Pop quiz!
- Thread starter Ian
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Ian
Notorious member
Yes, half inch grid. Scope has a 2 MOA black dot (when not illuminated) in the center and crosshairs that stop about the edge of the black ring, so anything behind the dot is a goner if I do my part but I like so split hairs sometimes. Time to go load a hundred of these now and give the scope a half MOA of windage.
Remember this is an LR-308 with ultralight furniture feeding itself from the magazine, so I'm pleased also that it does what it does with the soft alloy hollow points at the speeds I'm getting from the 18" barrel.
Remember this is an LR-308 with ultralight furniture feeding itself from the magazine, so I'm pleased also that it does what it does with the soft alloy hollow points at the speeds I'm getting from the 18" barrel.
Ian
Notorious member
Thanks, it's not great but acceptable for the circumstances. Holding a 1" group wasn't in the cards today.
I'm not getting clean cold flyers with powder-coated bullets and no bullet lube, IME the clean-bore shot goes into the group. Evidently the effect of warm bore condition is the same as clean and cold but not the same as dirty and cold. In any event it's close enough for oinkers and way better than most lubes will do at HV on the first shot.
BHN does make a difference, holes are same size this last time. Assume your chamber is larger than SAMMI, you have a 1/10" freebore and 30 thou. expansion to pass before bore size. 0.030" neck wall, 310 bullet and chamber neck is probably an extra 0.005 min. larger. Hard on the softer alloy with the faster powder. Fastest I've used in 308 is H335. Don't have any idea how a plastic lower is going to hold up in 308. Still very good results with what you are using. Kudos. An after thought, using small base sizer?
Ian
Notorious member
.003" neck slop with the PMJ military brass and .3098" bullets. I can chamber these bullets with just the gas check in the neck, throat is .310 and a couple tenths when clean, acts like .3100" dirty, the bullets have to jump a long way to the throat, like .150" or so.
I have no way to tell for sure but don't think the bases were bumping up before the throat even with the softer bullets. A primer and 70% fill of rice grains is enough to get the gas check well into the throat and the PC is slick. I think the softer alloy was failing due to land engrave wear farther up the barrel. This seems to be the #1 accuracy killer with greased bullets at hv and I have a working theory (you'll hear more about this soon) based on lots of observations over the years that the torque stress accelerates drive side wear and even if it doesn't lead the barrel something about gas leaking around the bullet destroys accuracy. I can assure you it isn't casting defects or bullet balance problems because velocity limitations still occur when bullets are breech seated and it's worse/lower speed the more brittle the alloy, but add a paper or polymer jacket and fit the bullet to the throat and ordinary cast bullets can do amazing things without "RPM" limitations. Slower twists and lapped barrels help too, but the slow twist still has drawbacks in the accuracy department. Process of elimination tells me that whatever limits accurate velocity (seems to be related to twist rate and caliber) is happening between the front end of the throat and a few inches past the muzzle because we have ways of fixing everything else and the right alloy will get you farther than the wrong one id not using some kind of jacket.
I have no way to tell for sure but don't think the bases were bumping up before the throat even with the softer bullets. A primer and 70% fill of rice grains is enough to get the gas check well into the throat and the PC is slick. I think the softer alloy was failing due to land engrave wear farther up the barrel. This seems to be the #1 accuracy killer with greased bullets at hv and I have a working theory (you'll hear more about this soon) based on lots of observations over the years that the torque stress accelerates drive side wear and even if it doesn't lead the barrel something about gas leaking around the bullet destroys accuracy. I can assure you it isn't casting defects or bullet balance problems because velocity limitations still occur when bullets are breech seated and it's worse/lower speed the more brittle the alloy, but add a paper or polymer jacket and fit the bullet to the throat and ordinary cast bullets can do amazing things without "RPM" limitations. Slower twists and lapped barrels help too, but the slow twist still has drawbacks in the accuracy department. Process of elimination tells me that whatever limits accurate velocity (seems to be related to twist rate and caliber) is happening between the front end of the throat and a few inches past the muzzle because we have ways of fixing everything else and the right alloy will get you farther than the wrong one id not using some kind of jacket.
Ian
Notorious member
I forgot to add, I'm not using a small base sizer, my chambers are sloppy enough as it is without making the problem worse. I use an RCBS gold medal match full length neck bushing sizer and bump the shoulder back to 2.002" on my Wilson gauge. The M1A pushes them out to 2.006 and the LR-308 pushes them to 2.004" along with my two .308 bolt guns which I headspaced myself.
The plastic lower is plenty strong, it's made out of a high temperature polymer reinforced with carbon fiber. The guys that produced it were packing so much fiber into it that they were running into viscosity problems even with high pressure injection so they had 3M make a resin which could take more heat and flow better. The only downside is flex in the receiver ring under recoil or when putting a lot of cheek pressure on the stock to drive it into the butt bag. If you don't do exactly the same thing every time it affects groups more than a solid stock or rigid aluminum lower will. What was screwing me up today was muscle tremors at the end of the string from holding the same pressure for too long waiting for the wind to luff.
The plastic lower is plenty strong, it's made out of a high temperature polymer reinforced with carbon fiber. The guys that produced it were packing so much fiber into it that they were running into viscosity problems even with high pressure injection so they had 3M make a resin which could take more heat and flow better. The only downside is flex in the receiver ring under recoil or when putting a lot of cheek pressure on the stock to drive it into the butt bag. If you don't do exactly the same thing every time it affects groups more than a solid stock or rigid aluminum lower will. What was screwing me up today was muscle tremors at the end of the string from holding the same pressure for too long waiting for the wind to luff.
F
freebullet
Guest
Really nice, Ian. I bet that's a nice mild fun to shoot load ta'boot.