Originally Posted by
Bass Ackward
Larry,
Time for another deep breath. Bear with me. Every time you have to stop and think about what I wrote, think about why you NEVER see a free floated military barrel.
Took the deep breath and have been thinking about it all morning. Yes I do know why we never see a free floated military barrel (except with todays application for SDM and sniper rifles). Bass, free floating barrels wasn't discovered (and accepted) to be a real aid to accuracy until the late '50s after the bench rest crowd proved over and over again that free floated barrels are more accurate. All of the surplus military rifles we have and shoot were made well before that discovery. Also all of the factory rifles were made with barrel pressure as that was the accepted way then. Those rifles were bedded with the techology that was known when they were made. That's WHY we don't see any free floated barrels military barrels.
You said yourself, jacketed accuracy can be found at low, medium or high velocity. Jacketed throws enough friction that as the speed increases, the vibration speed increases and the barrel whip is minimized. No bedding is needed because you simply alter your powder charge or your powder speed to get the vibration pattern needed for consistent shot to shot exits.
The wine glass with your finger routine. Slow and easy and the pitch is low and soft. Push harder and the pitch of the tone gets higher and louder. Ever play the geetar? A real lose string can be seen vibrating and drawn tight never seems to move.
With lead, you must stay with the slowest powder producing your best velocity or accept a lower velocity (RPM).
No problem with all of that, we're on the same page of the hymn book there.
So jacketed accuracy is a different bedding game. Stop here. You have to believe this or your wasting your time reading on. Lead bedding logic SHOULD be different. It doesn't "have to be" if you can get best accuracy within the harmonics range you want to operate in with the barrel you have. (HV 30 caliber seems to be 2200 to 2400 fps) Bedding won't help then if you are accurate and it could make things worse. But lead, you can only throw so much friction before your lube breaks down and you lead. SO if your best (most accurate) load effort CAN"T reach the proper harmonics, then you MUST change the oscillating frequency which may be more pressure or more pressure farther out. Bedding does this. The farther out vibration is controlled, the less pressure is needed. But if you don't tune your lead, you live with the result.
Guess I'm wasting my time but that's alright. You see all my rifles are not free floated. I have numerous military rifles that are still bedded as made. I do mostly bed all my sporters, varmint and target rifles. I've numerous times tried adding pressure to barrels to enhance accuracy. Only in a few instances over the years has it worked. That was most often with very thin barrels and the long (29 1/2") Mauser barrels when those were in sporter stocks. With barrels of normal contours they ALWAYS are most accurate with jacketed and cast bullets.
Now I'll give you a case in point. Years back I aquired a new 2 groove '03 barrel with 10" twist. Now everyone and their brother swore that 2 groove barrels are more accurate with cast bullets (they aren't by the way) so I thought I had the cat's meow for a cast bullet rifle barrel. I also had a M1916 (Oviedo made in '28) Spanish M93 action that had just shot out it's 3rd .308 Winchester barrel. I had converted it to cock on opening so the lock time was very quick. It had been a very accurate rifle with all 3 previous barrels shooting both J and cast bullets. I had dreamed up my .308 CBC then which is a shortened .308 case with an '06 length neck. case capacity was computed to be just enough 4895 to get a 311284 to 2200 fps. I had the 2 groove barrel set back, threaded for the SR mauser and short chambered with a tight necked .308 reamer and then a '06 reamer was used to lentgthen the neck. Conversion came out quite nice. Case were formed from unfired U.S. 7.62 cases with the necks turned for .002 clearence with a .311 bullet. However I quickly found that accuracy was best (1- 1/1/2 moa) with 311284 in the 1750 to 1850 fps range which is right smack dab in the RPM threshold. I was not yet aware of the RPM threshold as i didn't read about it until some time later. When I increased velocity, thus increasing RPM, accuracy deteriorated to 3-5 moa, That's exacty what that bullet (or 311041 or 311291) did in my 30-06 and one .308 which also had a 10" twist. To the point here; I have tried every concievable thing, including all manner of pressure to the barrel and nothing changes the accuracy. Yes , I also went to slower powders but the smaller case capacity didn't help. Point is, pressure to the barrel changed nothing as far as accuracy at any velocity when using cast bullets. I have also tried barrel pressure numerous times over the years on many different rifles. This was even recently with the RCBS 30-150-FN after I shot your 154 LBT bullets. Pressure of 2, 6 and 10 lbs did not change the accuracy one iota with those cast bullets at velocities from 1800 up through 2500 fps. Accuracy was still the best (1 1/2 moa consistantly with 10 shot groups) down in the RPM threshold and went to 5-6 moa (with 5 shot groups) at the high end.
