BallisticsGuy
Well-Known Member
6XC Load Development - Analysis Time
For those unaware, I've recently set up 2 identical barrels in 6XC for an experiment on barrel life. Both barrels were cut on the same reamer, profiled identically and I'm going to be setting it up so whatever load I choose has to perform nearly or actually identically in both. One will be mounted to my Coach's PRS rifle next week. One is already mounted to one of my match rifles, formerly known as "The Hot Dog Gun". Coach's rifle already has a 6XC barrel on it with around 1500 rounds down the pipe so it's within a few months of being done for anyway.
My barrel will be treated to HBN coated 115gn DTAC's at 29xxfps. Coach's barrel will be treated to Modern Spartan Systems complete cleaning system, including treating the bore as they direct with their "Accuracy Oil". We'll be tracking velocity, group size and throat erosion long term and using those measurements to report on barrel life. MSS suggests large increases in barrel life and increases in velocity when you treat your barrels with their products so I'm giving them the old Pepsi challenge. They're not paying me and we're not affiliated. I'm curious and a nerd and those are some pretty bold claims but if they're true then it'll save me about $500 a year on new barrels for my assorted match rifles.
This little case seems to really like being as full as possible and/or run a little hard and put away a little bit wet if you get my meaning. We broke in the barrel with 15 shots but as you can see from the data below, around shot #6 things stabilized. By round 10 I had warmed up the barrel a bit and was vacillating between baking rounds in the chamber while I wiggled around trying to get a natural point of aim and firing quickly when I was already at a good NPoA.
All discussions of load data and charge weights come with the "don't copy me and hurt yourself" disclaimer. Don't just run my loads, work up to them carefully. These are all on Norma brass, F210 primers, 115gn DTAC bullets and COAL at 2.8". Note X axis labels are actual shot number.
We hit the range with 80 rounds. 5 of each with 10 of the N550 and 4166 loads, just in case. After grinding out the first 15 rounds of Coach's match load (38.5gn H4350) to break in the bore and establish a zero; this was a BRAND NEW barrel after all, we took a little break and went to check the target. The new barrel shot to such a different POI than the prior barrel that it took quite a few shots just to get on steel at 100yrds. By round 10 we were on steel at what seemed like pretty close to POI=POA. Enough to move from steel to the BoxToBench Precision 100yrd Load Development Target and dialed the zero in on the cold zero aiming point. 5 rounds at the cold zero put us at 15 shots and we were already seeing each set of 2 bullet holes (because: adjust, fire 2, adjust) either touching or very close to it. We're pretty excited about the performance we're seeing out of the gate.
After the first 15 shots and letting the gun cool down I settled in to go for groups for record. Starting off we did the Coach's match load (CML). Then the RL-23 was run followed by N550 and IMR-4166. To wrap things up we came back to the H4350 and did the 39.5gn load then finished out our paper punching with 5 at 39 grains. After that I had 5 rounds left and wanted to drop a shot on the 900 yard target so we went up there and I rang the gong for 5 rounds of 38.5gn. There are called flyers (obvious) on some of aiming points. I wasn't in the most stable position and I knew it and you can tell.
Groups are nice and tight. There are called flyers there. The 38.5 group was shot out below of frame.
My velocities with Coach's match load are a solid 150fps above what Coach gets from his Enfield rifled barrel of the same length. Ok, to be completely transparent, it's not EXACTLY the same load. We do actually seat the bullets about .100 deeper for my new barrel than Coach's old barrel but I can't see 150fps difference from that. This is the polygonal rifling in full effect.
So now on to the powder results. H4350 you see the curves change shape as you fill the case up. To my eye it almost looks like someone's grabbed on to the right side of the graph and started pulling the string taut. Group sizes went down as powder charges went up but we're talking about going from a .75" group to a .71" group to a .3" group. The academic in me is crying out to be let loose with a scale and all of my reloading supplies to do a 1/10th grain at a time experiment. But, that's expensive and I have other matters to attend to. The experienced rifle shooter in me says, "You do realize that any one of those is sufficient for the 1000yrd stuff you're doing right?" The competitive rifle shooter in me says, "Take the 39.5 and let's go home and load ammo before you change your mind again."
Onward and upward. We still have loads to analyze. Many know that after my experience with it in .243AI and 6.5x55AU that I'm a big fan of Reloader 23's ability to make big velocity numbers and low pressures on highly overbore cases. It's sloooooooow burning and has been returning impressive velocities with reasonable pressures from very heavy for caliber bullets in relatively long bores from very overbore cases... as you would expect it to do if you are at all familiar with Boyle's Gas Law. We had no idea how much to start with so we did exactly what Coach did with it for my .243AI. We filled the case up to the body:shoulder junction, dumped it out and weighed it and put that much into 5 cases. It came out at 38 grains with no drop tube, just a funnel and a weighing pan.
