6mm BR & switchbarrel bolt gun

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Tue Jun 07, 2016 9:18 pm

A lucky group with the 75 grain bevel base. Deets on my coating shootout thread at the CBA forum.

Image

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sat Jun 11, 2016 5:55 pm

Here's a couple of bullets destined for someone's 6ppc. They're very similar to the Eagan MX2 design except Eagan's original truncated cone flatpoint nose has been replaced by a pointy ogive. The hope was to reduce the wind drift while otherwise maintaining the shooting characteristics of the MX2 (which shoots extremely well in the 6ppc in question). These bullets were cast of linotype because that's what the ppc shooter uses.

Will they stabilize in a 14" twist? Here's what the Miller stability formula predicts:

75 gr.
SF = 1.10 at the ppc shooter's 1100' elevation, 50F, 2200 fps
SF = 1.34 at my 4600 elevation, 50F, 2700 fps

77.5 gr.
SF = 1.07 at 1100' elevation, 50F, 2200 fps
SF = 1.30 at my 4600' elevation, 50F, 2700 fps
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The nose drops out a little on the fat side for the 6ppc's well worn throat, so I nose-sized them to fit my tight 6BR barrel. Here's how it looks after nose-sizing and seating to engrave slightly.
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I'm not a big fan of traditional bore riding bullets, especially at high velocities, but some people swear by them, so I wanted to try a few just to see what would happen.

The good news is that both bullets stabilized in my 14" twist, just as Don Miller predicted they would. The bullets made round holes in the paper. The bad news is they shot patterns, not groups. :? I can't say that I am surprised. Nonetheless it is a data point and as time allows I'll try to tweak this basic bullet design to see if I can get it to shoot better, though I'll prefer to use my normal alloy of hardened wheelweight or reclaimed shot.
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These linotype bullets did leave very light leading near the muzzle. The rest of the barrel was clean.

My interpretation of leading near the muzzle is that the bullet got too hot due to sliding friction. Another popular interpretation is that the bullet "ran out of lube." Those two interpretations are not incompatible.
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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sat Jun 11, 2016 8:05 pm

Today we tested different powders with the Hi-Tek coated 75 gr. BB.,

I have been using WC844 powder just because I had some sitting on the shelf, but 844 burns poorly with the slippery plain base bullets. Velocity variation for 10 shots has often been 200 fps or more. Velocity standard deviation runs around 1.9%, which I consider poor.

I shot one 10-shot group with each powder at 2700+ fps. While one good group does not prove that a load is good, one bad group is all it takes to prove that a load is bad! :lol: It was enough to weed out some powders that obviously were not suitable. Chart summarizes the results.

Varget was the only powder that demonstrated both decent accuracy and decent velocity variation, so I shot two more groups with Varget, which are included in the chart.

N133 showed some accuracy potential but did poorly on velocity variation. It might burn better with GC bullets.
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Varget averaged 1.86", not significantly better than WC844's 2.02" average-to-date.
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Varget seems the best powder that I've tried so far for the slippery 75 gr. plain base. Note that Varget was the slowest powder tested today.

Powders with a similar burning rate as Varget and WC844 (same as H335) that I haven't tried yet, might be H4895, RL12, AA2520, and RL15.

The bad news is that I am nearly out of Varget and it'll be a while before I can order more powder. :evil: In the meantime, I'll continue to shoot WC844. :x

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sun Jun 19, 2016 5:44 pm

I finally decided to keep the Kick-Eez recoil pad, so I went ahead and fitted it to the stock and refinished the stock (which had never been refinished after reshaping the stock to fit the rear bag). By the way, the Kick-Eez weighed 14 ounces before shaping, if I remember correctly, so if you are looking for a lightweight recoil pad to "make weight" on a competition gun, the Kick-Eez is not it. :D Unlike a lot of pads that are hollow, the Kick-Eez is solid sorbethane, plus it has a brass plate molded into it to hold its shape.

