CBTO, Freebore and Mag length

[Boozer]

Hi all-

I"m getting ready to order a Remage Barrel for a .223 build I'm working on. Northland shooters Supply offers 4 different Reamers: Std., Ackley Imp, Wylde, and Match. I have 3 goals I'm trying to achieve:

• Maximize case capacity, but also, not have round seated out so far to where there's less than ~1 caliber of seating depth.
• Ensure rounds will feed from an AICS pattern magazine (so OAL needs to be less than ~2.50")
• Ensure that the round set at the Max OAL (based on mag limits) will be able to slightly Jam, and still have room to chase the lands as the throat erodes a little.

I have the reamer prints from NSS. Boltface to end of leade/start of throat measurements:

223 Match - 90 thou freebore, 1.876", 1.5deg leade angle
223 Wylde - 78 thou freebore, 1.8504", 1.25deg leade angle
223 AI - 60 thou freebore, 1.8487", 1.5deg leade angle
223 Std. - 25 thou freebore, 1.812", 3.16deg leade angle

Here's my thoughts:
I intend to use the 90gr. Berger VLD to a ~2.50" OAL (maybe a little under to allow chasing the lands). From the dummy round, I would measure the CBTO. My understanding is that the CBTO measurements should be approximately the equivalent (ignoring a small error due to the leade angle) to the 1.8XXX measurements above. i would then subtract about .010 off of that measurement to allow for Jam, and then round down to the nearest Reamer. So for example, if CBTO measures 1.870, i would subtract .010 to get an "ideal" reamer length of 1.860, and then round down to the Wylde at 1.8504.

If this was 100% custom, I would just send a dummy round to the gunsmith and let him do all the math, but that's not the case, i have a choice of these 4 reamers.

Does that all make sense? It took me about an hour to type all this so that smoke didn't start rolling out my ears...

 

[Mikecr]

It all sounds right to me.
If your best tested seating ends up off the lands to begin, you will not ever need to 'chase the lands'.
In this case, initial best tested +/- tweaked group shaping would hold until the barrel dies.

 

[Boozer]

My understanding of the Berger 90 vlds is that they typically like to touch, or jam into the lands. There is a chance that I might find a sweet spot off the lands, but I'm not going to plan for that.

 

[Mikecr]

There never was actual basis for the notions of VLDs and best land relationships.
So there is no reason to treat them differently, and you can plan to use a best tested CBTO that's off the lands(OTL).
I use best OTL with 95 & 105 VLDs, no problem. I won't accept ITL relationships, so that I won't chase tail, chasing lands.