.880 and 1.840, lets call it 1.860 that would mean the PSA barrel is about 0.184 out of spec (long). Good thing your next batch of barrels is coming out soon.  Looking forward to trying a correct reamed barrel that has your blessing.  :D
Title: Re: Measuring freebore.
Post by: Retired on July 27, 2018, 07:08:59 PM
I see in a another forum, one of the members contacted Savage, said he had a rifle with a out of spec chamber [long freebore], when he told them it was the Valkyrie they asked for the rifle to be returned and they would correct the problem. Maybe some changes are starting to happen? 
Title: Re: Measuring freebore.
Post by: binazone on January 07, 2019, 06:58:06 AM
A bit late for a reply but I only just found this thread. I can confirm the PSA freebore dimension that Alaska guy found. I used both 88 and 75 ELDMs, base end first with my Hornady O.A.L. tool and got 1.890" and 1.893" respectively for results. Let's call it 1.892" or .216" over the SAAMI spec of 1.676".