Strange Day Good Day

[laowho]

Interestingly enuf rdnck, all, is that TexasMac posted the same essential question I've been askin myself re: barrel harmonics for BPCR vs. smokeless powders and barrels. With the exception of our heavier barrels possibly dampening the effects, but w/ all the opportunities afforded us to mix it up recipe-wise, I can't help but think there's still sumthin in the consideration of it for us:

http://www.assra.com/cgi-bin/yabb/YaBB.p...1275329603

Here's his post f/ 2010, wh/ never really got a response (or an intelligible reading)

"I recently sold a Browning .45-70 BPCR with the ½ octagon ½ round heavy Badger barrel to a guy that is working up smokeless loads to make it into a 300 yd scoped deer rifle. So far his best load has resulted in a 1/2” group at 100 yds., not too shabby. The load consists of a Barnes TSX FN 250 gr. bullet in front of 53.5 gr. Vv N-120 powder, and leaves the muzzle at around 2,500 fps. What he found is by varying the powder by as little as 1 gr. has a significant effect on accuracy. For example, by only reducing the powder by ½ gr. the groups start to open up to 3/8”, and at 51 -52 grs the group expands to 1.5 to 2” at 100 yds. He was surprised by how “fickle” the loads were regarding powder charge

So my response to the guy was:
Thanks for the details. Sure sounds like you’ve found one of the "sweet spots". No doubt you’re aware of the Browning BOSS system that uses an adjustable muzzle weight to fine tune the various standing wave vibrations (harmonics) in the barrel. If not, go to (You need to Login or Register to view media files and links). One can accomplish the same thing by varying the powder charge. Therefore I can easily understand how a small change can have a significant effect on accuracy, especially with the relatively light bullet and high velocities you are achieving. It's my belief that the same effect takes place when shooting black powder and heavy bullets in the same rifle, but with a 550 gr bullet at 1250 fps velocities, the rifle is not as sensitive to load changes as it is in your situation. Another way of looking at it is by increasing the velocity and reducing the bullet weight you’ve increased the overall frequency of the barrel vibrations, thereby reducing the wave length of the standing waves in the barrel. Shorter wave lengths (standing wave nodes closer together) result in a barrel more sensitive to changes; at least that's my belief.

So what do you guys think? Does my theory make sense or is it full of holes?

Wayne"

[bruce moulds]

doug,
I think you raise a very valid question.
I have used audette's ladder tests with fclass rifles for many years very successfully.
barrels are similar profile and length to my bpcr barrels, being 30 to 32" long, and 1" at the muzzle.
actions are trued and sleeved Remington and nesika bay. and of course barrels are free floated with tight necked chambers.
these guns will shoot most loads not too bad once a favoured seating depth is found.
ladders are done at 500 meters to spread out the shots on the paper.
as the charge increases in 0.3 gn intervals, the bullets go up the paper 'till a node appears, then start climbing again until the next node, and so on.
there are often 3 nodes in an acceptable velocity range, so you pick the widest one (most shots) all else being equal.
these ladders repeat.
no doubt a bpcr will behave similarly.
recent ammunition testing at 990 yards with a 40/72 showed groups going up the target with 1 gn powder increases peaking at 84 gns, then at 85 gns the groups started going downwards again. could not get more powder in the case so stopped there.
84 gns shot the best vert anyway so the load was decided.
this does suggest nodal activity, but other factors could have come into play.
no barometer readings were taken, so relative air pressure is unknown between groups.
sir henry halford makes mention of this, and the old time creedmoor shooters had a barometer on the range and knew what to do when pressure went up or down.
the powder compression was different to maintain the same coal. no chronograph that day to test muzzle velocity.
the fclass rifles are shot off a rest so shot to shot hold is virtually the same and a 32x scope with a target optimized for the dot is used.
hold and aim are good and consistent as is trigger letoff with jewell 2 oz triggers. also very quick lock time.
the bpcr is resting on the barrel, has iron sights, is lighter than the fclass rifle, has more barrel time, has a basic trigger system, has more recoil, and probably several other issues that give it a disadvantage.
however a similar result can possibly be achieved by shooting 10 shot groups rather than a single shot per load.
with the fclass rifle, you can have confidence to adjust the sight based on 1 shot.
with the bpcr you need probably 3 shots to make an accurate adjustment with confidence.
this reflects the relative accuracy of both systems.
a ladder test using different targets for each load and 10 shots per load would seem the best for bpcr.
10 shots is also a statistical number for group size.
range would need to be min 200 yds, pref 300, to see elevation differences of centres of groups.
aiming and shooting conditions need to be good for any ladder test to be meaningful.
please keep us informed with your most interesting journey,
keep safe,
bruce.

[laowho]

Thx Bruce,

And what you said @ the outset re: "all will shoot decently once a seating depth has been found" is what originally had me snagged and why I wanted to understand harmonics better, all the more so IF that seating depth has some independent advantage (e.g.., whether beginning on the lands is inherently preferred "all else bein equal." It never is, of course). Didn't wanna begin charge testing just to go back and start floating the bullet back and forth, specially since by then I'd be tryin to test for two things rather than one. I get that we have to find what the rifle likes, and that what I've been asking is old hat to you guys, but I didn't wanna be chasin my own tail or resort to some formulaic short cut vis a vis payin for OCW and QuickLoad. That, AND this stuff is just fascinating. (Later I have BC to delve into and will be revisiting your Shiloh thread posts.) And I'm still playin w/ the null point for cross stick placement. Got it narrowed decently to about 8.5" back. Cheers, Doug