Rifle Accuracy/PC/Hardness

[Bret4207]

Once the pressure is applied, I think a good deal of our hopes and assumptions go out the window. Felix called it "Dynamic fit". The idea of smacking something so hard our carefully crafted, carefully seated, lovingly created bullet turns into a blob of putty (obturation some call it) never made sense to me.

 

[Brad]

With the right alloy and design it works pretty well Bret. With the wrong design or alloy it really sucks. And there lies the problem.

 

[Bret4207]

"It only matters when it does." Bass had a good point there.

 

[fiver]

right, and your match of alloy plus pressure application matters.
your throat fit makes a bit of a difference but only if it helps or hinders you getting the bullet into the barrel without distortion or over powering the alloy.
the design and alloy strength have to work together for that first 1/2".
the PC comes in because it changes the timing of the whole thing.
you don't get to build up as much pressure to make the little changes to the bullet that you would without it.[I.E, it helps prevent or lessens the amount of slumping by moving forward easier]
it allows the bullet an easier path into the barrel.
and it prevents gas cutting.

the alloy underneath is free to flow and do whatever it does, but the PC changes the timing and protects the barrel from the lead and the lead from the barrel, plus from the gas.
that's why we see a lot of guys have such good luck pushing plain base bullets up into gas check territory.

 

[Bret4207]

"Changes the timing"!!! Eurkea! You just put a descriptor to an idea/issue in search of an answer/descriptor that was floating around in my so called brain. Makes perfect sense now. Thanks Fiver! Another tiny step in grasping a bit more of this game.

 

[Ian]

Bullet goes farther into the throat with less pressure when coated. That keeps the base from riveting but also increases the rate of space expanding behind the bullet which in turn stretches out the rate of pressure rise. It also lets you use a softer alloy.. Basically everything after the primer hit gets stretched out in time and happens further down the barrel. We add a little more powder to get back to where we were with velocity of uncoated bullets.

 

[johnnyjr]

FWIW- Ian stated "1.5 MOA at 300 yards", not 1.5 MOA at 100. If 1.5 MOA at 300 isn't good enough, well, best of luck.

Also FWIW- Ditto What Josh said about arrogance and condescension. We seem to have a lot of that showing up here. Based on personal experience, best to point it out publicly rather than in a PM lest you get put in time out with no recourse to respond. Be nice if all of us would grow a bit more respectful of the opinions of others. It's not just here either.

I know that's an old post might not even get a reply. That's good shooting at 300 yds. All many rounds were shot in that group. ? Thanks

 

[burbank.jung]

right, and your match of alloy plus pressure application matters.
your throat fit makes a bit of a difference but only if it helps or hinders you getting the bullet into the barrel without distortion or over powering the alloy.
the design and alloy strength have to work together for that first 1/2".
the PC comes in because it changes the timing of the whole thing.
you don't get to build up as much pressure to make the little changes to the bullet that you would without it.[I.E, it helps prevent or lessens the amount of slumping by moving forward easier]
it allows the bullet an easier path into the barrel.
and it prevents gas cutting.

the alloy underneath is free to flow and do whatever it does, but the PC changes the timing and protects the barrel from the lead and the lead from the barrel, plus from the gas.
that's why we see a lot of guys have such good luck pushing plain base bullets up into gas check territory.

Lol. Your choice of words cracks me up. I'm thinking of a shooter with a timing light over the muzzle of his gun. Is PC like lube? Slump is not good when you get old. Hard is better when there is proper fit. In all seriousness, I think lead has to be soft enough for obturation. Why? Everyone will agree that a bullet needs to expand to fit the rifling. Accuracy suffers. And has everyone pulled a seated bullet and measured the diameter. A lot of cases, the bullet diameter is less from being swaged by the case. I'd guess that a good load using a case with a thin wall will be larger if a load using brass with a thick wall where to be added to the mix.

 

[johnnyjr]

Interesting reply,but how many shots did you have in this 300 yard group?

 

[Mitty38]

Interesting reply,but how many shots did you have in this 300 yard group?

Can't answer for them. But I had been able to get under .75 groups at 100 yards in a PC'd bore rider 30- 06 with mentoring from Fiver. 5 shot group. Consistent.
So that would translate to 1.5ish at 200 yards. If I had had a 200 yard range available. Which I figure is pretty darn good for a stock Axis 2.

 

[fiver]

I think lead has to be soft enough for obturation.

it is.
there isn't a lead alloy in the solar system that isn't going to yield to a couple thousand PSI.

think about how we work it.
make the bullet big enough to slightly mush down and block the gas as it [bullet] fills the throat and then the barrels groove.
we use slow, slower, and slowest [relative] powder speeds to glide things along at speeds high enough to take a 19th. century persons breath away without even trying.

in the old days they made them small enough to fit the bore, and the powder made the bullet big enough to fit the groove diameter.
there wasn't a lot of options in the matter.

 

[fiver]

Burbank your timing light thing is kind of close but in the wrong place.

move it back to the first 1/2-3/4's of an inch of travel [from the base of the cases neck is a good place to start] and strobe it up into a series of pictures happening every 64th of an inch or so.

why would we start at the case neck?
because the case is moving right there letting loose of the bullet and where the problems start or are stopped by our actions on the bench.

 

[johnnyjr]

Can't answer for them. But I had been able to get under .75 groups at 100 yards in a PC'd bore rider 30- 06 with mentoring from Fiver. 5 shot group. Consistent.
So that would translate to 1.5ish at 200 yards. If I had had a 200 yard range available. Which I figure is pretty darn good for a stock Axis 2.

My axis shoots cast bullets good at 100.. want to try 200 next...but I need to make more bullets...

 

[Mitty38]

I had to get rid of a couple guns so the axis being a quick sell, at a bit more then what I paid, went.
Was a surprisingly accurate gun, as long as you supported the front from the bipod stud, and did not let the plastic stock hit the barrel.
I think a good bedded wood stock would have made that gun shine.

Wish I still had a large tapered cartridge caliber. Would love to do some more work with the Bore Rider. And with a few other bullet designs that lend theirselves well to .25 and up.

 

[popper]

Assume a SAMII correct chamber/good barrel. As said, neck dia close to chamber size. Fill the space (call this pre-bore) between mouth and bore to get/keep bullet centered in/on entering the bore. So. everything is/stays aligned. Base exits neck, expands some to fill pre-bore, then shrunk down to bore. Drive band(s) hit rifling. Tough alloy won't expand much in pre-bore. IMHO, at this point, PC can only help by having a flat base! Was some talk about trailing edge failure - NOT. Drag a putty knife across the putty, it leaves a 'dip' in the putty. Yes, gas can escape there. Lands don't 'cut' grooves, they displace metal to form the bullet grooves. And that metal tries to flow into the bore grooves! Tough alloy doesn't do that well so we over-size our bullets to force an excess of metal. Trying to fill that 'trailing' edge. At this point, IMHO, PC provides a higher thermal insulation coating that slows gas cutting. The rest of the ride, lower non-metallic contact friction (dust) reduces lead sticking. Tougher alloy is needed at HV to prevent basic shearing of alloy on the lands. Mr Gebson also notes the loss (and streaching of GC shank) using hard alloy. I suspect this is due to high temp (pressure temp) on the base with the added friction of the copper GC. I recovered the GC/base of a bullet - all that was left - the lead in the GC was re-melted (small ring) and rest was crumbly. GC was bent where it cut through 12ga wire. 308W @ 2400 fps, 165gr GC 4% Sb HT & PC.
I say tough, BHN actually measures the stess/strain curve at permanant displacement and is a SLOW measurement. The curve changes with impact fps.