6XC Load Development – Analysis Time
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. These are all on Norma brass, F210 primers, 115gn DTAC bullets and COAL at 2.8″.
After grinding out the first 15 rounds 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 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 so far.
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) which is 38.5gn of H4350. 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’s a called flyer (obvious) on 4 of the 6 aiming points. I wasn’t in the most stable position and I knew it.
My velocities are a solid 150fps above what Coach gets from his Enfield rifled barrel of the same length with the same load. Ok, to be completely transparent, it’s not EXACTLY the same load. We do actually seat the bullets about .120 deeper forn my new barrel than Coach’s barrel but I can’t see 150fps difference from that. This is the polygonal rifling in full effect. Less friction because you’re not engraving the bullet, you’re swaging then and then twizzlering them, if only ever so slightly.
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 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. Everyone knows that after my experience with it in .243AI and 6.5x55AU that I’m a big fan of Reloader 23. 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 if you’re seating to SAAMI/CIP lengths. 38 grains produced pretty nice velocity. A testament to the efficiency of the 6XC case setup. 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.
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 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.
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. 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 consistent pressure all the way without a huge spike of pressure in the case itself. My way is easier on brass but harder on barrel throats because there’s more grit 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 or later in your experimenting. That being the case and the fact that there was no data for IMR-4166 (which is around Varget/IMR-4895 burn speed) we elected to hot-foot bloody educated guess it. I calculated 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 so that’s 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.
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 super tight 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.
On the topic of barrel life. Common wisdom is somewhere north of 1500 rounds but under 2000 before it’s smoked. Well that’s about a year and a half or 2 worth of life. That’s from a conventional Enfield rifled gun. I run Columbia River Arms polygonally rifled barrels which have been giving me very long barrel life and I used only HBN coated bullets for the last 1000 rounds (it’s at north of 1300 rounds now).
My .243AI still runs like a laser. I only took it off because seating depth was longer than my magazine, but there’s plenty of bullet still in the case yet. I could take it another 500+ rounds if I was willing to single feed. Pushing 115’s at 3200 can’t have been gentle on it and the expectation was that by 1000 rounds it was going to look like 5 miles of rough road down the bore but it’s not. It’s smooth as glass still and makes tiny 1000 yard groups. So if we take this barrel life thing 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 2000 rounds before its death. Thanks to some combination of the HBN coating and the polygonal rifling.
Well then, I expect something like 4,000 rounds of life from my 6XC and something like 6,000 rounds from Coach’s since we’re giving his the Modern Sparts Systems Accuracy Oil treatment for its whole life. If I pull 4,000 rounds out of this barrel I’ll be surprised as hell. 3,000 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.