6mm BR & switchbarrel bolt gun

[mtngun]

Today we tried a harder (40 BHN alloy), polyurethane coating, and several different weights. Bullets were nose-sized to match the throat & leade.


Results were mostly an improvement compared to the uncoated 25 BHN GC bullets tested last time, though still shy of my goal of ten shots in 1 MOA at 2700+ fps. I assigned each target an ID# and I'll describe that load further down.


#T1: 57 gr. spitzer GC, reclaimed shot heat treated @ 470F (or HTRS for short), 3 coats oil-based polyurethane (or OBPU for short), 24.2 gr. WC844, loaded 2.044", seats to 2.028" when chambered. 2657 fps and 0.97% standard deviation.

#T2: same as #T1 except powder charge increase to 25.0 grains. 2789 fps and 0.58% standard deviation.

#T3: 65 gr. spitzer GC, HTRS, OBPU, 25.5 gr. WC844. Loated 2.157", seats to 2.137" when chambered. 2849 fps and 0.69% standard deviation.

#T4: same as #T3 except we increased the powder charge to 28 grains just to see what would happen. 3104 fps and 1.23% standard deviation.

After shooting the 3104 fps load, there was a little grey color in the corner of the groove in a spot a few inches long, mid-barrel.


Otherwise it was squeaky clean. On the whole this was an improvement compared to the uncoated 25 BHN bullets I shot last time.


#M1 & M2: 74 gr. spitzer GC, HTRS, OBPU, 26.5 WC844. Loaded 2.192", seats to 2.175" when chambered. 2833 fps and 0.68% standard deviation.

Taran's Predicted 10 shot group for each of today's loads:
-- 1.16" for 57 gr. @ 2789 fps
-- 1.46" for 65 gr. @ 2849 fps
-- 1.50" for 74 gr. @ 2833 fps
-- 1.60" for 57 gr. @ 2657 fps
-- 2.29" for 65 gr. @ 3104 fps

Conclusions:
-- the GC bullets seem to like the HTRS and the polyu coating so I'll stick with that combo until something better comes along.
-- accuracy deteriorated at 3100 fps but at least it still shot a "group" and fouling was minimal.

Things To Try Next Time:
-- vary nose-sizing depth.
-- vary the powder charge in the velocity range 2725 - 2850 to see if there is a sweet spot.
-- if I still can't hit my 1 MOA goal then I may fall back on Reloader 10 which seemed to be more accurate than WC844.
-- I have some more coated plain base experiments lined up when time allows.

[mtngun]

For some reason there were some loaded 74 gr. GC rounds leftover from last time, except with a half grain less powder to bring the velocity down to the 2700's, so I went ahead and shot them. Accuracy was about the same as last time so at least they're consistent. This is my first barrel that would consistently do sub-2MOA at 2700+fps, so I'm pleased even though I haven't achieved the magic 1 MOA yet.


Today's main goal was to compare water based polyurethane coating to oil based polyurethane coating. I chose plain base bullets as the guinea pig because plain base are more dependent on the coating than GC. The plan was to start out at 2000 fps and increase the velocity until it started shooting wild, and see which coating failed first. However, Mr. Murphy had other plans and as it turns out this barrel does not like the 62 gr. plain base developed for the 6x45 project, so accuracy was lackluster no matter the velocity.

Also the original plan was to make both types of coatings 0.001" thick, but Mr. Murphy intervened and gave us 0.001" thick on the oil-based and 0.0018" thick on the water-based.

Anyway, here are today's targets with an ID# for each target that will be explained below. My labeling system is "TOP ROW #1-4" left to right, abbreviated at T1, T2, and so on. The middle row is M1, M2, and so on. The lowest row is L1, L2, etc..

From now on I will abbreviate oil-based polyurethane as OBPU and water-based as WBPU.


#T1: 74 gr. GC, HTRS, ~40 BHN, , OBPU, 2.196" COL, 26.0 gr. WC844, 2768 fps, 1.23% standard deviation, 10 in 1.68", mean radius=0.55".

#T2: 62 gr BB, ACWW, ~11 BHN, OBPU, 2.075" COL, 15.0 gr. RL7, 1845 fps, 2.9% SD, 10 in 3", MR=1.04. Note the high standard deviation suggesting the pressure was too low to make RL7 burn well.

