Coating Shootout

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mtngun posted this 07 April 2016

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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mtngun posted this 07 April 2016

THUMBNAIL TEST:

Some people like to test coatings by smashing the coated bullet with a hammer, but I've never figured out what that is supposed to prove?    If there's going to be a problem with adhesion, it'll show up when you are sizing the bullets.   That goes double for me because I often nose-size bullets in addition to the normal sizing step(s).

But an easy test is to try scraping the coating off with your thumbnail.   

Hi-Tek can be scraped off with your thumbnail.    It sometimes flakes off while sizing, too.

To my surprise, both of the powder coats would scrape off with a thumbnail.   It took more effort than Hi-Tek, but it could be done.   I have not tried sizing the ESC-coated bullets yet.

Oil-based polyurethane sticks like glue and that is one reason I liked it.    Unfortunately, polyurethane has a modest melting point and I suspect that may be a problem if you push the bullets hard enough.

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mtngun posted this 07 April 2016

THICKNESS AND UNIFORMITY:

Previous tests with extra-thick (0.003” - 0.004") coatings yielded inferior accuracy.   Perhaps because the thicker coatings resulted in an unbalanced bullet?

I'm not sure what the ideal coating thickness is?    If the purpose of a coating is to protect the bullet from the hot gases, to prevent melting and gas coating, then thicker might be better, providing it is uniformly thick, and providing it adheres well.

One of the problems with the shake-n-bake method of powder coating (or other coatings) is that the thickness can vary quite a bit.   That may not matter for low-velocity plinking loads but my interest is in accurate loads for high velocity.   Hence I broke down and purchased an ESC gun for this test.   

The bullets for this test were coated as-cast from a 3-cavity mold.    All of the following measurements are for the bottom band only, based on a random sample of 10 bullets.

Uncoated/Unsized: -- Average = 0.2438" -- Lo = 0.2433" -- Hi = 0.2448" -- Average Out-of-Roundness = 0.0006"

One Coat ESC Super Durable Wet Black (polyester): -- Average = 0.2466" -- Lo = 0.2460" -- Hi = 0.2473" -- Average Out-of-Roundness = 0.0004” (less than uncoated bullet) -- Average Thickness = 0.0014"

One Thin Coat ESC Hi-Temp Bronze (silicone): -- Average = 0. 2455" -- Lo = 0.2446" -- Hi = 0.2464" -- Average Out-of-Roundness = 0.0004" -- Average Thickness = 0.00085"

One Heavy Coat ESC Hi-Temp Bronze (silicone): -- Average = 0.2462" -- Lo = 0.2452" -- Hi = 0.2470" -- Average Out-of-Roundness = 0.0005"  -- Average Thickness = 0.0012"

4 Thin Coats Zombie Green Hi-Tek: -- Average = 0.2452" -- Lo = 0.2442" -- Hi = 0.2465" -- Average Out-of-Roundness = 0.0006" -- Average Thickness = 0.0007"

Note the Hi-Tek was deliberately mixed thinner than usual because supposedly a thin mix adheres better than a thick mix, and I wanted to give Hi-Tek a fair chance to adhere, since one of my dislikes about Hi-Tek is that it doesn't adhere all that well.    Also, I baked it at 350F rather than the recommended 380F because it is my experience that 380F “burns” the coating, turning it a coffee color and making it more prone to flake off.

CONCLUSION ON OUT-OF-ROUNDNESS:

All of the ESC coated bullets were more round than the uncoated bullets.  :D  

The ESC method is a success in the sense that it produces a more uniform coating than the shake-n-bake method.   Coating-induced imbalance should not be an issue with the ESC method.

Even the Hi-Tek coating did not increase out-of-roundness, though it did have the highest extreme spread (0.0023") between the Hi and Lo diameters.     If I had used a more normal thickness of Hi-Tek mix then then out-of-roundness would have increased, based on previous experience.

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gpidaho posted this 08 April 2016

Very nice work mtngun. Holding your out-of-round dimensions to as good as or better than the as cast number is impressive. Gotta love the ESC guns. I haven't tried Hi-tek so Im watching with interest. I know Hi-tek has a very large following but I made the choice early to go with PC for my needs and it's worked well for me. Looking forward to the results of your tests. Gp

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mtngun posted this 08 April 2016

gpidaho wrote: I haven't tried Hi-tek so Im watching with interest. I know Hi-tek has a very large following but I made the choice early to go with PC Hi-Tek is “OK,” but nothing special -- so far, anyway.   It's downsides include marginal adhesion, cost, and it's incompatible with heat treating.  

I'm liking the ESC method though it does take money and time to get everything set up.   I'm hoping the silcone coat will be a winner, we'll see.

