Does all caps indicate how strongly you feel about this?
JK. thanks for the feedback. I like bugholes.
Looks like I need to consider even longer FB. What is your COAL?
thanks,
Jerry
That's good.
The 0.169" fb works well with the Berger 90 VLDs and the Sierra 90s. You can still load something like the Berger 80.5, but it will be seated fairly far out in the neck to keep it within about 0.020" of touching the lands. You can make a reasonable estimate of fb required for a given bullet relative to the 0.169" fb using either the bearing surface length or BTO measurements. Check out the following bullet dimensions from Bryan Litz's book, "Ballistic Performance of Rifle Bullets":
Berger 80.5 Fullbore bearing surface = 0.317", BTO = 0.589"
Berger 90 VLD bearing surface = 0.366", BTO = 0.622"
Sierra 90 SMK bearing surface = 0.393", BTO = 0.638"
To keep the 80.5 Fullbore boattail/bearing surface junction at about the same place relative to the neck/shoulder junction as a 90 VLD optimized for a 0.169 fb, you would want a fb of about 0.040" to 0.050" shorter, or somewhere in the neighborhood of 0.120"-0.130".
These numbers may be somewhat variable because of lot-to-lot variance in the bullet dimensions, as well as the ultimate seating depth you find to be optimum. For example, I am seating the 90 VLDs in the 0.169 fb chamber at 0.015"-0.018" off the lands. At this seating depth, my COAL is ~2.641". However, many people have found their optimal seating depth with this bullet to be about 0.010" into the lands. That is a difference of at least 0.025", and would yield a COAL of about 2.666". I find with the 90 VLDs seated at ~0.015" off the lands in the 0.169" fb chamber, the boattail/bearing surface is just barely above the neck/shoulder junction. In other words, at my seating depth of ~0.015" off the lands, I could have easily gone with a fb in the 0.195"-0.200" and it would have been fine.
The Berger 80.5 Fullbore and 82 gr, and the Sierra 80 SMK and 90 SMK, are all tangent ogive bullets. That means it's probably a fairly safe bet you can get them to shoot well at somewhere between perhaps 0.010" and 0.025" off the lands. The Berger 90 VLD is a secant ogive bullet, which is probably a big part of why many people find it wants to be seated into the lands. All you really need to do is make a reasonable estimate of the fb length you need for a particular bullet that gives you some leeway in either direction, so as to take potential differences in seating depth into account. If necessary, I would generally err on the side of caution (i.e. have the bullet seated a tick deeper in the case, which equates to slightly
shorter fb). It might increase pressure a bit more than necessary, but that's better than having a bullet seated way out with very little neck grabbing it IMO. You can usually make a very reasonable estimate with a few dummy rounds with the bullet seated such that the boattail/bearing surface junction is well above the case neck/shoulder junction, but somewhere below the halfway point of the neck.
