Reloading the 6.8 spc take 2

I have been reloading and learning and I thought I would share my results with you all. I worked up the loads published either in the powder or the bullet manuals, or in the case of SPEER 130 gr SPBT I used the data published by other reloaders as my maximum loads.

All the data have been collected over two shooting sessions in 43 and 64 F average temperatures. I used the Caldwell chronograph and the companion ipad app to record the speed of the bullets. I had troubles getting the chronograph to work consistently as clouds were moving in and out so some speed data is missing. Also whenever I wasn’t able to record at least five shots the standard deviation was not calculated. Similarly with estimating the group size if I did not fire 3 or more shots the group size was not estimated. The group size was estimated as the diameter of the largest circle that can fit over a group, or in other words, the distance between the two bullet holes that are furthest apart. I always used the center of a bullet hole to make the measurements.

All the shots were fired from the same rifle with a bison armory barrel: 20 inch stainless 1:11 twist 4 groves, 11 degree crown and rifle length gas system with SPCII chamber. (see http://www.mdshooters.com/showthread.php?t=139167)

I am reporting results for Hodgdon 322, Benchmark, IMR 8208 XBR all with CCI41 primers.
I loaded the Hornady 110 gr V-max, 120 gr SST and the Speer 130 gr SPBT.
The Hornady 110 gr V-max measures 1.0350-1.0385' in length with 0.37 BC
The Hornady 120 gr SST measures 1.117-1.120’ in length with 0.400 BC
The Speer 130 gr SPBT measures 1.0800-1.0905' in length with 0.449 BC
All are 0.277’ in diameter.
For all the reloads I used the Hornady 6.8 spc case once fired trimmed to 1.675’. Its weight is 119 grains and its water capacity is 36.4 grains.
All the bullets were loaded to 2.294-2.300’ case overall length.

Just for comparison below I am listing the speeds and groups for the factory loads I had on hand (all at 100 yards):

Hornady 110 V-max : fps: 2701 +- 39 group: 0.759’
Hornady 120 SST : fps: 2652 +- 17 group: 1.1’
S&B 110 gr FMJ : fps: 2733 +- 19 group: 0.751’

Now for some reason these loads were pretty hot in my rifle with the Hornady 110 V-max the hottest with a couple of cases failing to eject. Even though the S&B was the fastest it was the mildest of the bunch (probably uses a slower powder). Is it possible that factory ammo manufacturers stopped loading for the older SAAMI chambers or my 20 inch barrel makes such a difference?

Please note that the comments and conclusions I make only apply to this caliber and my rifle. You might experience different results in other calibers and rifles.

Speer 130 gr SBT and IMR8208XBR

I couldn’t find official data for this bullet for the powders I have so I used the load data from a 6.8 forum shooter as my maximum load. I discovered that H322 is too fast for this bullet (see discussion here http://www.mdshooters.com/showthread.php?p=3154776). Also I don’t have speed measurements so I am only showing IMR8108XBR data for this bullet. The bars are the mean speeds I recorded and the error bars are the standard deviations. The connected points are group sizes for that particular powder weight at 100 yards. The color green indicates when a case is filled at 100% capacity and orange indicates that it was a hot load with partially flattened primer. The dashed line is a linear fit to the first 5 points.

Note that bullet speed is linear to powder weight until the case is 100% full then compression causes the powder to burn more efficiently and changes the pressure curve shape, hence the larger than expected speed and increased pressure rate. Also overall the group size grows as the powder weight is larger and then drops when the powder completely fills the case. This might make a case for adjusting the bullet seating depth to maintain 100% capacity whenever possible.

This is a slower burning powder compared to H322 and Benchmark with a flat pressure curve, as long as it is not overly compressed (see my comments for the 120 SST). This bullet is the shortest 130 gr bullet I could find with 0.449 BC so it can pack almost as much powder as the 110 gr v-max. Because of its heavy weight it performs better with a slower powders with a flat pressure curve. I stopped shooting the moment I saw primers starting to flatten. In terms of pressure signs, the pressure even at 29 gr IMR8208XBR was not excessive when compared to the factory Hornady or S&B loads.

