Measuring freebore.

[Alaska_guy]

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
.010-.020.

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

http://www.midwayusa.com/product/462291/cerrosafe-chamber-casting-alloy-1-2-lb

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.

[Constructor]

Much easier way to do it posted elsewhere on this forum.

[Alaska_guy]

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.

[Alaska_guy]

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?

[Constructor]

If the measurement was to the forward most .224" diameter part of the bullet then I would say the freebore is long.

[Alaska_guy]

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. 

[Retired]

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? 

[binazone]

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".

[carnaby]

The method described here to measure freebore is in error. It will not measure freebore.

The statement "The 1.6767 dimension is the distance from the base to the start of the freebore." is incorrect. This is the distance to the end of the freebore. Besides that, to measure freebore in this way you would need an inspection pin that was precisely the diameter of the freebore, which is not 0.224", but 0.2246", and then this is the minimum. The maximum can be as big as 0.2266" given the SAAMI diameter tolerance of +0.002".

So an inspection pin that was precisely 0.224" diameter with a 1.5 degree leade , with a freebore diameter at SAAMI minimum of 0.2246 would give an OAL of 1.6882". A flat based bullet with a diameter of 0.2238", with a perfectly sharp base edge (not going to happen unless you machine or file it flat) would measure 1.6920".

Note that none of these measurements with a .224" inspection pin or .2238" flat base bullet measure OAL to the freebore. They measure OAL to the point that is the same diameter as the pin or bullet IN THE LEADE, that is the section of the rifling that is cut at 1.5 degrees.

JGS reamers are made to a diameter tolerance of 0.0005", which means the freebore diameter could be .2246 + 0.0005 = .2251". In this case, a .224 inspection pin would have an OAL to the leade of 1.6977" and a 0.2238 bullet would be 1.7015".

At the SAAMI maximum of 0.2266" the OAL to the Leade would be 1.7263 for the pin and 1.7302 for the bullet. A chamber that measures in this way is still SAAMI spec.

If you want to make an attempt at measuring OAL to the end of the freebore, you need an inspection pin set. Find the largest pin that will actually slide into the freebore, then check the OAL of a Hornady OAL gage using that pin. It will stop at or near the end of the freebore. Suppose your freebore is 0.226" diameter, then take a 0.225" inspection pin (or machine a cutting edge bullet flat at the sealtite band which is approximately 0.2255". This will stop very close to the end of the freebore and give a much better measurement.

For example, if the freebore diameter is 0.226" and the diameter of the sealtite band is 0.2255" then you should measure an OAL around 1.6862". When I do this with my barrels I get an OAL of 1.6880" indicating that the diameter of the freebore is more like 2.2261". This assumes that the lands are perfectly at 0.219" and all other dimensions are MINIMUM SPEC, which is highly unlikely.

This bunk about out of spec chambers not helping the 224 Valkyrie. Nobody seems to know what they are doing when attempting to measure freebore, and it cannot be done accurately by the method described earlier in this thread.

[trianglevelvet]

I just went through a giant process of trying to figure out using tapers and pins and calibrated ball bearings to get full chamber dimensions.......

And decided that this is why Cerrosafe was invented.
 
 
And these:

https://www.hornady.com/reloading/precision-measuring/precision-tools-and-gauges/lock-n-load-bullet-comparator#!/

https://www.sinclairintl.com/reloading-equipment/measuring-tools/bullet-seating-depth-tools/length-gauge-prod70272.aspx