Da..it, Don't shortchange me! 6.5 Grendel vs. 6.8 SPC

[sumpnz]

OK, that sounds a bit more reasonable, though I would still expect more than 100fps difference, probably more like 150-200fps. As I understand it, with these high intensity overbore cartridges you should expect (rule of thumb) about 30-40fps difference for each 1" of change in barrel length. At a 5.5" difference that would indicate 165-220fps as the expected difference.

What I'd like to see is some chrono data for the 108, and 123 grain bullets from the 18.5" and 24" tubes from the same day. Say 5-10 shot averages, with standard deviation and 100 yard group sizes if possible.

Edit to add: I'd also like to see the 6.8SPC tested alongside the 6.5 Grendel so that we can do an apples to apples comparison without trying to account for changes in temperature, humidity, and pressure.

 

[TX65]

Sumpnz,

The 6.5 Grendel, like the 6 PPC and 22 PPC are not high intensity overbore cartridges. As a rule of thumb, 25 fps is a good approximate, but that is not always correct.

TX65

 

[Grump]

I mis-spoke a bit:

Grump Quote: "I'm still not sold on the 144-gr 6.5mms. It's virtually the same amount of metal overall as the M80 ball bullet, and has a much more loopy trajectory. Goes to slow for me.

that should be a more loopy trajectory as compared to lighter bullets in the Grendel, not as compared to the M80 Ball. I'm lobbying for as much IMPROVEMENT over M80 as we can get! Well, without winding up with a .243 Winchester with short barrel life...

In general, the costs, materials and velocity loss of even the 123-gr 6.5mms may bring not enough* advantage over performance with the 100-105-gr bullets. The best compromise may lie somewhere between 100 and 123 grains of bullet...

We got some Grendel chrono data from different days--who has some SPC data?

 

[TX65]

Did some 6.5 Grendel range testing today using an Oehler 35 Chronograph

Shooting the Lapua 100 Scenar using N530, magazine length loading

28 Inch Barrel - 2900 fps
24 Inch Barrel - 2850 fps

The Nosler 100 Ballistic Tip and 100 Grain Partition were running 2850 fps out of a 24 inch barrel.

Edited to add..

The Lapua 100 Scenar has a .444 BC and is currently on the shelf at numerous dealers.

In an 18.5 Grendel, the 100 Scenar performed at 2775 fps using N530 in a magazine length load.

 

[Zak Smith]

We got some Grendel chrono data from different days--who has some SPC data?

The 6.8SPC Load Data page is linked off my FAQ. See below ...

The control point is the preproduction Remington 115gr shoots 2700-2750fps from my 18" bbl. Note that the BC of the 110gr VMAX is 0.37.

-z

 

[TX65]

Zak,

Since Remington used an OEM powder that is not available over the counter, do you have chrono data from loads you have done using cannister powders (ex,, H335, N133....) and bullets that are available to end users for magazine use (ie ... Hornady 110 VMAX)?

Also, please tell use what type of chrono you use, whether it is a triple or double screen and what screen spacing you use.

TX65

 

[Zak Smith]

TX65,

That is eactly what you'll find on my Load Data page if you follow the link as instructed.

Before I am innundated with questions here, please be sure to read it all carefully.

-z

 

[Zak Smith]

The two chrony's I used for much of that data were a Shooting Chrony Beta Master model, and a Prochono Digital. Default screen spacing. I always shoot some BH75 from my 223 AR15 to make sure they're working.

I wll pony up for a Oehler and build a chrono box after RM3G and ITRC...

-z

 

[TX65]

Thanks Zak, it helps me understand how you measure velocity...

Reading your data, your best velocity with a handload and the 110 VMAX is 2635 fps from an 18 inch barrel.

Would you happen to have the interior width of the case at the base of the powder column?

 

[Zak Smith]

Reading your data, your best velocity with a handload and the 110 VMAX is 2635 fps from an 18 inch barrel.

Well, let's put it this way-- within the limits of the tools I have available, that's the best I have some reasonable assurance is "safe." I have gone well above these, but without some actual pressure data, I'm not going to keep doing it.

I have only listed loads I have personally chrono'd in my rifle and the internal ballistics modeling reports less than MAX pressure. This doesn't guarantee it is under MAX pressure, but that along with the lack of physical indicators of overpressure is a pretty good set of clues.

If your comments about QuickLoad being inaccurate with regard to pressure apply to 6.8SPC, then these could be really conservative. I have no way to tell.

Would you happen to have the interior width of the case at the base of the powder column?

Sorry, no. I don't have the right kind of saw to section a case. The powder volume is around 35.4gr H20.

 

[TX65]

Zak,

You strike me as an intelligent guy so I assumed that the loads you posted are safe... The purpose of my question was to learn of the performance obtainable using off-the-shelf components available to everyone.

Regarding the width of the powder column, my interest in that number is to calculate bolt thrust levels at various pressure levels.

From your measurement of H20 capacity, the 6.5 Grendel and 6.8 SPC have the same H20 capacity.

In a discussion I had with the developer of Quickload, the internal ballistics calculators that allow you to estimate pressure and velocity are just estimators / indicators to give you some ideas of where a load is headed. As you and I discussed offline, Quickload cannot take into account variables in throat dimensions especially on non-SAAMI spec chambers.

 

[Zak Smith]

My point about velocity & safety was that: I'm pretty sure the max loads I'm posting are safe, but it might be that we can go higher safely with available components. Until we have pressure barrel data, I'm staying on the safe side.

I haven't actually measured the water capacity myself. That was a number I got from someone else.

I sectioned a case normal to its axis. The inside width at the bottom of the case is about 0.354", but that's just above the curved transition from base to wall. The case walls are approximately 0.033" thick 0.325" above the case head.

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