Here is how the rifle currently rolls. The white thing on the front of the scope is a "modifier ring," just a piece of plastic with a 1" hole drilled in it. It acts similar to a camera aperture to increase the depth of focus, but at the cost of less light transmission. It doesn't reduce mirage at the target but I used to struggle with keeping the reticle in focus on mirage-y days and the modifier ring solved that problem for me. The loss of light is not a problem during the day but I do unscrew the modifier ring if I'm shooting in the late evening, when there is little mirage anyway.
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Getting back to the lousy velocity spread. I did scrape together enough money to order some more Varget, as well as a pound each of some more powders to try (H322, H4895, H4166, Benchmark, and Reloader 15). In the meantime, I wondered if a heavier bullet would make WC844 burn better? So I scaled up the 75 gr. BB to 80 gr.

Note the bevel on the 80 gr. is smaller than the bevel on the 75 gr. That's because it obturated a bit during the push-thru sizing step prior to coating. I'm not sure if that matters, but on the next batch I'll try a different sizing routine to try to reduce the unintentional obturation.

OAL was 0.817" and weight was 81 grains, after sizing and lubing. Miller Stability factor in a 14" twist is:

SF = 1.38 @4600', 50F, 2500 fps
SF = 1.17 @sea level, 50F, 2500 fps

So it should be plenty stable at my elevation. :)
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I used the same nose-sizing and seating settings as for the 75 gr. bullet. As a result the extra length went into the case while the part sticking out of the case remained the same diameters and length as the 75 gr..

It shot just as well as the 75 gr. if not a little better. This is the rifle's best average to date for 4 consecutive groups! :cool: Unfortunately the ES did not improve much. The first group (on the left) was 1.15% standard deviation, but then the next three groups averaged 1.78%, not a whole lot better than the 75 gr. bullet. Nonetheless the groups were round.
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The BHN was not actually verified for this batch. This lot of reclaimed shot has run from 10 BHN up to 14. I've been adding it to the pot directly out of the bag without making any attempt to blend all the bags together, so that may be a source of variation.
UPDATE: I measured one of the bullets from this batch and it was 14.7 BHN.

Things To Try Next Time and Down The Road
-- retest the 80 gr. BB to see if the 1.35" accuracy is repeatable
-- verify and possibly tweak the BHN. Would 14.5 or 15 BHN shoot better?
-- test the 80 gr. BB with Varget

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Mon Jun 27, 2016 7:45 pm

Today I re-tested the 80 gr. bevel base Hi-Tek coated bullet that averaged 1.35" in the June 19 post. The goal was to see if the accuracy was repeatable.

No, not as good as last time, and there was at least one crazy flier in the 2nd target from the left. :x
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As to why it could not repeat the 1.35" accuracy with the same batch of bullets, your guess is as good as mine. The load was identical to the June 19 load and range conditions were good.

Until better evidence comes along I'm going to assume that the fliers are caused by the occasional coating failure of some sort. At this point I don't know if the alleged occasional coating failure is something that can be fix or if it is the nature of the beast when shooting plain base bullets at 2700 fps. :lol: :lol: :lol:

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Wed Jun 29, 2016 6:15 am

Following up on the MX2-EXP that shot patterns in my barrel and not-so-hot in Paul's PPC. Paul noted that the tapered nose engraved more than his modified Eagan nose. While normally more engraving is a good thing, the metal displaced by the rifling has to go somewhere, otherwise the entire bullet gets squeezed like a tube of toothpaste. Anyhow, that was the thinking behind today's bullet, the MX2-EXP2.0 -- put shallow grooves on the nose so that the metal displaced by the rifling can be pushed into the grooves without distorting the entire bullet.

To keep the comparison apples-to-apples, I did not lube the nose grooves. Load was 26.5 WC844, nose-sized & seated with the same settings that I have been using for all my heavy 6mm bullets lately. Cast from reclaimed shot and oven treated so they were hard.
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It sounded good in theory, but the EXP2.0 did not shoot any better than the original EXP. The 2826 fps velocity was higher than intended, perhaps due to the high BHN.