The best group was shy of 1 inch which was better than the 1.68 inch I got in the past with H322. The speed standard deviations were also pretty tight when compared to the H322 powder. In the future I might also try the Hodgdon Benchmark powder.

Edit: I am attaching a picture of the case heads that can be an indication of pressure for this data. Light to heavy powder from left to right, each corresponding to the points on the graph. Note the two loaded rounds had larger amounts of powder but were not fired as I wanted to be conservative this time round. As you can see no pressure signs. This powder is very mild as long as it is not compressed too much. You will see later that with less powder and lighter bullet (see the 120 gr one) the pressure signs are more pronounced.

Also note the flattened tip of the first round: it was in the magazine before I stopped shooting and was flattened due to recoil. Lead is pretty soft and flattens pretty easily. I played around with smaller COAL, down to 2.280' but I still get flat tips with this bullet. I guess it wasn't designed for the tight AR magazine...

Hornady 120 gr SST and IMR8208XBR

The IMR8208XBR didn’t perform as well with the Hornady 120 gr SST bullets. Even though I got groups less than 0.9 inches the speed standard deviation was pretty high and I got strange speed patterns when the powder was compressed >100%. As you see from the plot after exceeding 100% capacity at 28 gr the speed goes up at a faster rate and group size goes down as expected, but for 29 and 29.5 gr the speed remains effectively the same, but pressure goes up at a much faster rate and at 29.5 I only shot one round as I observed a partially flattened primer. This behavior puzzled me and here is what I think happened: because the 120 SST is so long compared to the other bullets I loaded, it compressed the IMR8208XBR powder quite a bit changing its pressure curve, forcing it to burn faster and shifting the peak towards the breach. This raised the pressure but not the speed.

The best group was shy of 0.9 inches which was better than the factory loads but the average speed and standard deviation were not as desirable.

Edit: I added a picture of the fired rounds left to right low to high mount of powder. Because of the strange bullet speed behavior, the moment I saw a flattened primer I stopped shooting. This powder behaves erratically when compressed too much.

Hornady 120 gr SST and Benchmark

I was impressed with the performance of Benchmark with this bullet and as you will see later with the 110 gr bullets. However for this group of loads my chronograph was misbehaving and I only managed to record the speeds and std of the last two powder weights. As you can see the groups get tighter as the case fills up at 100% capacity at 28 gr, the standard deviations are tighter and the groups don’t open up when the powder is compressed. I saw no pressure signs for this series of loads. This might be a good powder for this bullet for weights between 28-28.5, as groups are almost constant giving confidence that things will not change dramatically when the temperature is lower or higher.

Hornady 110 gr V-max and Benchmark

Benchmark gave impressive performance with the 110 gr bullet as well. Very linear velocity vs. powder weight, very low standard deviations overall and tight groups with the best group < 0.4 inch (much better than the factory loads), and velocities close to the factory load. Even with 30 gr I have not seen any pressure signs, with no swipes, and normal looking primers. This is a good overall powder for this bullet.

Hornady 110 gr V-max and IMR8208XBR

My chronograph started acting out again and only managed to get 2-3 speed measurements per load. Nevertheless, this powder also seems to be performing well with this bullet with no pressure signs. It is not as efficient as Benchmark, but the groups are tight.

Hornady 110 gr V-max and H322

Again my chronograph was acting out and only managed to get 2-3 speed measurements, except for the last load where no speed measurement was taken. However given that for 29.5 gr the case is filled at 100% capacity I expect the speed will still be in the linear domain, hence looking at the dashed line fit the speed should be about ~2683 fps. Again I saw no pressure signs.

The speed is not as high as the Benchmark for the same powder weight, probably because Benchmark is slower and accelerates the bullet longer in the 20 inch barrel.

Summary and conclusions for my latest loads

These two graphs summarize the load data for comparison purposes. This first one shows velocities and group sizes for all the loads described here. The second one shows velocities and standard deviations for all the loads. It is useful to present the data this way as I can pick a velocity I want and determine the powder weight for all the powders I have so that I can produce the same performance, assuming that the group size and pressure are acceptable.