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mtngun posted this 08 April 2016

THE OVEN TEST:

Place coated bullets in a heat-treat oven and gradually ramp up the temperature until the coating melts or otherwise fails.

Most of these coatings are polymers that don't have a definite melting point, instead it gets soft, begins to sag, the bullet laying on its side becomes oval instead of round, and eventually there's nothing left but a puddle.    I defined “failure” as either a very flat bullet or else a puddle.

Oil-based polyurethane did poorly in the oven test, failing at 700 F.   It's a shame because in other respects it's a cheap & easy coating that works well up to a point.

Polyester powder coat (Powder by the Pound Super Durable Wet Black) did not hold up well, melting at 700 F.

At this point I don't know whether the melting temperature has anything to do with the shooting performance, but I suspect it may.    We'll see.

By comparison, gilding metal melts at 1800 F (if you believe the internets).   So even the hi-temp silicone jacket is still wimpy compared to a gilding metal jacket, plus our coatings are much thinner than a  gilding metal jacket, too.

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RicinYakima posted this 08 April 2016

My question is: Does a cast bullet ever get to 700 F. when shooting? On the face of it, for the amount of time it is exposed to powder gas, could the bullet get that hot? Ric

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45 2.1 posted this 08 April 2016

RicinYakima wrote: My question is: Does a cast bullet ever get to 700 F. when shooting? On the face of it, for the amount of time it is exposed to powder gas, could the bullet get that hot? Ric

It's kinda of obvious it does from the pictures of flame cut bases he has posted. That is an indicator that the base needs to be protected from the gas.

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mtngun posted this 08 April 2016

RicinYakima wrote: My question is: Does a cast bullet ever get to 700 F. when shooting? On the face of it, for the amount of time it is exposed to powder gas, could the bullet get that hot? Ric No one knows ... but as 45 2.1 mentioned, there's those recovered bullets I have posted with the bases that sure look melted and gas cut.    It's well understood that any damage to the corner of the base can cause fliers.   

The bottom line is that jacketed bullets can be pushed up to 4000 fps with good accuracy yet plain base cast bullets struggle much above 2000 fps.     Why?   Why can't we push plain base cast bullets 4000 fps?    Exactly what causes them to fail?   Inquiring minds want to know.  :D  

Why do coatings pretty much eliminate lead fouling in most loads?   Well the obvious answer is  the coating keeps the lead away from the barrel, so there is no contact.    But why doesn't the coating foul the barrel, then?   What is different about the coating material compared to lead?    

It's possible that the melting temperature of the material has nothing to do with bullet failure.   Maybe something else is going on that I am overlooking.    I try to keep an open mind.     In the meantime I continue doing crazy experiments, looking for answers.   :D

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RicinYakima posted this 08 April 2016

"It's possible that the melting temperature of the material has nothing to do with bullet failure.   Maybe something else is going on that I am overlooking.    I try to keep an open mind.     In the meantime I continue doing crazy experiments, looking for answers.   :D" I have questioned several engineers that designed rocket nozzles for space launches. The most common opinion is that it is “molecular abrasion" or the products of combustion ripping the material off the bullet from leakage. That is how “lube” works, sacrificial material instead of the bullet. Only after it is abraded from the bullet does it vaporize to be laid down as leading in the barrel. NO PROOF, BUT THEY ARE SMARTER THAN ME! I am following closely to see if the “scratch off” ease has any relationship to shooting effectiveness. This is all interesting stuff you are working on, so keep going. Best wishes, Ric

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OU812 posted this 08 April 2016

Maybe you can compare ease of cleaning between types. Hi Tek Powder(two kinds by the way powder vs liquid)leaves behind lots of coating residue inside of barrel after firing. Inside of barrel looked foggy and did not look clear after firing 10 rounds. Patch a little difficult to push down barrel. Accuracy was not TOO bad.

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mtngun posted this 08 April 2016

RicinYakima wrote: I have questioned several engineers that designed rocket nozzles for space launches. The most common opinion is that it is “molecular abrasion" or the products of combustion ripping the material off the bullet from leakage. That is how “lube” works, sacrificial material instead of the bullet. Only after it is abraded from the bullet does it vaporize to be laid down as leading in the barrel. NO PROOF, BUT THEY ARE SMARTER THAN ME!    I am following closely to see if the “scratch off” ease has any relationship to shooting effectiveness. This is all interesting stuff you are working on, so keep going.  Best wishes, Ric The tentative plan is to start each coating out at 2200 fps with a clean barrel,  gradually walk velocities up, and note the speed where each coating begins to shoot “wild.”      Maybe some sort of trend will emerge, or maybe not.   We'll see. It's conceivable that the coating may hold together, yet enough heat is transferred through the coating to cause the bullet to melt and shoot wild, anyway.     If that is the case then I don't know how to fix it other than a thicker and less heat conductive coating material, such as a paper patch.

