Hornady GMX opinions....

[Handsome Rob]

I guess this is 50/50 reloading/hunting related, but does anyone have any real world experience with Hornady's GMX, specifically in .308?
I'm struggling to find any load data. Hornady says to use the same data as the SST bullet, but GMXs are much longer & take up a LOT of case space. Even at SST middle range loads I'm getting a bit of powder crunching. I daren't even try max loads!
I'm limited by magazine length (2.840"), so seating closer to the lands is not an option.
I'm wondering whether I ought to try loading to the next SST size up (165gn data for 150gn bullets)

I'm trying to load 150gn to achieve 28-2900fps from a 20" barrel, which looking at my old 168gn bullet data shouldn't be a problem, with Varget or 4064 however they're both fairly bulky powders & I'm concerned about pressure spikes.

[Hozzie]

Rob, I ran some numbers in QuickLoad and I don't think you will get to 2800 with Varget or 4064.   Can you provide some additional detail.  what is the Case length and can you get me the average number of grains of H20 each case holds.  This is really the only way to get exact numbers.  I am using a COAL of 2.840, but if you plan to load shorter than that, let me know.  You can see by the graphic below that this load with Varget is already above case capacity, but this is based on knowing nothing exact at this point.

 

Edited April 12, 2016 by Hozzie

[Handsome Rob]

I'll have to grab a sample & find out the average h2o volume, but I've got the cases sized at 2.125" (I think!)

[Handsome Rob]

Welp, it seems like 44.5-45gn of 4064 is the ticket in my Lightweight Hunter!

I shot 4 shot groups with each load & 44.5 & 45gn gave me sub 1" groups at 2831min-2869max velocities. No trouble with extraction, primers looked good. I was expecting pressure signs right off the bat, but right up to 45.5 (still a mid range SST load, but compressed with the GMX) everything looked great!
Top velocity at 45.5 was 2938
I've got a little room to play with seating depth (I underestimated on mag length: I CAN run right out to 3"!) & I reckon ½" isn't out of the range of possibilities, although as a pure 300yd & under elk gun, 1" is more than good enough.

[MacGyver]

This thread is worth the price of admission.

[Hozzie]

Rob, I know you know what you are doing, but it looks like you are actually over pressure even if it isn't showing it in the case.  Just want to provide the info so you have it.  I used the case length you gave at 3.0 COAL with 44.5g of 4064.  Pressure is above 64,000psi.  You might have a bit more case capacity than shown, but even then it doesn't go down much.  Shorter COAL really pushes the pressure.  At 2.850 it went above 70,000 PSI.   You can click to get a larger image. 

 

I will say, when I measure everything exact in my loads and put it in here, I am usually well within 60fps and more commonly 40fps, so I trust this.  I am paranoid about pressure is the only reason I am questioning some of the numbers.  Don't want to see anyone get hurt.

 

Edited April 14, 2016 by Hozzie

[Handsome Rob]

Dammit.

Thanks Hozzie. I did think I may be pushing things, but nowhere near that hard!

Could you possibly run me up a full spectrum of powders & velocities on QL?

What exact info do you need to do that?

[Hozzie]

I am not sure to be honest on running full spectrum velocities.  If you can give me exact (averages) Cartridge OAL, Case OAL, bullet length (may be different than what is default), and H20 capacity, I can easily run 2 or 3 different loads of 4064 and Varget for comparison.  I can also try some other powders.

 

Not pushing, but I suspect you are like me in that you don't want to spend $150 on QL, but let me tell you, it is some of the best money spent of anything I have bought for reloading. 

[Handsome Rob]

Thanks again Hoz.

I'm going to resize, trim & anneal a bunch of matching headstamp brass over the weekend so I've got a little uniformity to work with.
I don't know why I never considered it before, I guess it just didn't occur to me, but I've never actually sorted my .308 brass. It's not a round I shoot for match accuracy so I just have a bucket of mixed headstamp stuff.

That'd go a long way to explaining my velocity differences I guess. I just checked what I had loaded yesterday & there is Remington, Hornady, LC, Winchester, Norma, Nosler & Federal stamps!

I'm going to stick with 2012 Lake City (seeing as I have hundreds of them) from now on.
I'll pm you the specs as soon add I have them prepped

[Hozzie]

Sounds good.

[Alleycat72]

I have QL on my computer. If you need to look at it one day I can bring it to the range and let you play with it.