Plastic Coating and Bullet Size

[Dimner]

In the "Elvis 61 grain" Ian mentioned something that caught my eye:

Stubborn, plastic bore fouling at high velocity and accuracy problems at low velocity together with bore leading are the usual complaints with slicks but it doesn't always happen. I personally think that keeping coated monoliths closer groove diameter than throat entrance diameter is a key point to managing lead and plastic bore fouling. Those who go about the coated cast bullet thing the same way they do conventional lubricated bullets by making them 2, 3, or even 4 thousandths larger than groove diameter nay not have very good results, especially with slicks.

(emphasis is mine)
Caught my eye for two reasons:

1) I'm that guy when doing conventional lubed bullets goes 2-3 thous over groove diameter. It's always worked great for me in 30 cals, therefore that has translated to how I have been approaching my PC bullets for my 223 wylde AR15. Doh!

2) I have been struggling with accuracy with my 223 wylde ar15 load. Chasing variables in the most pathetic, non-scientific manner. So I am trying to get back to basics. Seeing Ian's quote leads me to believe that I am far over the size I should be.

So with that little confession... I get the feeling that there has been more discussion about powder coat bullets and their relationship with bore size or topics on how the PC behaves during it's initial obturation and then path down the barrel. Could someone point me to these threads? Before I make a post about all my attempts and struggles, I would like to read more about PC and how it interacts with the bore. Basically, I have been (wrongly) treating PC as a direct substitute for conventional lube. I need to fill that knowledge gap. How does PC behave from primer strike till exit from the barrel?

 

[Ian]

This might help:

My quest for speed and accuracy with powder-coated cast bullets

Sometimes the internet is wonderful, sometimes near useless, it all depends on what you're looking for and what's already been done and posted. With regard to getting good accuracy with powder-coated cast rifle bullets at normal copper-jacketed speeds, the reports are fair to dismal and only a...

www.artfulbullet.com

 

[popper]

I use PC for 9, 40, 30/30,308W & 3 300BO. My alloy is 2-4% Sb, some Cu added for rifle. I run 850 to 2400, softer alloy for slower. I size normal 1-2 thou over except BO which is tight chamber so just over bore. No leading. Make sure neck tension is 2-3 thou. and bullet is not swaged smaller when loading. A sharp edge will scrape it off so flare mouth when loading. PC is tough and ductile coating but when it hits a 'sharp?" edge, it scrapes off. Don't think anybody really knows how it acts in barrel, if alloy is too soft it will still strip in rifling. It does 'wear/sluff' off in the barrel but cleans like powder residue. I have seen it plated with lead at the 'lube' star muzzle so some does 'melt/vaporise' - maybe from burning powder at muzzle?

 

[Ian]

Before I make a post about all my attempts and struggles, I would like to read more about PC and how it interacts with the bore. Basically, I have been (wrongly) treating PC as a direct substitute for conventional lube. I need to fill that knowledge gap. How does PC behave from primer strike till exit from the barrel?

I think the consensus among the honest is that we haven't really proven definitively how normal, lubricated cast bullets behave inside the rifle barrel, and those have been around in one form or another for centuries.

What can be proven through secondary indicators is that powder paint makes the bullet more slippery so it has a lot less friction with the case neck, throat during engraving, and throughout its trip down the bore. In order to make the stuff work, it has to remain on the bullet and not get scraped off in tje throat, which is where size becomes important.

At high velocity, the bullet tends to do better if it has a slightly loose static fit in the throat, i.e. it wants a little space around and some jump to the lands so it can wiggle around and find bore center. The coating is just barely tough enough to let the bullet find its own center (dynamic fit) if the nose shape is right for the throat. Once engraved, the coating provides some separation between bullet and barrel like any other jacket, and also strengthens the bullet's surface at the critical land drive-side interface so the bullet can handle more acceleration and velocity than normal for a lead-alloy projectile and still maintain a true enough form to be reasonably accurate at copper-jacketed speeds.

I have found that the alloy of powder coated bullets can be much softer than what we would normally use in a given application. Your guess is as good as anyone's as to why.

Review the thread I linked above and you'll find near the last few pages a summary of what I learned.