(It's the same with jacketed and thus the 115 grin Speer reference in the 8.66 twist. While people believe it's the twist rate overstabilizing, it's the increased barrel FREQUENCY from speed (the finger on the wine glass again) making bullet harmonics and bullet exit at a single point (accuracy) even MORE difficult. We prooved in several rifles that light bullets COULD BE shot MORE accurately than heavy for which the 8.66 twist was designed. All we did was change bedding.)
I get quite good accuracy with that Speer bullet out of my M95 Mauser. I also know of several other 7mms that shoot that bullet well. Would appear more to me that you have a barrel there with a lot of internal stress. What most call a bad barrel. We still agree that pressure was helpfull there.
Now, when a lead bullet reaches the point that lube breaks down and it .... grabs, that barrel and the vibration pattern is changed dramatically. (You pushed your finger harder on the wine glass and got a louder sound) So you get a flier. If you bed so that that change is trivialized, then you get less of a flier. Or so that is my logic. Same thing for minimizing humidy effect on lube and temperature, and metal fouling too. This is why militaries grab their barrel and hold it so as fouling alters accuracy, the effect is trivialized. A benchrester simply cleans his gun. Bedding does externally what a powder or charge change does. It's as simple as that. Maybe not as well, but well enough for lead.
I pretty much agree with here but I'me not sure how you think the defects caused to the bullets aren't affected by the rotaional spin of the centrifical force of the RPM when the bullet leaves the barrel. That is what we're talking about here. That is where accuracy is primarily lost; the defects in the bullets (from what ever reason) are accentuated by the increased rotational speed at higher velocity and accuracy is not as good as it is at lower RPM. If a lube breaks down and the barrel is leading I don't really care what direction the barrel whip throws the bullet it is not going to be accurate. Have you ever had a leaded barrel shoot accurately? I haven't.
Works as long as you aren't destroying bullet because of fouling, then all bets are off. You have to understand once you reached that point. Based upon my 35 Whelen tests last month with 4-6 BHN harder bullets and how that affect accuracy, I am going to get a .308 sizer and harden my bullets a tad and see if I can't shoot HV this winter.
I think you're forgetting the adverse effect of obturation at high accelleration. Even if the lube doesn't break down and the fouling is minimal adverse obturation if cause by higher acceleration will cause imbalances in the bullet and accuracy will be adversely affected by the higher RPM. I will be most interested in the results of your tests. I too will test this winter. I will run identical loads through 3 very accurate .308 rifles; a 10" twist M788, A 12" twist M70 Target and the 14" twist M98 Palma rifle. I will keep you posted.
When you slow the twist rate, you are in effect cutting pressure that alters the vibration pattern to allow higher velocity (RPM)s before the lead becomes unstable changing vibration again. Why not bed?
I've seen numerous tests that show the pressure between a 10" and 12" twist barrel is essentially the same. Any measured difference fell between the shot to shot variation of pressure. Not sure how this is with a 14" twist though. I am probably going to put my money where my mouth is and get a piezo (SP?) pressure measuring system for Christmas. Hopefully I'll have an answer by next Summer.
One more point: Larry "So what's that prove? Nothing much except your bullet shoots much more accurately down in the RPM threshold. That is just what I have been saying."
No all that prooves is that your gun shoots that bullet more accurately at that velocity. Free floated aren't ya?
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