So far for the 130 gr Speer bullet IMR8208XBR works the best, for both Hornady 110 and 120 gr Benchmark works the best. For the 110 gr bullet I was able to find more accurate loads than the factory loads for the same speed. For the 120 gr bullet I was able to find fairly accurate loads but at a lower speed than factory. I am curious what kind of powder Hornady is using for such a long bullet to achieve 2652 fps with normal pressures.

Very nice work this time. I cant verify charge levels as im not at home but I can confirm they are a fair amount higher than my loads.

Benchmark as shot well for me but the one I think you are missing and need to try RL10X. It had thr best results I've obtained to this date.

Otherwise nice work and nice write up.

stu929 said:

Very nice work this time. I cant verify charge levels as im not at home but I can confirm they are a fair amount higher than my loads.

Benchmark as shot well for me but the one I think you are missing and need to try RL10X. It had thr best results I've obtained to this date.

Otherwise nice work and nice write up.

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Thanks,
I try not to load higher than what manuals recommend. The only place I find powder that is not too far from my house is the National Gun Show in Chantilly. For some reason they never carry RL10X... nor Accurate powders. I ll keep looking...

Im not trying to win any speed races just find accurate loads so if I find one on the low side im fine with that and dont load up to max. Save some powder and some recoil. All the hunting ill do with it is at 100 yards or less so not worried about max speed that much.

stu929 said:

Im not trying to win any speed races just find accurate loads so if I find one on the low side im fine with that and dont load up to max. Save some powder and some recoil. All the hunting ill do with it is at 100 yards or less so not worried about max speed that much.

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Same here. I wanted to collect this data to explore the whole space. My hunting round will be the one with the best accuracy and lowest std.

Glock nice write up again. I keep an eye on this because I will be loading for a 6.8 eventually. I was down at Maryland ReloadR in Pomfret last week and he had Accurate powders. Give Bob a call to see if he has what you need.

Again, get some Silver State Armory brass. More room in the case than any other brass out there.

jimbobborg said:

Again, get some Silver State Armory brass. More room in the case than any other brass out there.

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Why would you do that?

jimbobborg said:

Again, get some Silver State Armory brass. More room in the case than any other brass out there.

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Higher case capacity = lower pressure for same powder weight.

I am waiting for their price to drop a bit. The other day I bought 200 rounds of S&B for $15 per 20 rounds. Since I have been watching the price of 6.8 ammo, I have never seen lower prices. It seems they will drop some more.

By the way do you have any data on grains of water capacity for SSA brass?

According to my notes:

• Hornady: Case Length: 1.675 = 36.6 grains of water
• S&B: Case Length: 1.679 = 36.4 grains of water
• Federal: Case Length: 1.680 = 36.0 grains of water

Overboost44 said:

Glock nice write up again. I keep an eye on this because I will be loading for a 6.8 eventually. I was down at Maryland ReloadR in Pomfret last week and he had Accurate powders. Give Bob a call to see if he has what you need.

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Thanks for the tip. That's a bit of a drive from here, but there are lots of sightseeing in the the neighborhood to justify the drive.

I understand what the difference in volume causes just not sure why to do it. Just load them a little lighter.

Word of caution though. Heard a lot of people unhappy with hornady brass recently due to shorter case life and failures after just a few reloads.

Lwrc has great prices on 6.8 brass...30c a piece shipped.

stu929 said:

I understand what the difference in volume causes just not sure why to do it. Just load them a little lighter.

Word of caution though. Heard a lot of people unhappy with hornady brass recently due to shorter case life and failures after just a few reloads.

Lwrc has great prices on 6.8 brass...30c a piece shipped.

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Yes, I agree about keeping the pressure and the powder low. I did notice that the latest Hornady cases were much harder to resize than ones I bought in January. Also there were significant differences between two boxes of ammo in terms of speed and group sizes.

Have you visited the 6.8 Forums? http://www.68forums.com/forums/ They have all of the data you could ever want on the 6.8 SPC, including internal volumes of all cases, loads, and why you want to use SSA brass versus the rest.