 

[popper]

Something that might be helpful. Xcel program showing FORCE on the base. fps is NOT accurate, just an indicator. Distortion and fps are function of force.

cal	0.3​	0.223​	0.4​	0.45​	0.357​
area	0.471​	0.35011​	0.628​	0.7065​	0.56049​
psi	45,000​	45,000​	45,000​	45,000​	45,000​
force	21,195.00​	15,754.95​	28,260.00​	31,792.50​	25,222.05​
time	0.0005​	0.0005​	0.0005​	0.0005​	0.0005​
mass	170​	55​	165​	220​	240​
fps	436​	1003​	599​	506​	368​
cal	0.3​	0.223​	0.4​	0.45​	0.357​
area	0.471​	0.35011​	0.628​	0.7065​	0.56049​
psi	20,000​	20,000​	20,000​	20,000​	20,000​
force	9,420.00​	7,002.20​	12,560.00​	14,130.00​	11,209.80​
time	0.0005​	0.0005​	0.0005​	0.0005​	0.0005​
mass	170​	55​	165​	220​	240​
fps	194​	446​	266​	225​	163​

 

[CWLONGSHOT]

Ok here is what I have to add... I resist as there is many here with so much more in-depth knowledge than I. But I am likely as experienced as many others.

Im not much in all that scientific, mumbo-jumbo with charts circles a d arrows explaining how what where and why...

what I know is what I experience.

I have seen zero evidence that powder coating would or could in any way effect obturation.

To the fouling what you need to remember every barrel is different. Some copper foul while others do not. Some Lead and again others do not. NO ONE CAN TELL YOU UNEQUIVOCALLY that PC bullets will stop all leading worries. BUT, if it was a problem it wouldnt be as popular as it is growing to be.
Now personally speaking. I have pushed 20:1 alloy past 2000 fps in a few guns with zero leading AND ZERO BULLET FOULING. I have hit 2400 with some loads also without isues related to powdercoating.
Another advantage is durability to pressure and heat. Places where we once looked to a gas check. With powder coat I still feel a gas check is valid its just no longer needed as your cast past super sonic speed. Now with good properly applied and cured powder coat to can shoot to at least 2000 fps before requiring a GC. Again my own expereince but know for myown use I install a gc on any bullet design made for a GC.

Your experiences surely could vari my own expereince is widening almost daily as I cast and shoot more fireams and turn friends on to same. Thus far in my very few years powder coating I have probably had 30+ firearms shot with my bullets. Super so ic and sub so ic. Rifles revolvers a d semi auto guns and ONE S&W pref center gun in 43 Mag has shown leading. BUT KNOW it was a 250g RCBS bullet on 10g of Unique. So NO WAY HOT!!

I will put my $$ where my mouth is and extend this to any here wanting to try. You pay shipping, Ill send ya some PC bullets, of appropriate caliber, for you to try.

Good luck and God Bless

CW

 

[fiver]

I have seen zero evidence that powder coating would or could in any way effect obturation.

the coating goes where the lead goes.
it's main bonus features are gas cutting and it's slick.

why it lets you use a bit softer alloy is the 64$ question, but I bet it's tied into the diameter thing and why they seem to work closer to jacketed size.

 

[Ian]

I have a theory based only on results from a lot of shooting that the limiting factor to accurate velocity is more a matter of drive side wear than anything else. Crooked starts and base riveting are minimized when the small/slick bullet can slip right to the center of the bore due to low friction and soft alloy providing minimal resistance to engraving force and thus minimal force of the powder pressure on the base during the initial launch. However, there's more to that story because I played with breech seating a number of years ago using ordinary lubricated, gas-checked bullets of various alloys and STILL couldn't reach full jacketed speeds without getting bad antimony wash and loss of accuracy. Land engrave abrasion is the only explanation I can come up with for the accurate speed limit once the crooked launch was eliminated by breech seating. Further, adding a paper jacket, copper jacket, or plastic jacket cures the accurate velocity limits so what other explanation is there but preventing land engrave wear?

 

[popper]

Couple yrs ago I calculated the radial force due to base pressure and rifling, I'll try to find the numbers again. IIRC ~ 30% was radial for most twists. Leaves 70% for acceleration of bullet. used 4 thous depth and 6 groove; 308 only and just 'starting' numbers.

 

[Joshua]

So I may be off base here! If I’m understanding Popper’s chart correctly, because of lower base force, smaller diameter projectiles can be pushed harder.

I have seen many references to people shooting PC’d without GC .223 bullets really fast. I didn’t understand why you could push these so much faster than say .30 or .35 cal bullets. It makes sense to me now. The amount of base force is so much lower as you reduce diameter, enabling the thin PC to survive the entire length of the barrel, at velocity that wouldn’t work in larger calibers.

 

[popper]

Pretty much but 223 is much lighter bullet also. Also shows why 357 & 45 are 'low' pressure loads. Again while back I smashed 40SW bullets with hammer - they are really warm when hit. Figured it would take 50ft 10# drop hammer to get to base force = to powder force. Figures my 5# maul overhead swing was about same as what the base sees on the chamber. Careful with that test, they ricochet a long ways fast. After shooting a lot of PCd cast in 308W carbine I notices a lead/PC buildup at the muzzle with flash hider. Concluded that hot PC and lead 'dust' deposited there. Accuracy went down so I took the F.H. off-messed up good crown.
I'd HiTek (green) some 40SW and recovered. Coating left was whiteish. Stuff gets hot and abrades off.