#T3: 62 gr. PB, ACWW, ~11 BHN, OBPU, 2.090" COL, 16.0 gr. RL7, 1970 fps, 0.58% SD, 10 in 3.15", MR=1.10".

#T4: 62 gr. BB OBPU, same as other BB loads except 18.0 gr. RL7, 2202 fps, 1.34% SD, 10 in 1.82", MR=0.64".

#M1: 62 gr. PB, OBPU, same as other PB except 19.0 gr. RL7, 2344 fps, 0.96% SD, 10 in 2.33", MR=0.75".

#M2: 62 gr. BB, OBPU, same as other BB except 20 gr. RL7, 2439 fps, 2.25% SD, 10 in 3.35", MR=1.11". Odd that the SD was high, maybe it was starting to lead? I did not check with the borecam.

#M3: 62 gr. PB, OBPU, same as other PB except 21 gr. RL10, 2460 fps, 1.6% SD, 10 in 12.98", MR=2.83". Poor accuracy and high SD again suggest that it was starting to lead, but I did not check with the borecam because I was short on time.

#M4: 62 gr. PB, WBPU, same as other PB except 16 gr. RL7, 2015 fps, 1.52% SD, 10 in 2.53", MR=0.79".

#L1: 62 gr. BB, WBPU, same as other BB except 18 gr. RL7, 2212 fps, 0.83% SD, 10 in 3.91", MR=1.08".

#L2: 62 gr. PB, WBPU, same as other PB except 19 gr. RL7, 2329 fps, 1.0% SD, 10 in 3.61", MR=1.06".

#L3: 62 gr. BB, WBPU, same as other BB except 20 gr. RL7, 2448 fps, 0.84% SD, 10 in 1.95", MR=0.56".

#L4: 62 gr. PB, WBPU, same as other PB except 21 gr. RL10, 2520 fps, 0.67% SD, 10 in 1.99", MR=0.70. Note that both SD and accuracy were much better than the OBPU with this powder charge, suggesting again that the OBPU was leading, while this WBPU did not lead.

Conclusions and Observations:
-- this was supposed to settle for once and all whether water-based was as good as oil-based, but instead neither shot well because apparently the barrel does not like the 62 gr. bullet.
-- that said, the oil-based went wild around 2450 fps while the water-based maintained 2 MOA accuracy to 2520 fps. But, remember the water-based was 0.0018" thick while the oil-based was 0.001" thick, so the added thickness may have been the determining factor. Aargh !
-- the jury is still out on water-based poly. This test also raises questions about the optimal coating thickness. Previous tests determined that 0.001" thick was more accurate than 0.003", presumably due to less imbalance, but that doesn't mean that 0.001" is optimal. Hmmm.....

Things To Try Next Time:
-- perhaps the barrel would prefer a 74 gr. plain base?
-- if I can find a plain base it likes, then I'll redo the water vs. oil shootout.
-- on GC, tweak nose-size depth and neck tension.

[mtngun]

50 mph winds today so I stayed inside, cast some experimental 75 gr. bevel base bullets, and made a 0.239" M-die spud so I can experiment with more neck tension. I had been using a 0.241" spud.

[mtngun]

Today we're back to tweaking gas check loads, using a 60 grain spitzer -- actually 58 grains when cast of reclaimed shot -- so it's nearly the same as the "old" 57 grainer except the ogive is a tiny bit longer.

I arbitrarily chose a 25 gr. charge of WC844. The powder charge has not been optimized yet, perhaps that will happen next time.

Bullets were cast with reclaimed shot, oven treated at 470 F for approximately 40 BHN, and given 3 thin coats of oil based polyurethane (OBPU) about 0.001" total thickness.

The goal was to see how much nose-sizing, if any, is optimal. The nose die matches the throat and leade, resulting in a "glove fit." First we tried 1-diameter bullets, then we tried light nose sizing (called "taper #1"), then a little more nose sizing (taper #2), and finally as much nose-sizing as could be done and still make contact with the leade when chambered (taper #3). All bullets were seated 0.020" into contact, or in other words the bullet seats an additional 0.020" deeper when it chambers.




