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Does swaging a cast bullet improve accuracy? - Mountain Molds

Does swaging a cast bullet improve accuracy?

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Does swaging a cast bullet improve accuracy?

Postby mtngun » Mon Jan 02, 2017 1:51 pm

Some people believe that swaging improves the accuracy of cast bullets. The theory is that cast bullets often contain internal voids that make the bullet unbalanced, and that swaging generates enough pressure to collapse those internal voids.

There may be other benefits from swaging -- squaring up the gas check, creating a taper to match the throat, or changing the shape of the bullet nose. But I can square up the gas check and create a taper using a lubrisizer nose die at modest pressures, so I don't need to swage to do those things. For the purposes of this thread, I'm only interested in whether swaging improves accuracy by collapsing internal voids.

The plan is to compare the accuracy of swaged bullets to unswaged bullets. Both the swaged and the unswaged bullets will be final sized in the same nose die so that they both have the same fit in the throat.

My entry-level reloading press is not strong enough swaging, so I decided to use a 1 ton arbor press. I made a crude 7mm swaging die for the arbor press as shown. In hindsight, I would have made the die somewhat differently, but that's par for the course when you attempt something for the first time. If it turns out that swaging really does improve accuracy then I'll eventually make a better die and get a press more suitable for swaging. Nonetheless this crude die will get the job done for this shootout.
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The punch holders slip over the die in order to guide the punch in straight. In hindsight that was not necessary because the arbor press already pushes in a straight line, but nonetheless it works. The die & punches were machined from drill rod, then heat treated and tempered.
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My first test of the die for basic functioning used bullets that had already been heat treated, lubed, and nose sized, just because those were the only bullets I had on hand. :lol: It's necessary for the grooves to be filled with lube prior to swaging otherwise the grooves will collapse in the swage die and then it would not be an apples-to-apples test.
Image

The ejector punch is 3/32" so bullets exit the swage die with a small flat point.

The bullets also ended up with a slight hiccup at the start of the ogive -- that was unintentional, due to the way I machined the die and is one of those things I would do differently next time. :roll: Nonetheless we'll live with the hiccup for this test since it doesn't change the functional aspect of the bullet much.

I'm thinking that for the shootout I'll swage air-cooled bullets in order to more easily collapse any internal voids. That means I'll have to lube prior to swaging, then after swaging remove the lube so that the bullet can be heat treated. Yes it's a lot of extra work but the idea is to end up with a bullet that is exactly like the unswaged control bullet other than having its internal voids collapsed.

How much pressure is applied by swaging? Quick and dirty measurements suggest that my 1 ton arbor press has 30:1 leverage. I can apply 100 - 150 pounds to the handle so that works out to 3000 - 4500 pounds applied to the die. A 0.285" diameter bullet has a circular area of 0.0637939 square inches, so that works out to 47,000 - 70,000 psi. A 35 BHN bullet's yield strength is roughly 17,500 psi (500 x 35) so even a heat treated bullet should yield in the swage die. Since I'm hoping to swage air-cooled shot for the shootout, they'll yield at 6000 psi.

By contrast a RCBS lubrisizer has roughly 13.3:1 leverage when sizing in a nose die. I'm guessing I typically apply 20 pounds force to the handle (if you put your weight on the lubrisizer handle you're apt to bend the handle or even break the casting as I did one time :oops: ) so that works out to 4200 psi, not necessarily enough to make the lead flow in a hard bullet.

For some reason the Idaho weather in January is not favorable to shooting :lol: so it may be quite a while before I can perform the shootout.

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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Tue Jan 17, 2017 4:25 pm

I decided to make a new 7mm swage die because of the cosmetic imperfection on the ogive of the first swage die. The 2nd die turned out better. :)

The smoother finish of the new swage die seems to allow the lead to flow easier, and I could easily extrude lead out the ejection pin hole, even on 35 BHN bullets.