On shot #5 the chrono died and a hole appeared in the chrono display. Oops! :o Yet the shot "felt good" and landed within the group. Could it be that the GC came off?
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A nice clean 6mm hole. :(

On a side note, a couple of days ago I noticed a 6mm hole in the chrono's LED light. It didn't hurt the light but at the time I wondered if it might have been caused by an errant gas check? The last gas check bullets fired prior to noticing the hole in the light had been the EXP bullets, so in hindsight the gas checks had been coming off the EXP's as well as today's EXP2.0.
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The errant gas check was found just inside the chrono.
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The bad news is that I killed a 12 year old chronograph, the good news is that I learned something about cast bullets -- and learning stuff is what this project is all about. :)

The EXP2.0 gas checks were a snug fit going on, requiring a firm push, and then crimped on solidly when sized. So why did at least one check come off?

In the beginning, I used 0.244" or 0.2444" bullets in this barrel. Then somewhere along the way I switched to 0.245", which didn't seem to hurt at the time, particularly with plain base.

I'm wondering if the entrance to the throat -- which was 0.245" when the barrel was new -- is catching the 0.245" gas checks and pulling them off? Maybe try sizing 0.244" or 0.2445" and see if accuracy improves with GC bullets?

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Wed Jun 29, 2016 7:12 pm

The smallest 6mm sizing die I have is 0.2435", so I re-sized the 0.245" EXP2.0 bullets in that die. Due to spring back they were coming out 0.2443", so I ran them through the sizing die a second time and they came out 0.2440".

I dropped the powder charge to 26.0 grains which should produce ~2750 fps.

The theory was that the 0.245" gas checks might have been snagging on the entrance to the 0.245" freebore, and that a smaller check diameter would reduce snagging. But the 0.244" bullets still made a shotgun pattern. :(
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After 10 shots, I stapled freezer paper to the exit of my chronograph tunnel to detect any errant gas checks. 3 shots were fired through the paper shooting blind and relying on the bags to keep the rifle pointed in the right direction. Those 3 blind shots are included in the group above -- perhaps they are the 3 shot cloverleaf? :D :D :D

At any rate, 2 out of 3 gas checks definitely parted company with the bullet. There was a 3rd hole in the paper that might have been a gas check but it was a ragged hole so I couldn't be sure. So perhaps 3 out of 3 gas checks came off and for sure 2 out of 3. No wonder it's making shotgun patterns ! :x

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The fact that 0.244" bullets still lost their gas checks does not eliminate out the possibility that checks are snagging at the entrance to the throat.

Action Items

-- take the barrel off and remeasure the freebore. I've done that before (my notes say the freebore is 0.245") but I'll do it again to make sure I didn't miss anything.
-- drill out the primer pocket on a fired case so I can insert the bore scope with a fired case in the chamber, and see what the transition between the case neck and the freebore looks like.
-- as time allows, make some bullets with no lube space ahead of the gas check. I'm skeptical of the "hydraulic pressure pushing the check off" theory but nonetheless I'm willing to give it a try.
-- possibly make a tapered brass lap to lap the transition between the chamber neck and the freebore.
-- if all those things fail to fix the problem, and if I don't come up with any better ideas, I may set the barrel back and rechamber, and try to cut a more tapered transition into the freebore.

Isn't this more interesting than shooting jacketed bullets into one ragged hole? :D

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Thu Jun 30, 2016 9:32 am

More real data:

A quick and dirty way to measure the freebore diameter on these short cartridges (when the barrel is removed from the action) is to insert gage pins and then use a magic marker to mark how much of the gage pin protrudes from the breech. This is not super accurate but it gets you to within 0.001" (throat impressions and chamber casts can also be off 0.001", but that's another story).

A 0.244" pin definitely enters the freebore. A 0.245" pin only enters a little bit. These are minus pins, by the way. Hence the entrance to the freebore is at most 0.245".
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Some people suggested that hydraulic pressure caused by the lube in the GC groove could have been pushing the GC's off. I used a toothpick to scrape most of the lube out of the GC groove on three MX2-EXP2.0 bullets. These bullets were sized 0.245", by the way. Due to springback their GC's measured 0.2454".

The alloy is reclaimed shot oven treated at 470F, so these are hard bullets.
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The 3 bullets were loaded on top of 26.0 gr. WC844 for approximately 2750 fps, and fired blindly through cardboard at 16 feet.

