Reloading Tech: Wiring up a 6.5 Grendel

Happy Thanksgiving everyone!!

I hope all spent time with family and friends, and most importantly realized that we all have so much to be grateful for!

I’ve had the PressureTrace™ II system for a few years now and I’ve found it handy in examining the pressure characteristics of my reloads.  It’s been particularly handy in determining the pressure differences between two projectiles of the same weight using the same powder charge.  I’ve only used the system on my bolt guns up to now. I finally wired up my new 6.5 Grendel AR pattern rifle build, the details of which I can share in another post.

In terms of published data there still isn’t that much out there compared to other cartridges, but new data is coming out all the time.  In my estimation Alexander Arms® produces the best factory ammunition for the 6.5 Grendel since it was Bill Alexander who spear headed the design of the round.

Factory 123 gr Lapua Scenar Load

I wanted to use the Alexander Arms factory loads to compare against my loadings for my Grendel.

Since I had some time off yesterday and it was raining, I decided that I would get some load testing done under cover with the range all to myself.

Conditions:
Temp: 42ºF
Baro. Pressure: 29.83
RH%: 97%
Equipment:
Pressure Trace II System
Magnetospeed Chronograph

***CAUTION The data presented below is by no means meant to represent data published via SAAMI approved testing or test methods, and may not reflect the true pressure; this pressure testing is only meant as a comparative tool***

If you are curious as to what a Pressure Trace system is, check out the vendor website at Shooting Software

The barrel I am currently running is a Shilen Stainless Match HBAR 20″ 1:9″ twist featuring a rifle length gas system.  Its worth noting that most bullets in the 123 gr weight aren’t optimal in a 1:9″ twist barrel due to marginal theoretical stability, with the exception of the Hornady 123 gr Match ELD projectiles.

On to the data:

SAAMI MAP (Max Average Pressure): 52,000 psi

Pressure readings I’ve collected from Factory Alexander Arms 123 gr Scenar rounds following an estimated offset of 13000 psi added to original pressure value measure by the pressure trace system.

I’ve chronographed these rounds in 75-80ºF outside temps and they are cruising at about 2510 fps ±10fps out of the 20″ bbl, which is a fair amount faster than at 42ºF as we see here, so I think my estimation on the offset was correct. My aim is to establish the summer time pressure readings of the factory loads as my working maximum pressures.

Pressure readings from IMR 8208 XBR with a 26.5 starting load (not shown here) in 0.5 gr charge increments out to 28.5 gr (also not shown because of pressure signs of previous loadings.

Its clear that I’ve reached high pressure characteristics and any increase in charge weight would unnecessarily strain the Grendel and could eventually approach an unsafe condition.  I now believe I’ve established a safe maximum of 26.0 gr of IMR 8208 XBR, at least in these low temps. Surprisingly the charge to pressure correlation peaked at a lower charge weight than I would have expected based on published data out there for a similar weight projectile.  Published IMR loads with the 123gr Sierra Match King peak at a maximum charge of 28.5 grains (compressed).  I may have to re-adjust the charge downward during the summertime temps to stay consistent with pressures if I decide to stay with this load.  This is another reason why it’s always smart to back off by 10% (I started 7% from max). That being said, this is an accurate powder with 3/4″ groups.  I’ll likely study other powders as well for comparisons sake, such as Hodgdon CFE223 and Accurate 2520, both known for velocity production.  There is little data on CFE223 use, this is where the PressureTrace™ II system will shine, now that I have a baseline.

26.5 gr 8208 XBR 123 gr Hornady ELD Match vs Factory Alexander Arms 123 gr Lapua Scenar

Based on the images above, the Alexander Arms rounds are close in appearance to when I’ve fired them in the summer time, so these marks may be more associated with chambering.  One thing I’ve noticed is that sometimes just chambering a round produces some ejector swipe marks.  I think the marks on the reload to the left suggests that I should back off to 26.0gr and call it a day.

If your range has a covered firing position and it’s raining out, get outside and work your craft and understand your gun/ammunition combination, you just might have the range all to yourself

Look out for a companion piece to this article, as I purchased a 24″ barrel made by Satern Barrels and sold by Brownells, testing will commence with this shortly using the pressure trace system.

Until next time….

See you at the range!

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