 

[Ian]

I found the formula for land pressure after many years of searching and did the calculation for a four groove, .30-caliber with .004" rifling depth, 10" rate of twist, 22" barrel, 2400 fps muzzle velocity, and the MP 30-180 Silhouette bullet.

10,000 PSI give or take is what I came up with. That would be about 40K peak psi for a .30-'06 case, so 25% of peak pressure.

So the alloy must be able to resist 10K psi on the sides of the bullet in that instance.

But wait........there's more.

2400 fps linear velocity (discounting the helix) against barrel steel makes HEAT. So does being squished through the throat (Popper's observation I can verify, smash a bullet and burn your fingers picking it up). So does the powder gas but specific heat of lead is very high so it doesn't have enough time to warm more than the gas check.

Lead alloy melts at 550-600⁰F. At 200⁰F it is at about half strength where the strength is needed the most.....at the drive side of the lands at the highest point of velocity in the barrel (discounting the highest point of acceleration stress on the lands, which is back near the chamber). See why we get antimony wash and HV leading near the muzzle?

The only reason cast bullets work at all is the instantaneous velocity is low when the pressure is highest (1.5-3" past the case mouth) and the pressure has dropped 75% or more at peak velocity (at the muzzle) inside the barrel.

 

[Dimner]

I'm not an unintelligent fella, college degree in information technology/computer science, have worked for 20+ years in the field.

But for the life of me I feel like there is some sort of baseline knowledge I'm missing to be able to understand most of what is being said here after popper's chart. Ian, I read through your thread. Very informational. Went through your lessons learned. I'll be trying a bunch of those as I reset my project and start testing again.

So with all that gents, Here's the plan so far:

Bullet: NOE 225-70-RN - a plain based round nose
Alloy: clip on wheel weights with tin added to get the alloy to 1.7% tin
Powder Coat: Smoke's bacon grease and John Deere green
PC Application: Shake and Bake with air soft BBs
PC Bake: 425* preheated convection oven for 24 minutes
PC Quench: Immediate douse in tap water at faucet temp.
Aged: 7 Days

Bullets are weight sorted to 71.0 +/- 0.1gr

Three groups of bullets for each powder coat color:

1. 226 sized before powder coat, followed by 226 sized post powder coating
2. 225 sized prior to powder coating, not sized again after powder coating
3. 225 sized prior to powder coating, followed by 225 sized post powder coating

My goal is for the hardness to be near 13-15bhn. Does this seem a good place to start? Previously I was ice quenching post PC. Resulting in an aged hardness around 18-20bhn. But based on the past reading from you gentlemen, sounds like that was far too hard for this Application.

The reason for testing 2 different colors is that they coat a little differently and possibly a different thickness. I can post pictures if anyone is interested.

I can go into specifics of brass prep if it is of interest to anyone. As well as the specs of the AR that I will be using, but the barrel is easily capable of 1 moa with jacketed handloads.

Another portion that I could use some input on is the starting COAL of this bullet. My testing will not include any magazine loading. I slip the round right into the chamber for each shot and then release the bolt.

 

[Joshua]

Failure of bullet engraving! What a complicated formula that I will never endeavor to write. Mass vs engraving surface area vs base force wich is tied to overall diameter, with mitigating factors being not just overall hardness, but alloy fluidity based on alloy composition and heat treatment, combined with lot and batch variance of PC, with cure time and temp being another possible variable.

All the above does not even look at bullet size and shape as it relates to dynamic fit.

Dimner,
Why did you choose a bore riding bullet?

 

[Ian]

But for the life of me I feel like there is some sort of baseline knowledge I'm missing to be able to understand most of what is being said here after popper's chart.

25-40 years of tilting at windmills? It's tough to delete all that and try to break it down, but really I don't think you're missing as much as you may think. You only need to understand a few things to make this work, all the knowledge gained from decades of what did not work is only important to keep moving forward.

I tell everyone starting out that they need to know first and foremost what their rifle's throat looks like and measures. This requires a cast of some sort, like an impact impression, Cerrosafe cast, Sulphur cast, etc, but most won't do it for various reasons (not mentioned by the cast bullet handbooks?) I don't know. Anyway, at the very very least you need to know what your throat entrance diameter and bore diameters are. Pull your upper and remove the BCG. Take one of your coated bullets fhat is the largest diameter and drop into the chamber. Take a brass rod and tap it into the throat about 1/8" and knock it out with a cleaning rod. Next, tap a bullet into the muzzle nose-first and knock it out with a cleaning rod. Measure the land impressions with caliper jaws and the slug from the throat at the point that the throat cut it. That will give you throat entrance diameter and bore diameter.

