Question on timing a new bolt

[robertb]

There have been reports that 155 Scenars in a factory barrel don't do as well due to their dia. You may have better luck with SMKs out of your LTR.

 

[eshell]

Adam, I don't have any numbers, but the length, profile and ogive of the 155 Scenar (specifically) is extremely close to the 175 SMK. The actual contact point to your rifling is such an exact/critical number that is dependent on several chamber dimensions and leade angle, it is hard to predict (for me) how they'll set up in any given gun. As suggested by robertb, it might make the most sense to focus on the 175 SMK and simply try the 155s at some point.

There have been reports that 155 Scenars in a factory barrel don't do as well due to their dia. You may have better luck with SMKs out of your LTR.

Very true, they can be extremely sensitive to factory ("generous") throat geometry and your rifle will typically either love them or hate them. I have found Scenars more "forgiving" than equivalent Berger VLDs in both the .308 and the .264 diameters. If they are compatible with your ball seat/leade diameter, their inherent accuracy potential is excellent and the BC vs weight advantage in the .308 Win is very real.

 

[AdamZx3]

Thanks that sounds like a good thing to do, try the 155's at some point.

One thing I'm not real sure about is what exactly the ball/seat, leade, is. Where can I find information that explains more about this? I thought the throat length was all I needed to know. From what I can gather with the search tool, Leade is where the leading edge of the throat is sloped in a little?

I hope the DVD series "Building a Target Rifle" by Steve Acker will shed some light on that....it's back ordered for a while on smartflix.

I will be chambering a Krieger barrel, but i'm going to try and get the reamer ground to the specs for the SMK cartridge, and reload later on (and maybe use the SMK for matches)

 

[eshell]

. . . One thing I'm not real sure about is what exactly the ball/seat, leade, is. . .

The "ball seat" area is also often referred to as the leade, and might be considered the "freebore", but, the term "freebore" is usually associated with an exaggerated length such as seen in the original Weatherby chambers. The ball seat is the unrifled area just forward of the case mouth, that transitions into the rifle's bore via the area called the "leade" or throat, where the rifling begins at a slight taper from "no rifling" to "full depth". The leade angle is the included angle that is sometimes cut to various configurations provide best fit for certain bullets.

This diameter of the ball seat area is the product of an engineering compromise: If it is small enough and tight enough to provide zero clearance and 100% perfect support/guidance of the smallest/shortest bearing bullets, it is possible to develop dangerous pressures with bullets on the larger side, and/or with any fouling that will reduce necessary clearance. This area is usually designed to provide working clearance for bullets that will run, in the case of a .308, typically between .3075" and .3085" or so, yet provide minimal opportunity for the bullet to yaw. . .

In the context used here, a larger diameter ball seat/leade area can allow certain bullets with smaller diameters and shorter bearing surfaces (VLDs & Scenars) to develop in-bore yaw, and begin the entry into the rifling in a yawed attitude that negatively affects BC and accuracy. Larger diameter bullets with longer bearing surfaces (SMKs) will have less room to yaw and produce more reliable accuracy in a wider variety of rifles. A rifle may be chambered well within customary specs and tolerances, but, if the combinations of throat geometry is such that a VLD or Scenar can develop a little in-bore yaw, it may not deliver the accuracy one would get with an bullet like an SMK.

If you take a look at this handy reamer drawing, it is the area labeled "lead" and shown as a diameter of .2435" (giving it a clearance of .00025 on each side of a true .243 diameter bullet):
http://www.6mmhot.com/6mmHOT_Reamer__85-107_gr_Bu.html

Where can I find information that explains more about this? I thought the throat length was all I needed to know. From what I can gather with the search tool, Leade is where the leading edge of the throat is sloped in a little. . .

Precision Shooting magazine has had several very good articles on this, and here is one on-line reference:
http://www.riflebarrels.com/articles/50calibre/throats_50_bmg.htm

 

[AdamZx3]

Thanks Eshell!!

That explanation helped a lot, as well as the other artilcles. It is much appriciated!