Today's target. As you can see, the general trend was that more nose sizing was better. Taper #3 turned in the best group of the day. Since I only fired 10 shots for each taper (except 21 shots for taper #1), no one group is statistically significant, however I believe the 61 total shots fired were enough to "prove" the general trend.


Conclusions:
-- the full monty Ardito-style taper, with only the gas check seated in the case, seems most accurate, just like John Ardito said it would be. So that will be my new standard for the GC control load.

Things to Try Next Time With GC:
-- optimize the powder charge (as long as it's 2700+ fps)
-- try sorting bullets by weight ?

[mtngun]

Today I gave coated plain base bullets another whirl. So far this barrel has only been able to shoot coated plain base up to 2300 - 2400 fps before they would crap out and start shooting wild. By contrast the Shilen 6x45 would go up to 2570 fps with coated plain base, averaging about 2 MOA for ten-shot groups. Since the 6BR requires less pressure to achieve the same velocity as the 6x45, you would think the 6BR would beat the 6x45, but that has not been the case.

Since this barrel did not like the 62 gr. PB, I will try a 75 grainer today. Air cooled wheelweight (~11 BHN) coated with oil base polyurethane 0.001" total thickness. I'm not sure that 0.001" thickness is optimal but it's an easy number to remember.

I had hoped that the 75 grain bullet would create enough resistance to make WW760 burn well. Quickload said that 25ksi would be required to drive the 75 grainer 2570 fps, the same pressure, same velocity, same powder, and same coating as the 6x45 with its 62 grainer at 2570. You would think that if the pressure, velocity, powder, and coating were the same, they would shoot the same?

Anyway, since the GC bullets liked the full monty Ardito taper, I tried a full monty Ardito taper on the 75 gr. BB, pushed by 29.5 gr. WW760.


But WW760 did not burn well, with 2.2% velocity standard deviation at 2563 fps. I consider anything over 1% standard deviation to be poor in a rifle. 0.5% might be a good number if the powder were in its optimal pressure range.

The WW760 load shot wild. The poor velocity standard deviation cannot explain this, so I assume the coating is burning through and allowing the base to melt and gas cut.


So I tried different powders, and less nose-sizing.


Reloader 10 burned tolerably well, with 1.12% velocity standard deviation, Yet the bullet still shot wild. The camera missed one bullet hole that the arrow is pointing to.


WC844 burned decently, with 0.94% velocity standard deviation, yet the bullet still shot wild. Obviously the coating must have failed. Satan has possessed my bullets !


Note that all of today's "groups" were dispersed in the same diagonal pattern. I dunno what causes that? If the bases were melting and gas cutting you would think the dispersion would be random?

All of today's loads showed some grey color in the corners of the grooves, particularly 2" - 10" beyond the throat. The rest of the barrel was mostly clean. While a little grey color is not the end of the world, I consider any fouling to be undesirable. It suggests that the bullet was beginning to fail and gas cut.


Conclusions:
-- the same coating that worked up to 2570 fps in the 6x45 is failing at 2300 - 2400 fps in the 6BR. I don't know why.
-- WW760 has yet to burn well in any load in the 6BR. Loading density was only 80% so that may be part of the problem. Some powders tolerate reduced loading density, others don't.
-- I've tried enough different bullet weights, different nose-sizing schemes, and different powders to rule out those things. By the process of elimination, coating failure seems to be the reason for the wild groups at 2500+ fps.

Things To Try With Coated Plain Base Next Time:
-- thicker coatings.
-- retest powder coat and Hi-Tek. I've already tested them in the 6x45 but maybe this barrel has different taste.
-- keep searching for the ultimate coating.

Those coating tests will take time and may not produce immediate results, so I'm going to put plain base on the back burner for a while. The tentative plan is to finish tweaking a decent GC "control" load, then move on to the other calibers in this switchbarrel M700. Once I have a reliable control load then I can always return to the 6BR to do experiments, which is the general idea behind this project.

[mtngun]

Assuming I eventually settle on a gas check "control" load, one of the first experiments planned is a groove shootout. I've already done similar shootouts in other cartridges so I doubt that the results will be surprising, but nonetheless it's always good to have more data.

The tentative plan will be to shoot these two "extreme" designs along side my current 60 gr. GC.. If there had been room, I would have used even more extreme designs, but there is only so much real estate available on a 60 gr. bullet.

