While driving to work this morning, I was thinking about the accuracy bug that I've been chasing in the 308 rifle and why I still don't have consistency.
I have been a staunch advocate for direct impingement (DI) gas systems as they lend themselves to great accuracy. I've never bought into the myth that DI guns jam because of fouling, or that they jam more than any other firearm in general, and I have plenty of experience with the M16, AR15, and LR308 platforms to back that up. Sure, it fouls the action with soot, but it doesn't lead to the jams that uninformed/inexperienced people whine about on various internet forums. Discounting the defective magazine nightmare that I had with the LR308 when I first started shooting it, the only jam in one of these platforms that I can recall was 100% shooter induced. I was on the rifle range at Camp Pendleton, and I clicked off a shot on the 300 yard line which was ejected back into the action because my pinky was blocking the ejection port. This was before the days of forward grips and all, so we pulled in tight to the magazine well with our support hand.
As far as the heat involved with DI rifles, I've never worried about that too much as I don't even run my carbine like a carbine. When you never fire more than ten rounds or so at a time, why do you need to worry about snapping your bolt from all the high volume fire that will never happen? It's just never been an issue, or so I thought.
My epiphany occurred this morning as I was deducting that with the LR308, my groups always, always, always open up as the gun gets hot. I don't subscribe to the theory that it's because of the barrel; there's plenty of information and evidence about modern high quality barrels and how the point of impact does not shift when they get hot. Within reason, of course. No, my groups open up as the gun heats up overall, even if the barrel is not too hot to touch. I keep all of my targets for later analysis as I'm dorky like that, and I have lots of proof of why this is happening, and just hadn't put it together until now.
Gun powder is fickle stuff that performs differently depending on the climate where it is at. Temperature seems to me to be the big factor, as loads that you so carefully tested and approved of in the spring under clear, 60 degree temperatures go all screwy when the mercury hits 100+ degrees. Probably the main reason why Hodgdon's Varget is so popular is that it is extremely resistant to temperature changes (climate change?), but even the mighty Varget has its limits. I found this out this summer after one really hot day when I took the empty shell casings home after firing max loads of Varget with 175 grain bullets, and noticed that the cases wouldn't fit in a shell holder. Seems they had swelled a little too much. Looking at my data book shows the first five round group to average 2,547 fps, and consecutive groups going 2,603 fps.
The conclusion I came to was that it's not the barrel temperature that's causing it, but the chamber temperature. DI operating systems direct hot gasses straight into the bolt, right behind the case head. It's like hitting the back of the case with a blow torch every time you fire, and then the next unfired cartridge has to sit in that hot chamber with a glowing hot bolt until you shoot again. That is what is causing inconsistencies, and that is why the fifth round in a five round group in my warm rifle sometimes opens up. Checking through my data book, I see a variance of up to 90 fps with Varget loads, so who knows how screwed up things get with Reloader 15 or other temp sensitive powders. I would bet money that this phenomenon is why my gun will stack five rounds of 168 grain Federal Gold Medal right on top of each other, than the next group will be like buck shot.
If you think about it, you can eject a just fired .300 Winchester Magnum case out of a bolt action rifle straight into your hand, and it's kind of warm, but then try to pick up a .308 Winchester case off the ground that was just fired from a DI type AR rifle and it will immediately remove skin. They come out that hot even if it's the first round of the day. What I'm saying is that as I shoot, I'm basically cooking the round that I'm about to fire, and this increase in temperature causes pressure and velocity to climb which opens up my groups, and it gets worse with each round fired.
The solution is to either fire three round groups before letting the gun cool for fifteen minutes, use a powder that's impervious to temperature, or go with a gas piston system. The gas piston system does put reciprocating parts on top of the barrel, which can effect accuracy, but reading through the gun forums shows the technology is such that this isn't really an issue any more. The piston is getting very popular, to the point where even the Israelis are thinking about them.
I will have to take the route of least resistance and try out some of the new IMR 8208 XBR powder to find out if it is the magic solution for all of my DI problems. Looking on the IMR website shows that 8208 XBR is "totally insensitive to changes in temperature." Oh, we will see about that!
Now, I just have to find some. Last night I swung in to Ganderous Mountainous to see if they had Varget, and I found the store's condition to be exactly the same as it has been for years: deplorable. I hate that store. Not only did they NOT have any Varget, they didn't have any 175 grain Sierra Match Kings, no 165 grain Sierra Game Kings, no .223 Remington shell casings for a Stony Point OAL gauge; to tell you the truth, they really don't have much of anything. Time for a trip to Green Top!
I have some chain saw and machete work to do to perfect where I am going to test these rounds, and I'll get back to you with a range report when I get it done. If this doesn't work, I'll be looking to acquire a piston system for both ARs.