30-06: buffer, crimp, diameter, 2005 conclusions
Some of these experiments may seem like I'm going over old ground, but bear with me, I do have a plan, believe it or not.
I ran out of WW760 but had a jug of Ramshot Big Game taking up space on the reloading bench. Big Game has a burning rate and load density practically identical to WW760, except Big Game produces the dreaded secondary spikes. It would be a good test to see if shotgun buffer eliminates the spikes.
I sized the nose of the usual 180 gr. loverin in a 309" die. Actual nose diameters were 0.3085 - 0.3090". The bottom half of the bullet was sized 0.314". The reduced diameter nose is necessary to fit in the tight freebore section, while the 0.314" bottom helps to fill up the sloppy chamber neck. Even with the nose sized to 0.309", the bullet would run aground in the forcing cone before reaching the rifling. Apparently the forcing cone is 0.3085" or thereabouts.
The same 180 gr. loverin was sized in yet another 0.309" die. The actual diameters were 0.3080" - 0.3085", though, and this bullet could be seated out far enough to engrave the rifling. This is the third time that I have tested skinny bullets. An 8mm throating reamer has been ordered, so this was the final chance to shoot skinny bullets in the skinny throat.
Some of the 0.3085" bullets were roll crimped into one of the lube grooves, just as hard as I dared. This experiment was motivated by an intelligent caster who wondered if my oversize bullets shot better because the extra engraving force improved ignition. If the extra initial force is the key, then the roll crimp on these skinny bullets should provide a similar "extra force" and should shoot as well as the fat bullets.
0.3cc of Graf's generic shotgun buffer filled the case up to the bottom of the neck.
All loads used:
54.4 gr. Big Game
heat treated mystery metal HVR lube
COL = 2.940" fat bullets, 2.980" on skinny bullets
Here's the 0.3085" trace, without buffer.
Here's the 309/314/buffer trace.
Here's the 309/314 trace, without buffer. During this string, the strain gage adhesive began to fail, but the "good" traces average about 62,000 psi on the primary peak.
Here's the 0.3085/crimp trace, without buffer. The gage is definitely out of whack now.
Here's the jacketed validation load shot at the end of the session. Normally it would be around 54,000 psi. Yup, the gage is dead.
Let's summarize the results. 10 shots for each load.
[mrow] load[col]avg. FPS[col]std. dev. FPS[col]avg PSI[col]5-shot MOA
[row] 0.3085" [col] 2672[col]0.55%[col]60,000[col]8.75"
[row] 0.3085" / crimp [col] 2702[col]0.57%[col] ??? [col]8.90"
[row] 309/314 [col] 2701[col]0.52%[col]62,000[col]3.15"
[row] 309/314/buffer [col] 2731[col]0.53%[col]67,000[col]2.95"
What I've learned this year
improve accuracy or standard deviation one iota ...... leading me to believe that my fat bullets shoot better because they fit
better, not because they ignite better.
bullets sprayed all over the target just like they did in previous tests. The standard deviation suggests that ignition is not a problem.
appears to increase pressures about 5000 psi, though I can't be certain due to the adhesive failing in the middle of this session. The shotgun buffer did not reduce the secondary spike. So far buffers are not looking good, but I want to do more buffer experiments before drawing a conclusion.
bullets grouped from 2 to 4 inches with a variety of loads, all summer long. Not great, but at least they were groups. Definitely, positively, absolutely better than the skinny throat-diameter bullets.
Why 0.314"? Because 0.316" bullets are sometimes difficult to chamber unless the neck wall thickness is carefully controlled. For everyday use with unturned cases, I think you need to allow 0.0025" clearance in the chamber neck, which is about how much there is with a 0.314" bullet.
tapered bullet in a taper throat
is supposed to be the holy grail of cast bullet fit, but I was never able to achieve a consistent fit with a tapered nose, even after carefully sizing the noses in a tapered die. A taper-inside-a-taper produces a fit that is either extremely tight or quite loose, with very little in between. I think it is better (and easier) to have a one-diameter nose inside a one-diameter freebore, lightly engraving a tapered leade. Today\'s two-diameter 309/314 bullets and one-diameter 3085" bullets chambered with consistent light resistance. That's the way it should be.
Conventional wisdom says "size the bullet to fit the throat", but that's turned out to be yet another old wives tale. The 0.006" oversize bullets shot better today, and previous tests showed slightly better results at 0.009" oversize -- despite the fact that the oversize bullets must surely be shaving lead and deforming as they pass the sharp-edged shoulder at the entrance to the throat. The 309/314 bullet is just a bandaid -- the real solution is a chamber that's an ideal fit for a 0.309" - 0.310" one-diameter bullet. The SAAMI 3006 chamber has a neck that is too sloppy and a throat that is too tight, IMHO.
this gage was glued on with Loctite 4211. As discussed on another thread, Loctite 4211 is not recommended by RSI, and I have learned the hard way that 4211 does not stick reliably to the strain gage. The gage worked well all summer, until today. In general, the RSI Pressure Trace works great if the gage sticks, but I have been plagued with adhesive failures.
this gun does not show so-called "pressure signs" until primers fall out and cases seperate, which happens around 75,000 psi. Loads that I used to think were safe, actually produce 60,000 - 70,000 psi, and sometimes more. This gun cannot exceed book velocities at normal pressures, even though sometimes it can digest a couple grains more powder.
we are still missing many pieces of the puzzle, but it's starting to look like the spike is happening after the bullet exits the barrel. I say that for two reasons 1) the barrel shortening test where the spike kept moving backwards as the barrel was shortened and 2) the mica filler test that graphically indicated that the bullet was exiting at 1.0 - 1.1 millisecond. That contradicts theoretical estimates of the exit time which generally put the 30-06 exit point at 1.2 - 1.3 millisecond, but remember that there may be some error in the way the PT triggers. If the trigger has a 0.1 - 0.2 millisecond delay, that would explain the discrepancy.
Now, if the spike is outside the barrel, are we off the hook? No, because the spike still puts stress on the barrel -- maybe not as much because then there would be some pressure on the outside of the barrel
, too -- but still worrisome. High pressure at the exit may still affect the flight of the bullet. One way or the other, the secondary spike is not a good thing.
The loverin design is not as sensitive to bullet lube as some other designs, but nothing that I have tested to date has outperformed Rooster HVR in full throttle loads.
1) An 8mm Manson throating reamer
has been ordered, because if I am going to shoot oversize bullets, why not shoot them in an overize throat? The 8mm Manson is supposed to create a .3125" - 0.3130" freebore. I don't know if that's optimal, but let's just see if it moves things in the right direction, and go from there. I'll have to regrind the reamer pilot to fit a 30 caliber bore, but I think I can do that.
2) A custom chamber & barrel
with the neck and freebore cut for a 0.309" bullet, and a 1-12" twist instead of the usual 1-10". It may take a few years to save up for the custom barrel & reamer.