Also note that the differences in the length of the check shank.

[mtngun]

Today's plan was to tweak the powder charge for the 60 gr. GC spitzer that we have chosen for our GC "control" load. Things did not go as planned.

To begin with, the first prototype of the 60 gr. GC mold had a machining boo-boo that made the front band drop out 0.246" or so, much fatter than intended. When sized, the fat front band became longer. That mold had some other issues as well so I made a replacement mold. The replacement mold casts much better but still was not perfect (all these spitzers that I am experimenting with are R&D prototypes that are still being fine tuned and debugged). Bullets from the "bad" mold have a slightly longer front band than the "good" mold. The ogive on the "good" mold is a little different, too. So we were actually shooting 2 different 60 grain bullets today, which I'll call "good" and "bad" for lack of a better name.


Also, remember that this rifle has a pair of spring plungers on the forend, intended to dampen vibrations. Until today, the spring plungers were arbitrarily set to "contact plus 2 turns." The intention was to eventually experiment to determine the best setting or if they even helped at all. Today we finally began those experiments.

Unless otherwise noted, all of today's loads used the 60 gr. GC, cast of reclaimed shot, oven treated 470F, 3 thin coats oil-based polyurethane, HVR lube, CCI #41 primer, and WC844 powder. Bullets were nose sized to be a glove fit. They were seated with only the GC in the case and about 0.020" into the lands.

Here is the target. The groups are labeled in the order that they were shot so you can perhaps decipher the method to my madness.

Groups #1-3, walking up the powder charge with the "bad" bullet. Spring plungers set at +2 turns

Group #4, drop the powder charge to 24.8 gr. with the "bad" bullet.
Group #5 same as above except with "good" bullet.

Group #6, drop powder charge to 24.6 gr. with "good" bullet. Spring plungers removed.
Group #7, same as above except with "bad" bullet.

Group #8, return to 24.8 gr. with the "bad" bullet. Spring plungers set at +1 turn.
Group #9, same as above except with "good" bullet.

Group #10, same as #8 except with spring plungers at +2.5 turns.


The good news is that I set a new personal best for high velocity cast.


Over 100 high-velocity rounds were shot without cleaning the barrel. This is what it looked like afterwards:

Just a tiny bit of grey in the corner of the grooves, starting about 2" past the throat, and extending to the middle of the barrel. Most people would consider this not worthy of mention but I don't like any fouling at all. It suggests to me that the gas seal is not 100% perfect in this region, probably due to high pressure.


The latter half of the barrel was squeaky clean.


Conclusions:
-- minor tweaks to the powder charge made little difference while using the spring plungers. Apparently the spring plungers really do dampen vertical barrel vibrations.
-- but .... 5 out of 8 groups that utilized the spring plungers had more horizontal dispersion than vertical dispersion. I'm suspicious that the spring plungers are causing problems horizontally, even though they do seem to dampen vertical dispersion.
-- group #7, with the barrel free floated, showed noticeable vertical stringing. But it may be possible to find a "node" that eliminates that vertical stringing.
-- the "bad" bullet averaged 1.79" vs. 2.04" for the "good" bullet. It's starting to look like the "bad" bullet shoots better than the "good" bullet, but there were too many other things going on to be sure.
-- in any event I am out of the "bad" bullets so will use the "good" bullets from now on. We'll be experimenting with different bullet designs later on, anyway.
-- after thinking it over, I'm inclined to abandon the spring plungers because they seem to cause horizontal dispersion. Even if I were to find a "sweet spot" on the spring plungers for this 6BR barrel, I might have to re-tune the spring plungers every time I switch barrels. That doesn't seem like a good idea on a switch-barrel rifle.

Things to Try Next Time or Down The Road
-- free float the barrel and once again tweak the powder charge in search of a happy "node."
-- if that doesn't produce results I might try tightening the barrel with a torque wrench.
-- sort bullets by weight, though I should probably wait until I settle on a "control" load.
-- experiment with tweaking the bullet design once I settle on a "control" load.

[mtngun]

Today's range conditions. It alternated between snow, fog, and sun, which made for ever-changing mirage.