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mtngun posted this 08 April 2016

OU812 wrote: Maybe you can compare ease of cleaning between types. Hi Tek Powder(two kinds by the way powder vs liquid)leaves behind lots of coating residue inside of barrel after firing. Inside of barrel looked foggy and did not look clear after firing 10 rounds. Patch a little difficult to push down barrel. Accuracy was not TOO bad. I'll certainly note the bore condition at the time the coated bullets “fail.”   

I'm still debating whether or not to lube the test bullets.     While most people don't lube their coated bullets, I have found that unlubed coated bullet may leave a bit of polymer fouling at rifle velocities.    Lube usually eliminates the polymer fouling and I've never seen any downside to using lube, other than the extra step at the reloading bench.

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Ken Campbell Iowa posted this 08 April 2016

concerning uneven plastic coating .. could we establish a crude baseline by intentionally unevenly coating some bullets ?

how about applying additional coats of plastic with a brush to only 1/3 of the circumference front to back and try a few groups . plus normal wax lube ... i think .

after the results we could predict what and why the effect will be ( g ) .

if lopsided coating doesn't make a difference it would simplify test preparation .

ken

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mtngun posted this 10 April 2016

After debating whether or not to lube the coated bullets, I elected to lube them with my usual HVR lube, because past experience tells me that sometimes coated bullets benefit from a lube, and that lube has never, ever hurt performance.   

All of todays bullets used air-cooled wheelweight, ~11 BHN, because that is what worked best with coated plain base in my 6x45 experiments.

All of today's bullets were nose sized to be a glove fit in the throat when seated with only the bottom band in the case.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.  :(

The black powder coat shot poorly with the starting load so I did not test it any further.

ESC HI-Temp Bronze Silicone Powder Coat A few of these bullets flaked at the base while sizing, including the bevel or sometimes the side of the base.   Most did not flake so I only shot the un-flaked bullets.

After shooting the silicone-coated bullets, the barrel was clean near the throat and in mid-barrel, but there were streaks of lead at the muzzle end.

Hi-Tek shot decently with the starting load.   Yea! :)

I was running out of bullets and the last group at 2740 fps was shot with 6 bullets that I had set aside because they had significant flaking on their bases.    That group did open up noticeably, but was it due to the flaked bases or due to the higher velocity?

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mtngun posted this 10 April 2016

Re: flaking, brittle powder coat

The internets are telling me that flaking and brittleness is a sign of incomplete cure.   Well, I followed the instructions but ... I will try curing for a longer time period and retest.

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mtngun posted this 10 April 2016

Now they tell me -- when the powder coat instructions say “bake at 400F for 15 minutes,” what they really mean is “let the part sit in the oven until it reaches 400F, however long that takes, then let it stay in the oven for another 15 minutes.”  :D  :D  :D  :D  :D

I want to give powder coat a fair chance to work before I give up on it.

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mtngun posted this 11 April 2016

Adding 20 minutes to the recommended cure time and switching to the flat push-stems was a success!    No significant flaking while sizing. :cool:

So why did the hi-temp silicone powder coat shoot poorly even with a decent cure?    I don't know.   There was no leading, only some black polymer fouling.     For whatever reason, it didn't work well, so I have to admit defeat and move on.  :(

After shooting the 2909 fps polyester coat load, the barrel was squeaky clean except for some very light streaks of lead near the muzzle.   Many people would be happy with a barrel this clean but I consider any leading to be a sign that something is going wrong. 

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mtngun posted this 11 April 2016

Guys, it's been a while since anyone has told me that my alloy is too brittle, that I should use a 2-diameter bullet, that I should use H322 powder, that I'm using too much lube, or that I should check to see if my barrel is bent.

Come on, you're slipping.  :D  :D  :D   What do you think of my coating test so far?

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45 2.1 posted this 11 April 2016

Hahahahaha............ It doesn't do any good to do those things. We are all waiting on your testing results. Until you give up, you won't try anything someone suggests.

On the heat cycle time for your coating, you need to look at the times used for heat treating..... those insure you have adequate soak time.

Keep up the good work........ so far it has matched what other people have done at that and higher velocity for a slow twist barrel. We do wonder what you could do with a gas check bullet though.

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muley posted this 11 April 2016

mtngun, keep up the good work, its interesting that u are getting these readings with air-cooled ww. most of us have thought that those speeds would lead to leading problems. what is the burn rate of the WC 844 ?

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