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Discussion Starter · #1 · (Edited)
If you have checked different 6.8 bullets please post your findings here. Then we'll get into the length they should be loaded to get the best accuracy.
Thanks

If you have not tried the Hornady 110 HPBT set to 2.26" and the 120sst set to 2.31" give them a try.

These are just off the lands in the barrel (ARP Chamber)I am using today.
110HPBT-2.26
110vmax-2.32
120sst-2.31
110NBT-2.37
110prohunter-2.33
130 Sierra SBT-2.388
115-SMK-2.3

IMO the Barnes bullets do not need to be as close to the lands. The 85TSX does not touch at 2.31 and the 110 TSX does not touch at 2.41
The Hornady 110HPBT factory load is why we can't shorten the freebore length further to increase accuracy.
 

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The 85 and 110 grain TSX bullets should touch the lands at the same COAL as they have identical ogives (full bullet diameter is at the front of the first band). Barnes recommends 0.050" off the lands with their monolithic bullets though I have loaded them slightly shorter at times. I also found 110 Vmax and 120 SST to have the same ogive and thus same distance to the lands. In the three ARP barrels that I have measured 110 HPBT distance to the lands with, they were between 2.335" and 2.380". Below is a matrix I have kept on maximum COAL for one of my ARP barrels. Lead core bullets have COALs listed 0.010" from the lands, monolithic are 0.050" from the lands.

 

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Discussion Starter · #3 ·
The 85 and 110 grain TSX bullets should touch the lands at the same COAL as they have identical ogives (full bullet diameter is at the front of the first band). Barnes recommends 0.050" off the lands with their monolithic bullets though I have loaded them slightly shorter at times. I also found 110 Vmax and 120 SST to have the same ogive and thus same distance to the lands. In the three ARP barrels that I have measured 110 HPBT distance to the lands with, they were between 2.335" and 2.380". Below is a matrix I have kept on maximum COAL for one of my ARP barrels. Lead core bullets have COALs listed 0.010" from the lands, monolithic are 0.050" from the lands.

A lot of good info there, thanks. Which bullets when loaded at mag length or under have you found good accuracy? In the past I have only loaded to 2.3" and under but I have found some mags allow 2.31 and that small amount has made a difference in some cases.
 

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Which bullets when loaded at mag length or under have you found good accuracy?
I believe adjusting COAL fine tunes the bullet's exit point at the muzzle and is an important "lever" to adjust when fine tuning a load in search of a rifle's accuracy mode. One should be able to get good accuracy with most bullets at COALs at mag length or shorter. Loading longer than mag length just gives you a bigger "lever" to fine tune a load. Here are the results of 3-shot groups at different COALs with Hornady's 110 BTHP on top of 28.6gr of AA2200 in resized Hornady brass and 450 CCI primers. The only change between groups was the COAL which was stepped in 0.025" increments: 2.270", 2.295", 2.320", and 2.345" (which was 0.025" off the lands for this rifle). Interesting to see that a COAL shorter than 2.295" resulted in improved accuracy. I expected some variation but the average velocity was very consistent between the groups: 2715, 2719, 2697, and 2715 fps respectively.

 

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Here is another example. After not being satisfied with load development with the 100 AB using varying AA2200 and H322 charge weights with a constant 0.010" off the lands, I decided to try shorter COALs. I picked a powder charge at the desired velocity but at least a 0.5 grain below max and shot 3-shot groups at different COALs, e.g., 2.410", 2.390", and 2.370". Others on the forum have said they can see results with as little as 0.010" COAL adjustment. I'm down to adjusting in 0.015" and still see noticeable changes. I stopped reducing COAL at 2.370" to conserve bullets in this situation but believe good accuracy could be achieved with even shorter COALS (beware of increased pressure with shorter COALS).

 

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If you have checked different 6.8 bullets please post your findings here. Then we'll get into the length they should be loaded to get the best accuracy.
Thanks

If you have not tried the Hornady 110 HPBT set to 2.26" and the 120sst set to 2.31" give them a try.

These are just off the lands in the barrel I am using today.
110HPBT-2.26
110vmax-2.32
120sst-2.31
110NBT-2.37
110prohunter-2.33
130 Sierra SBT-2.388
115-SMK-2.3

IMO the Barnes bullets do not need to be as close to the lands. The 85TSX does not touch at 2.31 and the 110 TSX does not touch at 2.41
The Hornady 110HPBT factory load is why we can't shorten the freebore length further to increase accuracy.
OP, while the idea is a cool one, I'm not sure that having a central repository of "OALs for bullets to touch the lands" would be good for anyone else other than yourself, because all of these measurements are good for your particular rifle only.
 

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I have only started to keep track of distance to lands since loading past mag length with the 3R Triad.
Even at 2.36 with the 16in Bison it was still so far the the lands with everything I just adjusted COAL without worrying and distance to the lands. This was with the projectiles listed below.
Only when I started loading out to 2.41 did I start to keep track of it.

Here's for the 22in 3R Triad barrel with the heavies I have been using.
 

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Discussion Starter · #10 · (Edited)
OP, while the idea is a cool one, I'm not sure that having a central repository of "OALs for bullets to touch the lands" would be good for anyone else other than yourself, because all of these measurements are good for your particular rifle only.
I have about 8 or 900 6.8 barrels at the moment and thought if we get enough of the guys that are shooting my barrels or SPCIIs we could narrow it down to some OALs that work in both types of barrels and help some of the guys that are newer to reloading.
Edited after checking figures again.
There should be about .034" difference between the ARP and SPCII chambers if both reamers are mid spec but, if my chambers are .001 under and the SPCII is .001 over there could be as much as another .038" difference in the location of the lands. Then if the reamers are at opposite ends with the SPCII being .001 under mid spec and the ARP being .001 over mid spec the SPCII could be .004" shorter than the the ARP.
Barrels from the same batch are cut with the same reamer and should be close.
My chambers freebore are .277 and the lands should start ramping up at 1.7964". The SPCII freebore is .278 and the lands start ramping up at 1.8113 according to the drawing Ben with Bison posted in late 2009.
If the SPCII ramp starts at .278 it is .0005 further away from the edge of the bullet on each side. We must figure a triangle with a 1.5* small angle and a .0005 opposite side making both the long side and hyp .019 and add that to the freebore length of the SPCII.
 

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Discussion Starter · #11 ·
I have only started to keep track of distance to lands since loading past mag length with the 3R Triad.
Even at 2.36 with the 16in Bison it was still so far the the lands with everything I just adjusted COAL without worrying and distance to the lands. This was with the projectiles listed below.
Only when I started loading out to 2.41 did I start to keep track of it.

Here's for the 22in 3R Triad barrel with the heavies I have been using.
Right, the jumps are huge with the high BC bullets unless like you we cut the mags. Even longer jumps with the SPCII chambers. Counting on the freebore to align the bullets and be accurate is a shot in the dark. That shorter case thing we talked about earlier comes into play with the long bullets.

One of the guys is trying to get a group buy together for the Oly UMAR lowers which would allow 2.4" mag length for the 270AR. That might be the ticket for the Bergers.
 

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My experience with the 110 BTHP has been very similar to Xman's targets, I run them at 2.265-2.270 and get just about .5 MOA in a PT&G spcii (spc reamed to spcii by me). Haven't played with really long like he has, but it opened up as I went a little more. Mag length and half MOA is plenty adequate for most all I do, even long range as hit percentage will not increase by double if I half the grouping size.

-Alexander
 
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