And of course the lube grooves need to be completely full of lube while swaging, otherwise the grooves will collapse.
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To eliminate the extrusion through the ejection pin hole, I had to add a pin that was just long enough to fill in the hole, but not so long as to protrude into the cavity. I suppose on a more conventional press-mounted swage die, the ejection pin could be adjusted to fill in the hole? In any event this setup worked satisfactorily on my arbor press, other than being slow to operate because of the need to switch pins back and forth.
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If I had to do it over again, I might attempt to use a 1/16" ejection pin rather than 3/32", but overall I am pleased with the results so far. 47,000 psi (100+ pounds force acting on 30:1 leverage acting on 7mm) is plenty to squish the lead bullets and collapse any internal voids.
Image

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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Sat Jan 21, 2017 1:22 pm

The January weather finally gave me what I was looking for -- an overcast, calm day in the 30's. In an attempt to eliminate mirage in the shooting muffler, I did not turn the heater on in the shooting shack. Instead I bundled up and shot in the cold. It worked! There was zero mirage and I had 100% confidence in my shots. Yea! :)

The control load for this test will be the which at that time averaged 1.51" at 3184 fps. After oven-treating & lubing, the swaged bullets were nose-sized to fit identical to my normal bullets. The idea was that it would be an apples-to-apples comparison to my normal bullets.

However, life always deals us a few oranges, and this test was no exception. For one thing, the swaging process creates a small flat point, and the flat point varies a little from one bullet to the next. I don't think the small flat point affects accuracy at 100 yards, but it did make it impossible to measure and control the COL and the jam in my normal manner. Instead I seated the bullet so that the usual amount was inside the case. I had no precise way to measure the resulting jam other than to verify that it was engraving slightly.

Another variable is that the swaging process bumped the base of the bullet up to 0.2855" - 0.2860", and even though it was subsequently run through a 0.284" push-thru die, due to spring back the base of the bullet still ended up 0.2846" - 0.2850", a little larger than my normal bullets

A third variable is that the temperature in the shack today was 35 - 40 F, vs. 60 - 70 F when the control load was fired on November 29. Hence today's velocities were 54 fps slower.

After only 20 rounds I gave up because the swaged bullets were obviously shooting wild. Clearly the swaged bullets were far less accurate than their unswaged counterparts fired on November 29. :cry:
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Also, the pexiglass window on my shooting muffler gained two new holes today, due to errant gas checks. At least one of the de-checked bullets landed within the main group. I did not spot the other new hole until after all the shooting was done so I don't know if its bullet landed within the group or not. I have more to say about the errant gas check phenomena but will start a new thread on the subject rather than hijack this thread.
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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Tue Jan 24, 2017 11:58 am

Why did the swaged bullets shoot wild? I don't know, but to shoot as wild as they did I figure either 1) they must have been extremely unbalanced or 2) the base of the bullet must have been extremely deformed in some way or 3) if the gas checks were coming off in flight or inside the barrel.

BALANCE: The whole idea behind swaging was to improve the balance, not make it worse. But, it's conceivable that swaging a grooved bullet could worsen the balance if the grooves collapse in some asymmetrical manner. However, I did not observe any asymmetrical damage to the grooves. Mind you, to shoot a 17" group the damage to the bullet would have to be really really really bad, so you'd think it would be visible to the naked eye.

DEFORMED BASE: Swaging squares up the base of the bullet. In theory that might improve accuracy, to the extent that it affects accuracy at all.

GAS CHECKS COMING OFF: Only 2 gas checks hit the pexiglass window, however. it's conceivable that additional gas checks came off but went through the hole in the window, or they came off after the bullet passed the pexiglass window. It's also conceivable, I suppose, that the check came partly off while it was inside the barrel, and was cockeyed when the bullet exited the barrel -- a cockeyed gas check would definitely cause wild shots.

At this point, I just don't know the answer, other than to say that I need to fix the problem with gas checks coming off. That may require a new barrel, or setting the barrel back and cutting a fresh chamber, so it may be quite a while. In the meantime I may, as a sanity check, shoot a couple of groups with unswaged bullets. If unswaged bullets still shoot 1.5 MOA like they did during the alloy shootout, then I'll know that the accuracy problem is definitely caused by swaging.

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Re: Does swaging a cast bullet improve accuracy?

Postby 1johnlb » Fri Jan 27, 2017 2:08 am


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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Fri Jan 27, 2017 12:59 pm


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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Sat Jan 28, 2017 4:19 pm

As a sanity check, I wanted to shoot a couple of groups with the "ol' reliable" non-swaged load. I used the "enhanced" reclaimed shot bullets because there were some left over from the alloy test. They were loaded with the same load and the same procedure that with the exception that they were final sized in a new die that left them 0.2838"- 0.2842", compared to the old die at 0.2846" - 0.2850".

Due to the problem with mirage when the shooting shack is heated, I shot without heat, and the shack was 32 - 35 degrees F. There was zero mirage and only 5 mph wind so I had 100% confidence in my shots. I did not chronograph the loads because I was afraid an errant GC might damage the chrono. Two warm-up shots were fired before group shooting began. I gave up after only a 5-shot group because it was obviously shooting wild.