All theoretical considerations aside, I decided to disable the spring plungers/barrel tuner because it just didn't seem practical on a switch barrel rifle. So today's goal was to optimize the powder charge with the 60 gr. GC bullet and free floated barrel.

Recall that there were 2 different molds for the 60 gr. GC, and both had imperfections. The "bad" mold dropped an excessively fat front band. The "good" mold dropped a slightly undersize ogive. It's ogive was a tiny bit longer, too. The differences were subtle but nonetheless seemed to affect accuracy -- the "bad" 60 gr. bullet shot good and the "good" 60 gr. bullet shot bad.

I ran out of good-shooting "bad" bullets so today I only shot bad-shooting "good" bullets.

Some background on my spitzer designs: I generally frown on long unsupported noses. Cast bullets need support! On the other hand, target shooters want a good ballistic coefficient to buck the wind. So I started out with very short ogive spitzers (see the 77 grainer below) and have gradually been experimenting with longer ogives, trying to determine how long the unsupported nose can be before accuracy degrades. The answer will probably "just depend," but in the case of the 60 gr. bullet the "good" mold seems to have reached that limit. Apparently the short stubby bullet can't quite support that particular ogive.


Here is today's target. I said earlier that it had stopped snowing, but naturally as soon as I began shooting it started snowing again. Sometimes the snow would stop, sometimes fog would roll in so that I could barely see the target, and sometimes the sun would peak through. Lather, rinse, and repeat. The mirage was constantly shifting, ugh.

The top row and the middle row used the bad-shooting "good" 60 gr. GC. The bottom row used the 77 gr. GC.

Yesterday the "good" bullet averaged 2.04", today it averaged 2.17". At least it is consistent -- consistently bad!

Until today I have always shot this rifle "free recoil," not touching the gun at all except the 2 oz. trigger. But the lousy accuracy of the bad-shooting "good" 60 gr. GC made me want to try holding the rifle, so I did. The first 5-shot group looked promising, but it turned out to not be repeatable. So I gave up on holding the rifle and went back to "free recoil." Less skill and concentration are required to shoot free recoil because all you have to do is touch the trigger. B)

For a sanity check, I shot a couple of groups with a 77 gr. GC (bottom row) that were leftover from a previous experiment. The 77 grainer had previously put 16 shots in 2.08" at a leisurely 2608 fps. Today it shot 1.5 MOA (other than one flier in some funky ever-changing mirage) despite a brisk 2800 fps. That tells me there's nothing wrong with the gun, it simply doesn't like the bad-shooting "good" 60 gr. bullet.


Conclusions:
-- today was frustrating because accuracy went backwards not forwards. But at least I am learning things and that's what this project is really all about.
-- I learned that it doesn't like the slightly longer, slightly less supported ogive of the bad-shooting "good" 60 gr. bullet.
-- despite the poor accuracy I did settle on the powder charge for the 60 gr. GC.. 25.1 grains seems to have a slight advantage for both accuracy and velocity variation.

Things to Try Next Time and Down The Road:
-- correct the 60 gr. ogive and retest.
-- if the 60 gr. still doesn't shoot then move up to a heavier bullet, but I'm pretty sure it will shoot if the ogive has a little more support.
-- try sorting bullets by weight.
-- experiment with different 60 gr. bullet designs

[mtngun]

Borecams at the end of today's session, 94 rounds without cleaning. The session ended with the 77 gr. bullets at 2800 fps. All of today's bullets were coated with shake-n-dry oil-based-polyurethane (OBPU).

It was cleaner than yesterday, perhaps because the long 77 grainers seal better? The only fouling was this grey color in the corner of the groove and only in a short stretch 1" - 2" past the throat.


The rest of the barrel was squeaky clean like this:


For the record the 77 grainers averaged 2802 fps and 0.41% velocity standard deviation -- that's very good standard deviation. The heavier bullet seems to help the powder burn better.

By comparison, the standard deviation for the 60 gr. bullet averaged 0.98% when pushed by 25.1 gr. of WC844. That's nothing to brag about but seems typical for the lighter bullets.

[mtngun]

The undersize ogive has been corrected on the 60 gr. GC.

That'll increase the bearing length a tiny bit, but not a lot. You wouldn't think it would make much difference. We'll see.