This photo was taken on the entrance side of the cardboard (the exit side holes were torn and harder to interpret). My interpretation is that the clean-cut holes were made by gas checks, while the fuzzy holes were made by bullets. I count 3 clean-cut holes and 3 fuzzy holes.
Image

Can we dismiss the "hydraulic" theory now?

OK, so what happens when you push a 0.2454" gas check into a 0.245" freebore? If there is a tapered entrance the gas check should size down to fit. But most freebores do not have a gently tapered entrance.

When I ground the neck reamer, I attempted to put a 45 degree chamfer on the end of the neck reamer. 45 or 60 degrees is pretty typical at the transition between neck and freebore. 45 degrees is not a gentle angle, though. Also, sometimes reamers push up a ridge of metal as they cut. As time allows I'll try to get some borecam photos of the entrance to the freebore, but I dunno if the Lyman borecam is up to the job. I tried yesterday but the photos came out too dark and grainy to make out much detail.

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sat Jul 02, 2016 10:38 am

I'm skeptical that crimp-on checks routinely come off in flight due to aerodynamic drag (but will do more testing with other cartridges to verify). It seems to me that something must be happening in the barrel to knock the check off. All I can think of is that the check is being grabbed by the abrupt corner at the entrance to the freebore. I could be wrong. The only thing I'm sure about is name, rank, and serial number. :D :D :D

So .... I decided to try putting a forcing cone at the entrance to the freebore. Worst case, if the forcing cone doesn't work out, then I set back the barrel and rechamber. It wouldn't be the end of the world.

I chose 11 degrees per side because 1) I wanted keep the length of the forcing cone to a minimum and 2) 11 degrees is commonly used for revolver forcing cones. Maybe 5 degrees would be better, I dunno, this experiment was just a stab in the dark with absolutely no guarantees.

I chose the max diameter of the forcing cone to be 0.250" if the reamer was run in all the way. 0.250" was another stab in the dark that may or may not have been a good choice. If there is a manual on how to do this, I haven't run across it. :D

D-bit reamer with a snug fitting pilot. To avoid the hassle of setting up the barrel in the lathe, I instead supported the rear of the reamer with a drilled-out case and cut the forcing cone by hand. With the reamer supported at both ends it couldn't help but cut straight.
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Borecam showing the new forcing cone (the shiny band). At the time I had no way to accurately judge the depth of the cut so I cut a little, examined the cut with the borecam, cut a little more, and repeated the process several times until it looked about right.
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It didn't occur to me while I was cutting the forcing cone, but later I snapped a 1/100ths scale with the borecam. If I apply the same scale to the above pic of the forcing cone then the forcing cone is 0.044" - 0.049" long. It only needed to be 0.013" long to reach the full 0.250" diameter, so in hindsight I cut further than necessary.
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Did the forcing cone make an iota of difference? 3 shots at 20 feet while pointing shotgun-style. 3 bullet holes and 1 GC hole.
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3 more shots fired without aiming, this time from 30 feet away. No extra holes in the target.
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It takes more than 6 shots to prove something beyond doubt, but it does appear that the forcing cone may have reduced the frequency of errant gas checks to only 1 out of 6 shots. Even one errant gas check is too many, though. :(

What about accuracy? The EXP2.0 bullet still shot a lousy 4.45" with the forcing cone.
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Conclusion: the forcing cone at the entrance of the freebore may have greatly reduced the frequency of gas checks coming off, but it did not completely eliminate the problem. It did not make a noticeable improvement in accuracy with this one bullet. Nonetheless the reduction in errant gas checks suggests that perhaps I was at least barking up the right tree ???

Things To Try:
-- annealed gas checks

-- will the forcing cone improve accuracy with coated plain base bullets? Was the coating being scraped off coated plain base bullets as they entered the forcing cone?

-- perhaps try 5 degrees rather than 11? A 5 degree cone would only need to be 0.030" long to increase the opening diameter 0.005".

-- try shooting GC bullets in other barrels at close range to see if they are hanging on to their gas checks.

-- any other things I should try?

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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sat Jul 02, 2016 11:47 am

As a sanity check, I'll be shooting other firearms at 30 feet from the target to see if gas checks are coming off. Here's a Marlin 357 mag. Note that the check shank on this particular mold has a "relaxed" fit, which is how I prefer GC's to fit.
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