Reloader 23 showed me with my .243AI that it likes a full case (I'm sensing a trend here with these slow burning magnum powders) and that it's pretty hard to put enough into a 6mm case based on a .473 case head to blow the damned thing up. 38 grains produced pretty nice velocity. A testament to the efficiency of the 6XC case. Still with 28fps SD's, 66fps ES and a .68" group of 5, it would "do" but I'd want to develop it more if I were to use it. We did find that RL-23 is a great option. Somewhere around 40 grains should give high 2900's at reasonable pressures even when seating bullets deepish. If I had another keg of it on hand I might tweak with it a little but RL-23 meters like garbage so that's a factor in the ultimate decision.
Those velocity spreads are too wide and too random.
Pressing on, we have N550. A double base NG/NC powder known for being a little temperature touchy after 90F and for being pretty darned expensive. 36.5 grains of N550 gave us a nice narrow 11.95fps SD's on ES's of only 29fps. Still a little tall on the variance but velocities were touching 2900 and pressures were VERY low. It also grouped a .2" group of 5 shots. Oh man am I tempted to increase my powder budget by 25%. We figure we could fill the case on this stuff somewhere around 39 grains at 3,000fps. But, I don't want to develop a load; much less an expensive one, if one jumps out at me and that H4350 load at 39.5 grains is hard to beat even with stupid tight groups.
The group was so tight I put a 6th round off to the side to make sure the bullets were actually getting there.
Now we cross into "Coach style load" territory. So far we've been on the very slow side of the slow side of the rifle powder spectrum which is the "Me" style of loading. Now we're going to cross the street where the Beatles fans turn into Stones fans and start dragging their knuckles. Not really. Just making fun of Coach.
The defining line between a "Me" style and a "Coach" style of handload is I like my powder to burn all the way down the barrel giving good pressure all the way without a the spike of pressure in the case itself. My way is easier on brass but harder on barrel throats because there's more incompletely burned powder coming out of the case neck this way. Coach likes his pressure to form in the case, for all the combustion events to happen in that space and then to use the built up pressure. He also tends to jam bullets rather than jump them where I jump them at least a little bit normally.
Making a Coach style recipe means you know you'll see pressure sooner than later in your experimenting. That being the case and the fact that there was no data for IMR-4166 (which is near Varget/IMR-4895 burn speed) we elected to hot-foot-bloody-educated-guess it. I calculated using wild assumptions, rules of thumb and back of the napkin approximations (and comparative analysis, experience and old fashioned guessing) that 37.5 grains was about the most we'd want to try and so we tried that. It came back hot enough to imprint my ejector hole on the brass of 2 out of 5 cases so that's looking to be at least 1 full grain too much juice. It did however make 3080fps with a 2fps SD and a 6fps ES for 5 rounds. Drop a grain or two and you're right up around 2950-3000fps. What a smoker though! Too bad the pressures were simply too high. We've seen this kind of performance from IMR-4166 in several rifles, even when we don't push pressures through the roof. Stupid tight SD's.
If it wasn't so over pressure I'd be working on tweaking this load. WOW it's fast.
Below you can see the velocities as they came out of the gun during testing. You can see it took about 5 rounds to season the bore and then it's pretty much standard load development wavy gravy until you get to 2 very specific sections whose extreme flatness gives away that something very cool happened there and needed to be paid attention to.
So while N550 turned in the best group and ok SD's, the extremely tight SD's and the almost as tight of a group out of the 39.5gn load of H4350 has won the day. I might mess with bullet jump a little but really, I'm happy. Best not to waste barrel life racing paper.
On the topic of barrel life. Common wisdom is somewhere north of 1500 rounds but under 2000 before it's smoked on a 6XC with H4350. Well that's about a year and a half or 2 years worth of life with my match shooting schedule. That's from a conventional Enfield rifled gun. I run Columbia River Arms polygonal rifled barrels which have been giving me very long barrel life and I use only HBN coated bullets.
My .243AI still runs like a laser and it's north of 1300 rounds now. I only took it off the rifle because seating depth was longer than my magazine would allow. There's plenty of bullet still in the case yet. I could take it another few hundred rounds at least if I was willing to single feed before the boat tail is up inside the relatively short case neck. Pushing 115's at 3200 can't have been gentle on the throat and the initial expectation was that by 1000 rounds it was going to look like 5 miles of dirt road down the bore but it's not. It's smooth as glass still, cleans quick and easy and makes tiny 1000 yard groups. So if we take this barrel life term to mean the point at which the boat tail is up inside the neck of a loaded case when seated equals cooked, my .243AI will have gotten something like 1500-1800 rounds before its final inglorious death based on throat erosion rates so far; like thanks to some combination of the HBN coating and the polygonal rifling. That's pretty darned good for something pushed as hard and shot hot regularly.
Well then, I expect something like >3,000 rounds of life from my 6XC and something like 4,000 rounds from Coach's since we're giving his the Modern Sparts Systems Accuracy Oil treatment for its whole life. If I pull 3,000 rounds out of this barrel I'll be surprised as hell. 2,500 rounds wouldn't go amiss though and would be well representative of my real expectations.
How will it all turn out? Stay tuned to find out.