For seating depth, bear with me here but try this. Your bullet is a two-diameter Barlow design which is probably the worst possible choice for your rifle, but it can be made to work up to a point. You need to be sure that the nose isn't too fat to slide up into the bore past the throat and that your sized diameter is smaller than the throat entrance by half to one thousandth of an inch. Ideally you will seat so that the front driving band of the bullet nestles about as far into the funnel of the throat as possible, which won't be far but it needs to be past the entrance by a few thousandths to ensure a straight start. If the nose is much more than .218" it may not let the bullet be seated far enough forward to get the first band started into the throat.

Here's about the best you can hope for with that bullet if seated to "proper" overall length:



More on bullet diameter and throat stuff to be found here, with a photo of slugs tapped into and back out of the throat so you can visualize what to measure and how to size bullets for your rifle:

https://www.artfulbullet.com/index....larger-than-the-throat-entrance-diameter.211/

You can see how if the bullet is coated and sized too big the coating and a little lead will get scraped right off, so make it just a titch smaller than throat entrance. .225 to .226" should be about right fir a .223/5.56. If the noses are too fat after coating to go in far enough for the front band to pilot up in the throat funnel, that's a whole 'nuther problem.

 

[Ian]

Along with making the bullet the right diameter for your rifle, you also need to get case neck tension correct, especially for softer bullets (13 bhn). I've used 10 to 19 bhn bullets in my 5.56s to 3,000 fps and didn't note any appreciable accuracy difference as long as they were 12-13 or better. 1.5 thousandths of interference fit is about right and you will need a bellmouthing neck expander to do this, otherwise your bullets will get squished by the case neck when seated and will be too small.

The best possible neck preparation is with a bushing neck sizing die which is selected so that it doesn't make the neck inside diameter too small to begin with. Full-length resizing dies make the necks really tight and re-expanding them to proper size for powder coated cast bullets introduces concentricity errors galore which is bad for accuracy and can cause one side of the bullet (and coating) to get scraped off in the throat due to off-center positioning from a wongo case neck.

 

[Dimner]

Dimner,
Why did you choose a bore riding bullet?

You know... thanks for pointing that out. I picked that bullet because because it's heavy and it wasnt the two other bullets (MP 65gr and 73gr NATO bullets) that I was having problems with. So maybe it's time to rethink even the bullet profile I am using. Originally, I wanted something heavy and somewhat long to stabilize in my 1:9 barrel as well as cycle the action. But right now, I am just looking for something 2 or less MOA at 100 yards. So I have no problem going a different route with the bullet.

With this new new new baseline set of testing (i think this is my 3rd testing reset on this project)... I'm open to using any profile of powdered coated bullet that I own as long as the test can give me useful information about:

• What bullet size my rifle likes
• What COAL my rifle likes

During this phase of testing, I'm not that concerned with velocity or a pressure curve that would cycle my AR. I can play with those elements later when I have an understanding of what bullet fitment and seated length works best.

So here are the 22 cal bullet molds I have on hand:

MP 227-65 NATO Gas Check

MP 227-73 NATO Gas Check

NOE 225-70-RN Plain base
(as shown previously)

NOE 225-62-FN (in the mail to me from NOE)

RCBS 22-55


Lee Bator Boolit
(we all know what that looks like)

 

[Ian]

1 in 9" is pushing it with 70 grains. The MP NATO bullet with the gas check would be the bullet I'd focus on first because it self-aligns better than any of the others and likes some jump to the lands (10-20 thousandths). Also, and you will soon find this important, the nose tapers and sizes of all the MP NATO bullets designed by Bob Kell will make powder coating thickness irrelevant.

 

[Dimner]

Ian, wonderful post and sketch, thank you very much. I have no hesitation about doing a pound cast of my chamber, but have not thought of a way to do it with precision in a gas operated rifle. Lever and bolt actions I have done a half dozen, but not in a semi auto. Is there some tips on how to keep the shell from jolting around while doing a pound chamber cast? If not, I'll resort to the method you describe in your post.

 

[Dimner]

1 in 9" is pushing it with 70 grains. The MP NATO bullet with the gas check would be the bullet I'd focus on first because it self-aligns better than any of the others and likes some jump to the lands (10-20 thousandths). Also, and you will soon find this important, the nose tapers and sizes of all the MP NATO bullets designed by Bob Kell will make powder coating thickness irrelevant.

I'd be happy to go back to the NATO bullets if that is what would give me the best chance for the information I am looking for. I was incorrect, I have a 1:8. Just double checked. So should I go 73 or 65? Or perhaps wait until we get a look at the chamber dimensions?