.308: Croud -Source advice sought re setback from the lands

[Campbell]

I am new to reloading .308.

My rifle is a Parker Hale 1200TX (7.62x51, No magazine) It shoots nice (thus far out to 600 yards) even with RG ammo & aperture sights.

I have prepped 50 cases with the mind force of OCD. :-[

Today, I confirmed that my rifle's max COAL (using a test FC case and a test Lapua Scenar bullet), is 2.900" (I did 6 tests)

The average COAL of my FG rounds (bought with my rifle and shoot OK) is 2.787. I.e. .113 less than my max COAL.

What setback/COAL would you advise me to start a load development at? My initial thought was to go back 0.020, but since milsurp ammo seems to be OK-ish should I reconsider, and if so, by how much?

Many thanks in advance.

Cam

 

[jlow]

I would not use COAL – cartridge overall length, but BTO - base to ogive length since spire point bullets tend to have significant variance in COAL but much more consistent BTO length.

You also cannot use COAL if you are measuring with one bullet and reloading with another since the shape of their ogive will be different and in the end it is using BTO measurement with the same bullet that you will get relatively accurate distance for when the bullet will touch/jam into the lands and that is when you will get abrupt changes in pressure - something to avoid if you are not prepared for it.

If you can measure accurately, I have used a starting BTO of 10 thousands to avoid any chance of jamming and go shorter from there.

 

[Keith Glasscock]

Crowd sourcing might get you into trouble when it come to loads. That said, your rifle likely won't shoot like mine, so I'd say to pick a spot and go from there. We are all guilty of having arbitrary starting loads.

 

[Lapua40X]

Re: .308: Crowd - Source advice sought re setback from the lands

As soon as you change bullets you'll change ballistics outcomes. As jlow and Busdriver aluded, advising you how to proceed with seating depth would, IMO, be a reckless endeavour. I too would recommend you abandon the COAL and rely on the CBOL to establish a starting point. CBOL isn't magic of course. Even using that measurement you'll find bullet lengths. weights and shapes will make a difference in how they perform and they will perform differently in your rifle than they might in mine. From the data you shared it would appear that your rifle likes to give the bullet a good head start (jump) before marrying it to the rifling but that too may be just one sweet spot over a range of nodes you may yet identify.
For the sake of clarification, could you define "FC" , "RG", and "FG" please.

 

[Campbell]

Many thanks for all of your replies.

As usual I am expecting you to read my mind!

What I have done is built a dummy-reference round using a prepped case and a chosen Lapua bullet and have set the COAL according what I measured using the chosen bullet. I am going to use a comparator to set all live rounds, but my question is about how far back from the lands should I start.

If I change type of bullet, I will of course re-measure for another dummy-reference round.

RG rounds shoot OK set back 113 thou, but that seems to high.

Many thanks again.

Cam

 

['Freak]

I single load everything so can start with the bullets seated out at the point of first ogive contact with the lands, or for short stuff and/or longish throats, as close to the lands as can be and leave the neck hanging onto at least ~ .020” of the bullet’s bearing surface.

 

[Campbell]

Many thanks to you all.

I have now built my reference cartridge with exactly the same COAL as measured in my rifle, and have taken a reference measurement from the ogive using a comparator.

Cam