Load development for same barrel length?

A

armybuck041

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Hey Guys,

I little while back I cooked up a nice load for my .308" Sig Cross with the 16" factory barrel. Essentially running a very spicy 130gn Barnes TTSX over 49.7gn of Varget at a COL 2.840". Was giving me a nice repeatable 1/2 MOA (5 shot group) at 100 yards and a nice velocity of ~3010fps with an SD of 8.6fps.

I love the Sig Cross platform enough that when the Sawtooth version of the same rifle with the 16" Proof Research carbon fibre barrel came out, I couldn't resist. It's just under 8lbs fully loaded with a Leupold Mark 5HD 3.5-18 in Leupold Mark 4 rings. A delight to carry. Shoulders beautifully.

So, got it all setup and headed out with a box of my pet load 130gn TTSX...

Long story short. Getting roughly 2 MOA with the same load. Fired about 40 rounds and called it a day.

Looks like I need to go through some load development again.

So my question is, given that it's the same length barrel, does the brain trust here at CGN think I skip the charge weight portion of the load development and just focus on seating depth? Or do I need to repeat the complete process of finding a charge weight node followed by finding the seating depth node?

Thanks!

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Interestingly enough, I just checked the COL to jam on this barrel and it’s longer than the magazine length, whereas the original Cross wasn’t. I’ll start making up some ladder powder charge loads using the magazine length as the COL (2.885”ish) and go from there.
 
49.7 gr of Varget seems like a lot of powder and is .5 gr over Barnes published recommended Max compressed load. What brass are you using and what does a fired case look like?
Barrel length has nothing do with what load will work in one barrel and not the other. One barrel is SS the other is SS/carbon wrapped. Think about a tuning fork. Barrel harmonics have more to due with accuracy than max velocity.
You can have 2 barrels made one after the other from a manufacturer, same length, same bore, same groove, same twist, same chamber, one barrel will shoot faster than the other.
 
As Maynard has stated, two barrels of identical dimensions will not automatically shoot the same load well.
You may be able to use your original data as a benchmark in some instances, but I would start from square one, myself, especially since you are right t the upper end of the load for that combination.
Cat
 
Thanks for the comments fellas. This all makes sense.

Case in point; chamber (jump to the rifling anyway) is quite a bit different on this new one compared to the previous one, not to mention, the barrel harmonics changing from barrel to barrel as you guys pointed out.

I put together a ladder of powder charges from the bottom to the top of the table (3 rounds for each 0.3gn increment) in order to start the process over again. Loaded them all to the max COL that functions in the PMAG that comes with the rifle (2.882") which is still well off of the lands.

Never had any issues with the load mentioned above. Agreed it's at the top of the scale, but I've had no appreciable pressure signs and I've fired well over 100 at this point. When I was going through the load development last time, it was evident that these 130gn TTSX liked to be pushed hard, at least in that particular rifle. Got two distinct nodes; one at around 48.7gn and the other at 49.7gn. The latter picked up approximately 150fps and the SD dropped down to under 10fps consistently. Given it's a 16" barrel, I wanted every last bit of energy I could safely and accurately pull from this load.
 
nodes don't exist, so I wouldn't devote too much time and components in trying different seating depths or charge weights.

Every barrel can like different bullets / powders. If you're set on the 130 TTSX, I'd try a few other powders and see if one stands out. If nothing works better, try a different bullet
 
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