Note that the last 2 shots, #4 and #5, had a normal POI and normal accuracy. It was the first 3 shots of the group that were wild. That makes me wonder if the rifle need 5 shots to warm up rather than only 2 shots?
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Aaaaaand there was a new hole in the pexiglass after firing the 7 shots. :cry:
Image

Conclusion:
Obviously there is something seriously wrong because even the "ol' reliable" load is now shooting wild. :cry:

Before I blame the rifle, I want to see if temperature is a factor. The wild shooting began when I stopped heating the shooting shack. So I am going to leave the rifle & ammo in a heated room overnight, then tomorrow I will quickly shoot another group with the warm rifle & warm ammo and see if that restores accuracy.

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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Sun Jan 29, 2017 8:58 am

OK, the rifle and ammo were left in a warm place overnight. The shooting shack temperature was 28F while the outdoor temperature was 20F despite sunshine. There was a 10 mph wind quartering across the range and there was a wee bit of mirage, but not enough to complain about. I carried the warm rifle & ammo into the shack, immediately fired 2 warm-up shots, and then immediately commenced group shooting, firing as quickly as I could load and aim, and not allowing the barrel to cool between shots as I would normally do.

The results speak for themselves. The warm ammo shot just as well as it did on

I had replaced the pexiglass exit with cardboard because I got tired of errant gas checks punching holes in the pexiglass. :lol: Well, there were no holes in the cardboard today
Image

Conclusion:
-- my cast bullet loads are worthless when the ammo is cold.
-- I'm guessing the lube (BAC) does not work right when it is cold.
-- for the time being, heating the shooting shack is the lesser evil, plus it's more comfortable. :lol:
-- but if I heat the shooting shack, then I must contend with terrible mirage in the shooting muffler. The colder the outdoor temperature, the worse the mirage.
-- so I may be out of luck until the weather warms up. I'm guessing the outdoor temp needs to be at least 45 - 50 F to bring the mirage under control.

Longer term, I may attempt to find a lube or a bullet design that is less sensitive to cold, but that could take a long time, and there may not be a simple solution.

Armchair Theory About Why Cold Hurts Bullet Lube:
I have no way to prove this but here's my thinking -- when any bullet is engraved by the rifling, the metal that is displaced by the rifling has to go somewhere. In the case of a jacketed bullet, the entire bullet gets squeezed and becomes longer. Nonetheless jacketed bullets seem to tolerate getting squeezed. Monolithic copper bullets do not like getting squeezed -- it increases pressure and hurts accuracy. So the trend in monolithic bullets is to have grooves. Rather than squeezing the entire bullet, displaced copper is simply pushed into the groove. Hence grooves are good things to have even if you are shooting a coated cast bullet that does not require lube.
Image

But what happens when a lubed cast bullet is engraved? Well, the lead is pushed into the groove. That displaces lube, which is an incompressible fluid. So where does the lube go? Most likely it squirts backwards, into the throat & neck area. Even in a benchrest gun there is still a little bit of clearance in the throat and neck, so there is room for the lube to squirt. Usually this squirted lube in the throat eventually gets melted and consumed by the hot combustion gases, but sometimes you'll end up with lube buildup in the throat, or on the neck of fired cases, particularly at lower pressures. I've had loads that would leave lube smeared on the case neck, and that's what first made me suspect that the displaced lube must be squirting backwards. Plus, if you think about it, there's nowhere else for the displaced lube to go.

OK, so at normal temperatures the displaced lube simply squirts backwards and then is consumed by the hot combustion gases. But what happens at cold temperatures if the lube is too stiff to squirt? I'm wondering if cold, hard lube makes the bullet act like a solid monolithic bullet, distorting the entire bullet? Also, it's possible that the displaced lube in the check shank, rather than squirting back into the throat, pushes against the gas check and pops the gas check off? I had experimented with removing lube from the check shank , and at that time it did not solve the flying gas check problem, but it's possible that there is more than one cause of flying gas checks, so I may have to revisit that theory.

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Re: Does swaging a cast bullet improve accuracy?

Postby mtngun » Sun Jan 29, 2017 12:29 pm

Getting back to the original question of swaged bullets -- we now know that my original test of swaged bullets, fired at 35F - 40F, is invalid due to the cold temperatures. So I will have to retest swaged cast bullets when the weather is more cooperative.


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