I've previously performed shooting comparisons of several different coatings in a 6x45 and in a 357 rifle, and those comparisons led me to settle on oil-based polyurethane, but disappointing performance with the urethane coated plain base in a 6BR is forcing me to try a little harder.
So this time I'll be doing some different kinds of comparison tests, trying to understand what characteristics make a good coating, and trying to understand how coated bullets fail. The shoot-out will include oil-based polyurethane, an electrostatic applied polyester powder coat, an electrostatic applied silicone powder coat, and zombie green hi-tek coating.
Today was my first time ESC'ing bullets, so I'm sure I have a lot to learn about it, but here is how I went about it. First, I made fixture to hold the bullets base-up, because I'm going to be testing plain base bullets and I suspect that it is important that the base receive a good coat. I used a 1/4” center drill to drill an array of holes in an aluminum plate. The depth of the hole was chosen so that the coating would stop at the top lube groove, that way any “flash” would end up in the lube groove where it would do the least harm.
Using this base-up method, the base ends up getting a thicker coat than the rest of the bullet. That's not necessarily a bad thing, though, depending on your favorite theory of how coatings work. I believe coatings function similar to a gas check, protecting the base from melting and gas cutting, so I'm happy with the base receiving a thicker coating.
The silicone powder does not stick as well as polyester powder so my first batch of silicone powder coat turned out a little thin. About that time I noticed that the air pressure was dropping down to 5 psi while spraying, so I adjusted the regulator up to the recommended 8 psi and sprayed a second batch, deliberately putting it on a little thicker. This is that second batch.
If the shooting results are black and white, I may be able to wrap up this shoot-out in one day. But if the results are “muddled” then testing may drag on for a while. We'll see.
I'm also performing an oven test to determine at what temperature the coatings “fail.” I'm not sure if that's relevant to the shooting performance, but it might be.
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I had <url=<a href=)
http://www.mountainmolds.com/phpBB3/viewtopic.php?f=8&t=561&start=10#p2468>already tested OBPU in this barrel and found that it began shooting wild somewhere around 2300 - 2400 fps depending on the coating thickness, the powder, and the pressure. But we gave it another go today using the 22.5 gr. WC844 load. Accuracy was poor so I did not waste time testing OBPU any further. :(
-- 0.0025” thick
-- 0.0014” out-of-round</p>
<p>The problem with a thick coat is that invariably it increases out-of-roundness due to variation in the thickness of the coating. That causes the bullet to be out of balance. :(</p>
<p>The BHN 11 bullets were less accurate than BHN 18 at every powder charge, though neither was very accurate and I didn't shoot enough to “prove” a statistically significant difference. </p>
<p><u>Conclusions:</u>
-- while I believe that both the bottom of the bullet and the sides of the bullet benefit from coating, apparently coating the sides is the more important thing at higher velocities. I guess it stands to reason that the heat from sliding friction increases as velocity increases ?
-- coatings thicker than 0.001” - 0.0015” don't help, and in fact may hurt accuracy by unbalancing the bullet.
-- BHN 18 may be slightly better than BHN 11 for coated plain base at 2800 - 3000 fps. At any rate, BHN 18 did no harm.
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Things To Try With Powder Coat Next Time:</u>
-- try to coat both the base and everything that is in contact with the barrel.
-- aim for a coating thickness of 0.001” - 0.0015)
-- how does it compare to quenched powder coat GC bullets?</p>
<p>A lot of shooting may be required to answer those questions. :fire</p>
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, except this time with mica added to the polyurethane. I had tested OBPU + mica in the 6x45 and found that the mica actually hurt performance, but there were some problems with the way I applied it that time. So I tried again, and this time the application went well. Yet the mica still hurt performance. I admit defeat with mica. :P
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. :D :D :D
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 70 minute coating did not shoot as well as the 12 minute coating.
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 is not enough, and that 0.245” is more accurate than 0.244” in this barrel.</p>
<p>The quest for the ideal Hi-Tek thickness will go on. :coffee</p>
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 they were nose sized less and 2) the Durable Wet Black powder had a little residual hybrid powder left over in the tumbler. I didn't anticipate that mixing powders would be a problem but in this case it was because the two powders seem to repel each other and segregate on the bullet, rather than form a homogenous mixture. I dunno if that affected the way the bullet shot? In any event accuracy was poor, either due to the funky coating or due to the different nose-sizing.</h1>
<h1>13 & #14 were back to the same Hi-Tek load as in #3 and #4. Accuracy was not great but at least Hi-Tek was consistently shooting in the 2” neighborhood. Consistency counts for a lot. :)</h1>
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<p>FYI “FB)
"going nowhere." :D :D :D
 and is likely to increase imbalance. Strive for 0.001” thick, a good cure, and minimal out-of-roundness. I've had good luck with 3 thin coats about 0.00033” thick each. </p>
<p>-- the main problems I've experienced with Hi-Tek were air-drying prior to curing, and curing. If there are still wet spots when you put it in the oven (the lube grooves take longer to dry than the rest of the bullet), the wet spots will bubble. Sometimes it will dry in 10 minutes while other times it takes an hour, depending on the temperature and on the coating thickness. You can speed up drying by setting the tray in a warm place.</p>
<p><b>Things To Try Next Time and Down The Road:</b>
-- work out the Hi-Tek mixing ratio to create my favorite 0.00033” thickness per coat. Thus far I've been mixing by the seat of my pants so I have no idea what the ratio is, and every mix has been a little different. This high-velocity stuff is forcing me to up my game and develop more methodical reloading techniques. </p>
<p>-- other than that, I'm comfortable with Hi-Tek as a coating, particularly for bullets that don't need to be heat treated. So I'm going to shift my attention to powder coat for a while and see if I can figure out why its 1.54” accuracy went away?</p>
<p>-- while load tweaking is not the focus of this thread, I am hoping that I can improve the accuracy of this 75 gr. bevel base bullet by trying different powders, by fine tuning the nose-sizing, and by fine tuning the neck tension.</p>
<p>-- since this load seems to be extremely fussy about BHN, it might be worthwhile to try 15 BHN. Or 14.5, even. :D</p>
<p>-- I think the next thing I will look at with powder coat is the cure. Does the cure time affect accuracy? </p>
<p>Thoughts? Suggestions?</p>
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<p>I would like to repeat this test with 2 coats of Hybrid powder so that poor accuracy can't be blamed on a too-thin coating. The rule of thumb for 2 coats is to cure the first coat for only 60% of the normal curing time. Perhaps I will try an 18 minute cure on the first coat followed by a 30 minute cure on the 2nd coat.</p>
<p>Thus far I have not been able to achieve repeatable accuracy with plain base bullets and powder coating. One batch will shoot good but the next batch will shoot bad. Perhaps my coating is too “ratty” -- for sure the 1-coat hybrid is too thin and splotchy -- or perhaps there is some other important variable that I am not even aware of?</p>
<p>Today's target.
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 of uncoated bullets at 2886 fps, the barrel was surprisingly clean. This photo shows the worst fouling I could find, just a little bit of grey wash in the corner of the grooves about 3” from the muzzle. </p>
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